Evolution

by FRANK H. T. R HODES The University of Michigan ILLUSTRATED BY

REBECCA MERRILEES

and

RUDY ZALLINGER

®

GOLDEN PRESS

•

NEW YORK

Western Publishing Company, Inc. Racine, Wisconsin

FOREWORD

How l ife arose a n d h ow m a n developed are two q u es­ tio n s that are as old a s man h i m s e l f, a s the cre a t i o n a c c oun ts o f m a n y civi l i z atio n s b e a r witn ess. But a n c i e n t a s i s t h i s con c ern , t h e i m p l icatio n s o f m a n ' s re l a ti o n s h i p t o t h e wo r l d o f l ivi n g th i n g s are a s s ig n ifica n t i n t h e S p a ce Ag e a s t h ey were i n t h e Sto n e A g e. Th i s boo k i s a s i m p l e a c c o u n t o f m a n's search for t h o s e ori g i n s a n d rel a ti o n s h i p s. I t d escri bes t h e h i storica l d eve l o p m e n t of the pres e n t t h eory of evo l uti o n , or d es c e n t with m o d ifi­ cati o n , the i n d i ca ti o n s that sup port it, its n a ture a n d m ec h a n i s m , a n d its res u l t i n th e l o n g h i story of l ife. The boo k c o n c l u d es with a secti o n on t h e m ea n i n g of evolutio n , for the th eory of evo luti o n h a s h a d a p rofoun d i m p a ct o n m a n ' s view of h i m s e l f a n d h i s re lat i o n s h i p to th e worl d in w h i c h h e l i ves. Evo luti o n a ry th eory provi d e s a powerfu l ex p l a n a t i o n of h ow l i fe d eve l o p e d , yet b e­ yo n d i t, a n d un a n swered by i t, l i e s t h e ul ti m a te questi o n o f w h y l ife d eve l o p e d . T h a t q uestio n , confro n ti n g a s i t does t h e l a rg er s i g n ifica n c e of l ife, t h oug h t h e a b stra c­ tive m ethod s of s c i e n c e provi d e n o a p propriate so l ution to it, is neit her m ea n i n g l e s s n or inco n s e q uentia l. for in our re s p o n s e to it, i n d ividua l l y a n d c o l lective ly, l i e s the future of evol uti on , a n d with it the future of ma n k i n d. I a m g ra teful to m y co l l ea g u e D r. A l fred S m it h w h o k i n d ly rea d t h e m a n u sc r i pt o f t h i s book. Fra n k H. T. R ho d e s

l£0;).q5:

GOLDEN, A GOLDEN GUIDE®, and GOLDENCRAFT® are trademarks af Western Publ i s h ing Company, Inc.

Copyri g h t © 1974 by Western P u b l i s h i n g Co m p a n y , I n c . A l l r i g hts reserv e d , i n c l ud i n g r i g h ts o f reprod u c t i o n o n d u s e i n o n y f o r m or by a ny m eo n s , i n c l u d i n g t h e m a k i n g of cop i es by a n y p h oto proc e s s , or by a n y e l ectro n ic or m ec h a n i c a l dev i c e , p r i nted o r w r i t te n o r ora l , o r record i n g for sou n d o r v i s u a l re p rod u c tion or for use i n a n y k now l e d g e ret r i eva l system or dev i c e , unl e s s per m i s s ion i n w r i t i n g i s o b t a i ned f rom t h e copy r i g h t p ropr i etor. P rod u ced i n t h e U . S.A. P u b­ ri s h ed by G o l d e n Pres s , N e w Yor k , N . Y . l i b ra ry of Con g ress Cata l og Card N u m ber: 74-76432

C O N T E N TS OVE RVIEW OF LIFE .

. .4-29

Di vers ity, develop m e n t a n d orig i n of l i fe; spon­ ta neous g e n era t i o n ; c l a s s i f i cation; degrees o f d i vers ity a n d d i scove ry of l i fe's lon g h i story; deve l o p m e n t of t h e theory of evo l u t i o n from Aristotle to La m a rc k to C h a rl e s D a rw i n a n d Alfred R u s s e l Wa l l a ce; t h e voya g e of t h e Beagle a n d p u b l icat ion o f O n the Origin of Species; t h e w o r k of G r e g o r M e n d e l , lea d i n g to t h e laws o f i n h e rita n c e a n d to the new synthesic t h eory of evo l u t i o n . .... 30-55 Con t i n u ity, u n ity a n d t h e n a t u re of l i fe; i n ter­ depen d e n c e a n d i m p l icatio n s of s i m i la r i t i e s­ a n a to m i c a l , e m b ryolog ica l , b i oc h e m i ca l , a n d serolog i ca l ; ad aptations; geogra p h ical d i s t r i b u ­ tion; l i v i n g species; c h a n g es i n species; fos s i l s a n d h i g h e r ta xa; m i ss i n g l i n k s a n d t h e fos s i l record .

I NDICATI O N S OF EVO LUTION .

. .56-1()3 I n herita n c e : Ce l l d i v i s i o n ; patte r n s a n d laws; proba b i l ity and m e c h a n i s m ; g e n e s ; c h e m istry (DNA a n d RNA), sources of varia b i l ity; reco m ­ b i nation a n d m u ta t i o n ; g e n e t i c d ri ft; i s o l a t i o n a n d m i g ra t i o n .

THE PROCESS OF EVO LUTI O N .

N a t u r a l Selectio n : Nature of n a t u r a l selection a n d its action i n living popu lations a n d fos s i l s ; adaptation a n d m i m icry; sexual selectio n ; n a t­ u ra l selection i n m a n ; m i s s i n g l i n k s . T i m e: T h e geolog i c t i m e scale , rates of a n d recipe of evo l u t i o n . THE COURSE O F EVOLUT I O N .

104-150 T h e p r i m itive eart h ; orig i n of l i fe; fos s i l s-th e o l d est; m a r i n e i n verte b rates-t h e oldest; l i f e o n l a n d ; l a n d , va s c u l a r seedless a n d seed bea r i n g p l a nts; a m p h i b i a n s ; t h e r i s e of a n d d o m i n a n c e of t h e re ptiles; a doptive rad iation; b i rds; evol u ­ t i o n a n d g eog ra p h i c d i st r i but i on of m a m m a l s ; pri mates a n d evo l u t i o n of h u m a n societies.

T H E M E A N I N G OF EVOLUT I O N

.... 151-155 I t s i m p l icatio n s ; perspective; t h e future of m a n .

M O R E INFORMAT I O N.

.156

I NDEX

.1 57

3

OVERVIEW OF LIFE Ea rt h tee m s with l ife . living cre a t u re s exist from ocean dept h s to t h e h i g h e st m o u ntain p e a k s, fro m e q u a to ri a l j u n g les a n d h o t m inera l springs t o t h e frozen p o l a r waste l ands, fro m t h e b l inding b rig h tness a nd a ri d i ty of t h e desert to t h e d a rk intesti n e s of a ni m a l s . I n e a c h envi ro n m e n t, unto l d nu m bers o f indivi d ua l o rg a ni s m s i n h abit eve ry n o o k and cranny of t h e a va i l a b l e s p a c e . Most a n im a l a n d p l a n t species conta i n a myriad i n d ivid u a l s . Thus the s u rface layer of m ost m e a d ow soil s conta i n s several m i l l io n a n i m a l s per a c re . Micro­ scopic a n i m a ls and pla nts exist i n u n cou n ta b l e n u m b e rs . O n e g ra m of soil may conta i n h u n d reds of m i l l i o n s of l ivin g t h i n g s . B i rds a n d in sects exist i n populatio n s so vast a s to con stitute loca l "plag ue" con d ition s . Aquatic l ife is n o l ess p rol ific. It is u n l i ke l y t h a t the e a rt h is u ni q u e in this respect . I t h a s bee n c a l c u l a ted t h a t t h e re m a y b e m i l l i ons of p l a nets in ot h er p a rts of the u n iverse ca p a b l e of s u p ­ p o rti n g s o m e form o f l ife. Eac h e nviro n ment s u p ports a n i m a ls .

a

distinctive co m m u n ity of pla nts a n d

6. Other invertebrates-2 1 ,000 5. Wormlik e phyla-38,000 4. Protozoans-30,000

3. Chordates-45,000 2. Mollus ks-45,000 1 . Art h ropods-900,000

MAJOR GROUPS (PHYLA) OF ANIMALS-OVER 1 ,000,000 SPE CIES

DIVERSITY OF LIFE i s shown by t h e existence of

m o re t h a n o n e m i l l io n ki n d s (species) of a n i m als a n d m o re t h a n 350,0 00 k i n d s o f p l a nts . An i m a l s ra n g e i n size from a few thousa ndths of a n inc h to m o re th a n 1 00 feet in l e n g t h . They represent a vast variety of ways of l ife-p a rasites, pred ators, herb ivores, swi m m ers, fl i e rs, crawlers, burrowers. S o m e spend their l i ves fi x e d i n o n e spot; others u n dertake sea sona l m i g rations of thousa n d s of m i les. In spite of t h e m a ny ki n ds of a n i m a l s a n d p l a n ts, t h ey rep rese n t o n ly a few ba sic g ro u p s ( p hy l a ) .

4. Algae a n d F u n g i-60,000 3. Mosses

and

L iverworts-

23,000

2. Ferns, Con ifers, et c.-1 0,000 1. Floweri ng

Pla nts-250,000

MAJOR GROUPS (PHYLA) OF PLANTS-ABOUT 350,000 SPECIES

5

THE DEVELOPMENT OF LI FE h a s a lways b e e n o n e of

m a n's g re a t conce rns . A n c i ent s a cred writing s of m a ny fa it h s d i s c u s s this q uestio n . T h e ea r l y c h a p t e rs of the Boo k of Genesis, fo r exa m p le, d e a l with the seq u e n ce of c re a t i o n , and Ad a m n a m e d t h e d iffere n t k i n d s o f a n i m a l s . T h e nee d t o c l a s sify l iving t h ings w a s p a rt l y p ra ctica l . So m e p l ants were poisonous, ot h e rs e d i b l e . So m e ani m a ls were h a rm fu l , ot h e rs we re n o t . Ea r l y m a n's s u rviva l d e pended o n h i s s k i l l i n recogniz­ i n g each kind . Ma n 's d a i l y e x p e rience and re l i g i o u s t ra d ition coinci d e d h e re : every a ni m a l a n d p l a n t t h a t h e recognized re p rod u ce d " a fte r i t s own k i nd . " Ma n's e a r l y life a s a h u nter b ro u g ht h i m in c lose contact with a n i m a ls, and a n cient cave pa inting s b e a r a record of h i s inte rest. Late r d o m estication of a n i­ m a l s and h a rvesting of c rops inc rea se d thi s concern. Creation of Ada m f r o m M i c h e l a n g e l o 's p a i n t i n g of Sisti n e C h a p e l Cei l i n g .

r )

6

I

ARISTOTLE, G reek p h i losopher, was a ls o one o f the fi rs t a n d greates t b i o l o g i sts. He wrote exten sive ly o n the c l a s s ification a n d s tructure of over 5 0 0 s pe ­ c i e s of a n i m a l s f r o m t h e M e d i ­ terra n e a n a re a . Ari s totle w a s a gifted observer, a n d described details of s uc h t h i n g s a s ch ick em bryo logy. H e accepted the sponta n eo u s g e n era tion of fl ies from p u trefyi n g m a tter, b u t was also concerned about the prob­ lems of h eredity.

A R I ST O T L E (384-32 2 B. C . ), p u p i l of P l a to and tutor of Alexand e r the Great, o b s e rved t h a t s pecies a p pea red to be unc h a ng ing . Cows p ro d u ced onl y cows; horses a rose only fro m horses. Between the two t h e re wa s a clea r d ivision. S p ecies we re c h a ra cte rized by t h e i r re p ro­ d u ctive isolation. Ind ivid u a l s d eveloped, a ccord ing to A ristotle, by the c a p acity ( psyc he) of e a c h to confo rm to the a rc h etype c h a ra cters of t h e s p e c i e s relations h i p s . H e constru cted a "la d d e r o f Natu re" s h owing t h e unity of p l an. In contra st to A ristotle's "vita l i st" views, the De­ m o c rita n s were " m e c h a n ists ." T h e y b e l i eved t h a t a n o rganis m's a ctivity wa s the resu l t o f t h e intera ction of the ato m s of w h i c h it wa s m a d e . Altho u g h vita l istic and m e c h anistic controve rsies sti ll persist, the s u p posed confl ict b etwe en s c i e n c e and relig ion being an exa m p le, the two views a re ofte n co m pl e m e n ta ry, not c o m petitive . In s o m e situ a tio n s , we need to u s e both ( p . 1 5 5 ) . T h e d iscuss ion i n t h i s book is m e c h anistic ("how" t h ing s d e ­ velop, not "why") , but that do es n o t m e a n that l ife h a s n o m ea n i n g a n d p u rpose .

7

THE ORI G IN OF LIFE

wa s

long regarded as a spon­

taneous eve n t : l iving thing s a rose fro m nonlivi n g m at­ ter. Althoug h the va rious g ro u ps of l iving thing s were b e l i eved to h ave b e en created in d efinite s eq u e n ce, it was su pposed that each k ind of ani m a l and p l ant a rose " fu l l y fo r m ed " fro m the d ust of t h e e a rth . S u c h a view involved n o obvious contra d i ctions . Flies, for exa m p l e , cou l d be seen to d evelop fro m m a g g ots, w h i c h a rose " s pontaneously" in decayi n g m eat. T h e s pontaneous g enerati o n o f l iving t h ings b e ca m e a universa l assu m ption. We sti l l s p e a k of d i rt " b ree d i n g " ve r m in. T h e view w a s a l so econo m ical : it involved only one category of expl anation . O u r c u rrent popu l a r vi ews req u i re not only a n explanation for t h e origin of l ife b u t a l so one for t h e origin of species. Early views o n the o r i g i n o f l i f e i n c l u ded o n e t h a t s u g gested s h ee p a rose from a pla nt. (Afte r Weinberg.)

8

of l iv i n g cre atures from n onlivi n g m a tter beca m e increasing l y s u spect in the seve nteenth c e n tu ry. Francesco Redi ( 1621-97), an Itali a n p hysici a n , beca m e convi n ce d that the m a g gots fou n d in m eat were d e rived n ot fro m the m e a t itself but fro m e g g s l a i d by flies . S PONTA N E OUS G E N E R A T I O N

f l i es, d e c o y i n g m ea t , a n d m a g g ots

p l a ced a "dead s n ake, s o m e fi s h , a n d a s l ice of vea l" in four o p e n - m o u t h e d fl a s k s , and then placed the same t h i n g s i n f o u r fl a s k s t h a t h e c l o s e d a n d s e a l e d . F l ies con­ sta ntly settl ed o n the m eat in the open fl a s k s , w h i c h beca m e w o r m y . N o wor m s a p peared o n t h e m e a t i n t h e s e a led fl a s k s . Knowi n g t h a t s o m e b e l ieved air to b e e s s e n tia l for g e n e ra· t i o n , Red i repeated t h e ex per­ i m ent, t h i s t i m e u s i n g a g a u z e cover f o r t h e " c losed" fl a s k s t o p rotect t h e m f ro m fl i e s b u t a l lowi n g a i r i n s i d e . Ag a i n , n o m a g g ots a p pea red o n t h e m eat. This d i scredi ted the most fa m i l ­ i a r e x a m p l e o f s po n ta n e o u s g e n e ra tio n . R EDI

meat decoy ing, b u t n o f l i e s o r m a g g ots

A R EFINED VE RSIO N of Red i's

experi m e n t was used by Pa ste u r i n the m id - e i g htee n t h c e n t u ry to d e m o n strate t h a t p u trefa ction a n d f e r m e n ta t i o n depend o n action of ai r - b o r n e o rg a n i s m s .

..,

Ope nf l ies a n d m a g g ots o n decoy i n g m e a t

Cove red w i t h g a u zeno f l i e s or m a g g o ts o n d e c oy i n g m e a t

9

A C LASS IFICATI O N OF LIFE was devised by Aristo t l e

i n t h e fo urth c e n tu ry B . C . a n d stood und i s p ut e d for n i n eteen c e n t u r i e s . This c l a ss i fi c a tion e m b r a c e d a co m pl ete g ra d a t ion fro m t h e l owest to t h e h i g h est o r g a n i s m-m a n . Fifte e n t h and s i xteenth centu ry voya g es of d i scov­ e ry a n d t h e inven tion of t h e m i croscope revea l e d a d i ve rsity of a n i m a l a n d pl ant fo rm and functio n un­ k n own to Aristot l e . With these n ew observatio n s, c h a n g e s in classification too k p l a ce . (1627-1705), a n En­ g l is h natural ist,· i n trod uced the present idea of species and h ig he r categories i n classifica­ tion . Ray s h owed t h a t groups of s i m i l a r s pecies co u l d be classified i n to sets, w h ich he called genera. This syste m is t h e b a s i s for t h e i n tern atio n a l o n e s ti l l b e i n g u s e d today. JO H N RAY

( 1 707- 1 77 8 ), a Swed i s h na t u ra l i st, devel ope d the present syste m a n d method of biological c l a s s i f icatio n (ta x ­ o n omy). H e u sed a u n iform system of c l a s s i f ication a n d no­ m e n clature. T h e 1 Ot h edition of h i s Syste m a Naturae ( 1 758) marks the beg i n n i n g of modern tax o n o my.

CARL LIN N A EUS

RELATIVE AGE OF CATEGORIES OF ANIMALS

s h ows a n in cre a s i n g sim i­ l a r i ty of e a c h g ro u p from t h e kin g d o m to t h e s pe c i e s . N ote t h e modern evo l u ti o n a ry bra n c h i n g in terpre t a tion o n t h e rig h t.

THE L I N N A E A N T Y P E CLA S S I FICATI O N

IN BIN OMIAL NOME NCLATUR E ,

t h e ba sis of the cla ssif i ca tio n developed by Lin n a e u s , each s pecies h a s two n a m es: t h e f i rst is the g e n u s to which it be­ longs; the seco n d is the spe­ cies. T h e Com m o n Rave n , for

exa m p l e , is Corv u s cora x , w h i l e t h e s o m ewhat similar Co m mo n Crow is Corvus brach yrhynchos. Lin n a e u s used this s h ort, a n d internatio n a l l y u n d e rstood clas­ sification to c l a s sify all of the species k nown a t that ti m e .

L I N N A E U S and m ost of his contem pora ries a s s u m ed

that e a c h sp e cies was d i stinct and unc hang ing , t h e i r d e g rees of s i m il a rity refl ecting s i m i l a rity to the a rc he­ types, o r m o d els, u pon w h i c h e a c h h a d been c reated .

11

THE VARYING DEGREES OF D IVERS ITY shown

by

d ifferent species s u g g ested to so m e e i g h teenth c e n t u ry stu d ents a conclu s ion bol dly d ifferent fro m that rea c h e d by li n n aeus and m o s t o f h i s contem poraries. P e r h a ps, it was a rg u e d , s pecies were not u n c h ang ing and i m ­ m u ta ble . P e r h a p s existing s pecies a rose b y a slow m o d ification of e a r l i er for m s . P e r h a p s d e g rees of s i m ilarity b etween s pecies reflected t h e i r d eg ree of rela tions h i p to co m m on ancestra l fo r m s . Perha p s c h ange, not constancy, w a s one essenti a l c h a r a cteristic of s pecies . Perh a ps species have evolved , o r unfol d e d , r a t h e r t h a n h a ving a p pea re d fu lly fo r m e d . Perh a p s t h ey a rose not fro m a sing l e c rea tive act b u t b y s l ow p rocesses of c h ange ove r l ong p e ri o d s of ti m e . ERASMUS DARW I N ( 1 7 3 1 - 1 802),

JEAN

g ra n d father of C harles Darwi n , w a s a ph ysicia n , poet, a n d natura list. H e was impressed by t h e exte n t of c h a nges in farm wit h i n t h e lifeti m e of indiv i d u a l a n ima ls (frog s , for example), by the i nfluence of s e lective breed­ ing i n horses and dogs, by d iffe rences d u e to c l i mate, a n d b y t h e close a ffi n i ties of t h e m a m ma l s-wh i c h h e rea soned i m p l i ed their commo n o ri g i n .

( 1 744- 1 8 29), Fre n c h s o l d i e r a n d biologist, w a s t h e fou n d e r o f t h e stu dies of i n vertebrate a n i­ ma l s . He stressed t h a t n o a b ­ solute limits s e p a rated o n e spe­ cies from a n oth er and that s pecies reta i n con sta nt c harac­ teristics o n ly i n u n c h a n g i n g en­ v i ronments. When the environ­ ment does c h a n g e , he a rg u e d , t h e i n creased u s e of some or­ g a n s a n d the relative d i suse of

BA PTISTE

DE

.r

LAMARCK

others lead to i n h e ri ta b l e cha nges. T h e g i raffe's long n e c k , f o r exa m p le , co u l d be b e s t ex­ plained by t h e l o n g -conti n u e d habit of reach i n g u pward t o feed on t h e l e a v e s of trees. By La m a rc k's t h eory, the rela-

tive devel o p m e n t of a n y organ respo n d s to i t s d eg ree of use. La marck's belief that ac­ q u i red c h a racteristics c a n be i n h e rited i s n o l o n g e r a ccepted , b u t h is recog n ition of evolution was of major i m porta n c e .

La m a rck's views s u g g e s ted t h a t g i raffes reach i n g u pw a rd be­ i n c rea s i n g l y ca m e l o n ger­ n ecked a n d tra n s m i tted t h i s c h a racteristic t o t h e i r offs pri n g .

T h e conce pts o f evo l ut i o n p ro p osed by Era s m us D a rw i n a n d by La m a rc k were n ot o n l y rejecte d b u t were a l so ri d i c u l e d by t h e i r scientific co n te m p o ra ries . Th is wa s beca u se of t h e excesses of s o m e i n te r p reta ­ tio n s p ro p osed by La m a rc k a n d h i s d i sci p l es, a n d a l so beca use m a n ' s eve ry d a y e x p e rie n ce p rovi ded l it t l e s u p p o rt fo r La m a rc k ' s t h e o ry of s p e c i e s deve l o p m e n t . I n sp ite of circu m sta n ti a l evi d e n ce, n o o n e h a d yet seen o n e species t u r n i n to a n ot h e r.

13

THE DISCOVERY THAT LIFE HAD A LONG HISTORY

d i d not com e until t h e e i g h teenth and nineteenth cen­ turies, when it b e c a m e genera lly recogni zed t h a t fos­ s i l s were the re m a ins of once-l iving ani m a l s and p l a n ts . Fos s i l s ind i cated that m a ny s p e c i e s h a d beco m e extinct and that m o st l iving species were of recent origin. If species were i m m uta b l e, how cou l d these c h anges in the pattern of l ife be exp lained ? D u ring the nineteenth centu ry, two o p posing schools of thought developed . CATASTROPH I STS a ttem pted to

recon c i l e the foss i l record with the early c h a pters of the Boo k of Genesis. T h ey reg a rded t h e Flood of N o a h as the l a s t o f a series of g reat worldwide ca­ tas trophes, each of which d e ­ stroyed a l l l iving t h i n gs. After eac h catastrophe, a n ew crea­ tion took pla ce, in w h i c h the earth was repopu lated by ani­ m a l s a n d pla nts of new a n d d ifferent species. These in turn were d estroyed, and their fos­ sil re m a i n s e n t o m bed in t h e s trata o f the next cataclys m .

GRADUALISTS m a i n ta i n e d that t h e foss i l record s h owed n o evi­ dence of wor l dwide catastro­ phes, a l t h ou g h it d i d s h ow m a ny exa m p les of local eros ion s u rfaces and c h a n g i n g e n v i ron m e n t s of roc k d e p o s i t i o n . A l t h o u g h t h ese c h a n ges a re often m a rked by t h e c u toff of o n e k i n d of fossil and its re p l a c e m e n t by a n other, th i s was a p i e c e m e a l , loca l , i rreg­ u la r process, not a worldwide o n e . New species ori g i n ated, accord i n g to g r a d u a l ists, by t h e s l o w m o d i fication of a n cestra l fo r m s . G E O R G E S CUVIE R ( 1 769- 1 832 ), a n outsta n d i n g Fre n c h a n atom ist and pa l eontologist, studied th e foss i l ve rte b rates of t h e Pa ris Ba s i n . T h e s u ccession of d iffer­ e n t species see m e d to h i m to i m p l y a se ries of u n ivers a l ca ­ tastro p h es, t h e last of w h i c h was the F lood o f N oa h . C u v i e r be­ l i eved that some species s u rvived to repo p u late the earth w h i l e other students i n voked a new creation after each of the catas­ tro p hes. As m a n y a s 30 catastro­ phes were proposed.

JAMES

H UTTO N

(1726-1797),

Scotti s h p h ys i c i a n , l a n d owner, a n d a g r i c u l t u r i st, l a i d t h e fou n ­ dations of m od e r n g eology. He recog n ized t h a t many rocks were the res u l t of eros i o n a n d deposition i n e n v i ro n m e n ts t h a t h a d modern co u n terparts. T h i s co ncept of uniformitaria nism s o u g h t to e x p l a i n t h e fea t u res of t h e earth i n term s o f pres e n t processes.

(1797-1875), a Scott i s h s o l d i er, l awyer, a n d g eologist, p u b l i s h e d T h e Prin­ ciples of Ge ology in 1830-33 T h e book, w h i c h ra n to twelve e d i t i o n s , had e n o r m o u s i n fl u­ e n ce . I n it, Lye l l esta b l i s h e d the s c i e n c e of g e o l ogy, j u st i f y i n g a n d a m p l i fyi n g H u tto n ' s con­ cept of u n i form itaria n ism . Lye l l f i rst used t h e word "evo l u t i o n " i n its prese n t se n se.

C HARLE S LYELL

N ew d iscove ri e s l e d to t h e g ra d u a l rejectio n of c a ta s ­ t ro p h i s m . Fi rst, t h e n u m b e r of catastro p h e s re q u i red to ex p l a i n t h e fo s s i l record ste a d i l y i n c re a s e d u n t i l t h e w h o l e syste m b eca m e u n wi e l d ly. I t beca m e c l ea r, a l s o, t h a t the rock reco rd coul d b e i n te r p reted s a t i sfa c to r i l y i n te r m s of p r e s e n t- d a y, o b serva b l e g e o l o g i c p rocesses ra th e r t h a n u n k n own catastro p h e s . In a d d it i o n , t h e "d i ____ 1=-uv'_ -' i a l " ro c k s t h a t l a y ove r t h e s u rfa ce of m u c h of Euro p e a n d North Am e rica a n d we re t h oug h t to b e t he re m a i n s o f Noa h ' s Flood we re reco g n iz e d a s g l a c i a l d e p osits . More a n d m o re evi d e n c e of conti n u ity ( o r evo lutio n ) of fo s s i l s wa s d e m o n strate d . D a rwi n a n d Wa l l a c e p ro p ose d a n a ccepta b l e m ec h a n i s m fo r t h e p ro c e s s of evo l utio n . 15

D ARWIN'S VOYAGE aboard t h e HMS Beagle c h a n g e d the worl d ' s viewpoi nt i n regard to evo l ution a n d the d evelo p m ent of s p ecies. U ntil t h e p u b l i cati o n of D a rwi n's On t h e O rigin o f Species, i n

CHARLES

1859, t h e i d e a o f evo l utio n was g e n e ra l l y rejecte d .

D a rwin was born a t S h rewsbu ry, E n g l a n d , o n F e b ru ­ a ry 1 2 , 18 09, t h e s a m e d a y a s Li n c o l n. After two ye a rs of m e d ica l tra i n i n g a t E d i n b u rg h , h e w e n t to Com ­ b r i d g e, w h e re h e g ra d ua ted i n 1 83 1 . Afte r h i s g ra d u a ­ t i o n , D a rw i n was a p pointed n atu ra l ist to t h e Beagle, a 240-to n , 1 0 - g u n brig, which was to u ndertake a s u rvey voyag e to South Am erica a n d fro m t h e re on a ro u n d t h e worl d . Th e voy a g e l a sted five ye a rs, a n d t h e i n ­ s i g h ts D a rw i n g a i n e d d u ri n g t h ose yea rs were t o b e ­ co m e t h e fo u n d a t i o n of h i s l i fe ' s wo r k . Da rwi n mode i m p o rta n t co n t ri b u t i o n s to t h e g e o l ogy of South A m e rica, the orig i n of cora l reefs, the re l a t i o n s h i p s betwe e n l ivi n g a n d foss i l a n i m a l s , a n d t h e struct u re , a d a p tation , a n d geog ra p h ic distri b u t i o n of a n i m a l s . I t wa s t h ese s t u d i es t h a t l a t e r fo r m e d t h e basis fo r h is evo l utio n a ry th eory . DARWIN too k the first volu me of Lye l l 's n ewly p u b l i s h e d Prin­ ciples of Geology o n the voya g e a n d was d e e p l y i m pressed by i t . Lye l l a rg u e d tha t t h e ea rth's s u rface had b e e n s h a p e d by s u c h n a t u r a l forces as river erosion , vo l ca n i c e r u p t i o n s , a n d c h a n g e s i n s e a leve l s . D a rw i n u s e d s uc h i d e a s i n u n rave l i n g t h e geo logy o f a re a s h e v i s i t e d , a n d t h ey i n fl u e n ced h i s t h i n k ­ ing a b o u t t h e orig i n o f s pecies .

C h a rles Darw i n , aged 31

16

The route of the HMS Beagle is show n on t h e m a p a bove. I t is proba b l e that Da rwin contracted C h a g a s ' disease du ring an inl and journey in South A m erica, m a k ing h i m a s e m i - i nva l i d later. VERTEBRATE S collec ted by Darw i n from Arg e n t i n a a n d elsewhere i n c l u d e d T oxodo n a h eavy e l e pha n t-sized m a m m a l t h a t looked m uc h l i ke a rhi noc­ eros. D a rw i n concl uded ( wro n g ­ ly) t h a t i t s h owed t h e two groups were c l o s e l y re l a t e d . D a rwi n d iscovered foss i l teeth of horses that had l ived a t t h e sa m e t i m e a s Toxodon a n d h a d b e c o m e extinct with it, a l tho u g h s u rviving i n ot h e r pa rts of the worl d . T h i s m a d e the idea of catastro p h i c w or l d w id e e x t i n c ­ t i o n a p pear s u s pect. FO S S I L

,

T H E S I M I LA R I TY of some fo ssil

verte brates, such a s t h e g i a n t a rm a d i l lo- l i ke Glyptodon, to form s s t i l l l i v i n g s u g g ested to Darwi n the idea of descent by evolution.

THE GALAPAGOS ISLANDS, l oca ted

i n the Pa c ific a bo ut 600 m i les west of the coast of Ecu a d o r, a re a d esolate g ro u p of 1 4 roc ky i s l a n d s , representi n g t h e re m a i n s of exti n ct vo l ca n oe s . T h e i s l a n d s a re s e p a ­ rated fro m e a c h oth e r by deep water, a n d n o wi n ds o r ocean c u rre n ts carry s m a l l a n i m a l s o r seeds fro m o n e t o a noth e r . T h e g e n eral a bsence o f m a m m a l s h a s a l ­ lowed g i a n t tortoises t o g ra z e i n safety, l i z a r d s t o be­ co m e s e a g o i ng, and fi n ches to exist i n n iches that else­ w h e re a r e occupied by other species. D a rw i n d i scovered that ea c h of t h e i s l a n d s , a l t h o u g h h a vi n g very s i m i l a r c l i mates a n d e n v i ro n m e n ts a n d b e i n g o n l y a b out 5 0 m i l e s a p a rt, h a s its own fa una and f l o ra - s i m i l a r to but d isti n ct fro m t h o s e of the n e i g h b o ri n g i s l a n d s . This s ug g e sted to D a rw i n that t h e s i m i l a r species m i g h t h a ve d eve l o p e d from a co m ­ mo n a n ce sto r rat h e r t h a n e a c h h a v i n g b e e n c reated sepa ra t e l y . The isl a n ds a r e of rece n t o rig i n , a n d t h ei r fa u n a , de rive d fro m t h e S o u t h A m e rica n m a i n l a n d , i l l u strates co l o n iz a t i o n of, a n d a d o ptio n to, a n e m p ty e n vi ro n ­ m e n t b y re l a tive ly ra p i d evo l uti o n .

I G U ANAS g row to f o u r feet l o n g a n d a re fears o m e i n a p p e a ra n ce , b u t t h ey are h a r m l es s h e r b i vore s , feed i n g o n s e a weeds. F o u nd o n l y \r'..:'i"J4f!: i n t h e G a l a p a g o s I s l a nd s , t h ey i n c l ude two rel ated s p e c i e s , o n e terrestr i a l , a n d t h e o t h e r mari n e. T h e latter are pow erf u l s w i m m ers, with we b b ed toes and a f l atte n ed ta i l to a s s i s t i n s w i m m i n g. Ea c h i s l a nd h a s i t s o w n ra c e , s h ow i n g m i nor d i f fere n c e s f ro m o n e gro u p to a n ot h e r.

w e ig h i n g up to 250 pou n d s g ra z e on veg eta­ tion, fi l l i n g a n i c h e occu pied i n oth e r p l a c e s by m a m m a l s . These tortoises a re fou n d o n ly i n the Galapagos I slands, a n d each m a j o r isla n d h a s its own variety. The variation within a stn g l e sp e ci e s of tortoise is so s i m i la r to that fou n d betwe e n species i n t h e Gal a p a g o s fi n c h e s t h a t Darwi n w rote, " I m ust s u s ­ pect t h at (th e fi nc h species) a re o n ly varieties."

G IANT TORTO I S E S

THE F I N C H E S of th e G a l a pa gos

Islands showed a g e n e r al s i m ­ i l a rity t o o n e a noth er a n d to th ose of t h e m a i n l a n d of South A m e rica, b u t t h e fi n c h e s of each i s l a n d d iffered s l ig htly fro m those of t h e n ext. T h e 1 3 d iffe re n t s pecies s h owed a perfect g radatio n , fro m g ro u n d ­ l i v i n g , seed-eati n g f o r m s with heavy, large beaks to tree­ dwe l l i n g , i n sec t eatin g fo r m s w i t h long, p o i n t e d beaks. -

D a rwi n w o n d ered w h y the s pecies, i f created s e parately, rese m b led one a n ot h e r a n d those of t h e m a i n l a n d o f So u t h A m erica, w h e re a s birds of t h e Ca pe Ve rde I s l a n d s , a t t h e same latitude i n the South At­ l a n tic, res e m b l e d t h o s e of Afri­ c a . " O n e m i g h t rea l l y fa n cy," w rote Da rwi n , " t h a t . . . o n e s pecies h a d been ta ken a n d m o d i fi e d f o r d iffere n t e n d s . " T h e s e a re i l l u s trated o n p . 8 2 .

The G a l a pagos Islands, s h owing route o f H.M.S. Beag l e .

19

THE SEARCH FOR A MECHAN ISM h a d b eg u n . C h a r l e s

D a rwin retu r n e d w i t h the Beagle t o E n g l a n d i n Octo b er of 1 83 6. The fol l owi n g July h e o p e n e d h i s fi rst n ote­ book o n The Transmutatio n of Spe cies. He was t h e n 27 y e a rs o l d . D a rwi n h a d see n how s m a l l v a r i a t i o n s co u l d b e s e l ected b y a rtifici a l b ree d i n g i n d o m estic a n im a l s . Could the sa m e tra nsform ations within a spe­ cies a lso occur between species so t h a t one u l t i m ately g ave rise to a noth er? D a rwi n 's observations s u g g ested that they could, but h e could not vis u a l i z e t h e m e th o d .

ALFRED

RUSSEL

WALLA C E

( 1 8 32- 1 9 1 3), British su rveyor a n d natural ist, i n d e p e n d e n tly s u ggested the t h eory of natura l se lection . A l ready con v i n ced of the fact of evo l u t i o n , he con­ ceived t h e idea of natura l se­ l ection w h i l e l y i n g s i c k with feve r i n the Moluccas i n Febru­ a ry, 1 85 8 . H e reca l l e d the Essay on Pop ulation, by Robert Ma l ­ t h u s, w h i c h h e h a d read twe lve years before. He w rote tha t he saw its a p p l ica tion to evo l u ­ t i o n " i n a fla s h o f i n t u i t i o n . " Wa l l ace w a s a l so a n out­ sta n d i ng p i o n ee r in the study of the geogra p h i c d i stri bution of a n i m a l s and its s i g n ifica n ce for the th eory o f evo l u t ion (p. 43).

MALT H U S (1 766-1834) was a n E n g l i s h c l e rg y m a n a n d that eco n o m i st . U n c o n v i n ced man is p e rfect, and d i s b e l i e v i n g t h e p robab i l ity of un ivers a l p e a c e , e q u a l i ty, a n d p l e n ty, pred i cted by the p o l i t i ci a n s a n d u t i l ita rian phi l osophers o f the e i ghte e n t h c e n t u ry, Ma l t h u s wrote a n a n o nymo u s " Essay on Popu lation " in 1 798. In it, h e sla ted t h a t h u ma n pop u l ation c a n not e x pa n d i n defi n itely. Pop u l ations e x pa n d at a geo­ m etric rate of i n crease w i t h wh ich f o o d s u p p l i e s c a n n ever keep pace. Fa m i n e , d i sease, and wa r, Ma l l h u s a rg ue d , wi l l l i m it t h e i n c rea s i n g size o f h u ­ m a n popu l a tio n s . ROBERT

Natura l selection i m plies that a n c estral g i raffe populations i n ­ c l uded n e c k s of various le ngths. More of the l o n g e r - n e c ked gi­ raffes s u rvived, a n d t h ey pro­ d u ced i n crea s i n g n u m bers of offs p r i n g that i n h erited t h e i r pare n ts' lon g e r nec ks.

"I n Octob e r, 1838," wrote D arwi n , " I h a p p e n e d to read Mal t h u s for amu seme n t. Bei n g wel l pre p a red to appreciate t h e stru g g l e fo r exi ste n ce , wh i c h eve ry­ w h ere goes o n , fro m l o n g conti n u ed o b s e rvati o n of t h e h a b its of a n i m a l s a n d p l a nts, i t a t o n ce stru c k m e t h at u n d e r t h e s e circ u m sta n ce s favo r a b l e va r i atio n s wou l d te n d t o b e p r e s e rved a n d u n favora b l e o n es to be d e stroye d . T h e re s u l t of t h i s wo u l d be t h e fo r m ation of a new s p e c i e s . " D a rwi n ca l l e d t h i s process " n at u r a l s e l ecti o n ." H e arg u e d t h at t h o s e parti cular i n d ivid u a l s b ette r -a d apted to t h e i r e n v i ro n me n t wou l d l ive l o n g e r t h a n t h e rest. S i n ce t h e offs p r i n g wou l d s h a re t h e i r p a rents' c h a r­ a cteri stics, over m a n y g e n erati o n s, t h o s e most favo r­ a b l e wo u l d te n d to pre d omi nate. D a rwi n m u l l e d ove r his theory, p re p a ri n g a bri ef outl i n e of it i n 18 4 2 , a n d a l o n g e r a b stra ct two years l ate r. T h e s e were n ot p u b ­ l i s h e d u n t i l 1858 ( S ee p . 2 2 ). For t h e n e xt fourteen years h e g a t h ered data for a fou r- vol ume treatise. These vo l umes were n ever p u b l i s h e d. 21

THE ORI G I N OF S PE C I ES. I n t h e summer of

1858,

D a rwi n received fro m Alfred Russel Wa l l a ce a m a n­ u s c r i pt e n titled " O n the Ten d e n cy of Va rietie s to D ep a rt I n d e fi n itely fro m the Orig i n a l Typ e . " Wa l l a ce h a d i n d e p e n d e n tly rea c h e d t h e conclusion that n at u r a l sel ection h a d pl ayed a m a jo r rol e i n t h e o r i g i n of n ew species. D i s m aye d , D a rwin offered to with d raw h i s own m a n uscri pt, but a j o i n t paper by the two men was rea d b efo re t h e l i n n a e a n Society of lo n d o n o n July 1 , 1858. On Nove m b e r 24, 1859, D a rwin p u b l is h e d t h e Origin o f Species-a b rief abstract, as h e c a l led i t , of h is views . The book created a sensatio n . The f1rst e d i­ tion of 1250 copies sold out o n the f1rst d a y of p u b l i ­ cati o n . Scie n tists w e re at f1rst d ivi ded i n t h e i r views . Oth e rs, wro n g ly as it n ow a p p e a rs, reg a rd e d the boo k as a d i rect c h a l l e n g e to rel ig ious b e l i efs. I n s u c h d iverse f1el d s a s p h i l osophy, h istory, a nth ropology, pol itics, and sociology, D a rwi n 's book ra ised p rofo u n d q ues­ tions. Th e d e b ate was wid espread a n d i ntense .

THE B O O K w a s ca refully w rit­ THE ORIGIN OF SPECIES Bf !lEANS OF NATURAL SELECTION,

Bf

CHARLES DARWIN, K.A.,

_............... _ .. �, ....................... , ...._ .... _.......... ..... _, ___ .......... ""... _,.. ™ .... . .......

LONDON:

.lORN MURRAY, ALBEM:ABLJ: IITREET.

22

ten a n d cog e n tly a rgued. T h e fi rs t four c h a pters described t h e res u l ts of d o m e s t i c s e l ection and bre e d i n g a n d then devel­ oped t h e idea of natu ra l se­ lectio n . T h e fifth c h a pter, de­ voted to t h e m echen isms of va riation and i n h erita n ce , is t h e o n ly part of the boo k that has s i nce been d iscredite d . T h e s ixth to te nth c h a pters d i s­ c u ss e d possible o b j e ctions to the w h o l e idea of evo l u t i o n , a n d t h e re m a i n i n g c h a pters treated poss i b l e evid e n c e for evo l utio n . S h ow n at left i s t h e title p a g e of t h e 1 s t edition .

Cartoonist's view, i n 1 8 7 1 , of Charles Darw i n Hux ley, ( rig h t ) w h o c h a m pioned his teac h i n g s .

( Left ) a n d T. H.

The i m porta n ce of D a rw i n ' s book on Th e Origin of Species, i s d iffi cult to exa g g era t e . It h a s b e e n ca l l e d t h e most i m p orta n t b o o k o f t h e n i n etee n t h c e n tury. More t h a n a n y o t h e r b o o k , b efore or si n c e , i t esta b l i s h e d t h e t h e ory o f evo luti o n -or "d esc e n t b y m o d ifica t i o n , " a s Da rw i n c a l l e d it. I n t h is, D a rw i n i n itiated a tra n sform a ­ t i o n i n t h e stud y o f t h e org a n i c worl d a s profoun d a s t h a t broug h t a bout i n t h e p h ysica l worl d b y N ewto n ' s work w i t h g ra vitatio n . I n b i o logy, evo l ut i o n p rovi d e d a p owerful n ew unify i n g pri n c i p l e , g ivi n g n ew m ea n ­ i n g a n d i nsig h t t o a ma z e o f confl i c t i n g d a ta a n d a l s o a n ew i mpetus i n every fi eld of i n quiry . T h e boo k's i nflu e n c e w a s n o t co n fi n e d t o b i o logy. I f l ife had a h i s tory, so had m a n , so had l a n g u a g e, so h a d cul ture . H a d t h ey too evolved? I f there h a d been org a n i c evo l uti o n , had there a l so b e e n i n o rg a n i c evo­ lut i o n in w h i c h the e a rt h , t h e sol a r system , the u n iverse, m atter, a n d e n ergy itself had u n d e r g o n e c h a n g e? A wate rs h e d of h u m a n thou g h t was esta b l is h e d . Ma n's view of the wor l d , of l ife, a n d of h i mself would n eve r be q u ite the s a m e a g a i n . 23

DARWIN'S THESIS for t h e ong m of new species

rested on th ree essenti a l fou n d ation s-two of t h e m d e m o n stra b l e fa cts, t h e oth er a n i n ference. Fi rst, D a rw i n stressed t h at variation existed t h rou g h ­ o u t t h e wor l d o f livi n g t h i n g s . N o two i n divi d u a l s of the s a m e s pecies a re exa ctly a l i ke . In size, p ro po rti o n s , colori n g , m e ntal a b i l ity, d i s position, physiolog i c a l p ro­ cesses, a n d m a ny other ways, each i n d ivid u a l i s u n i q u e . Furth e r m o re, m a ny o f these a n d oth e r featu res a re tra n s m itted fro m p a rent to offs p ri n g . Seco n d ly, D a rwi n a rg u ed that every s pecies over­ pro d u c es . More you n g are p ro d u ced t h a n ever su rvive, for the n u m be r of i n d ivid u a l s in a popul ation s h ow re l­ atively l ittle va riatio n . Th is overprod uction exists at every l evel i n the plant and a n i m a l ki n g d o m s . " Even s low- b reed i n g m a n , wrote D a rwin, " h as d o u b l e d ( his n u m bers) i n twenty-five yea rs, a n d at t h i s rate, i n less than a thousa n d yea rs there wou l d l itera l l y not be sta n d i n g room fo r h i s progeny. There m u st therefore be a very hig h rate of m o rta l i ty, a n d t h i s h a s been s h own to be so. In m a ny s pecies of birds and i n sects, 98 pe rce n t of t h e i n d ivid u a l s die b efo re m a t u rity . II

11

Swarm of locusts exemplifies a b u n d a n ce of l ivi n g thi n g s .

NATU RAL SELECTION

+

•

GRAZI NG HORSES

Darwi n 's "re c i p e " for evo l ution was the i n teraction of variation, ove rpo p u lati o n , and n a t u r a l s e lectio n ; i l l u s trated h ere i s evo l u tion of h orses ( See p . 5 1 ).

Third ly, D a rwin a rg u ed that m a ny c h a racteristics of p l a nts a n d a n i m a l s were a d a ptati o n s to the e n ­ vi ro n m ents i n w h i c h t h ey l ived . T h e p rotective col o r i n g o f m a ny a n i m a l s wa s c l e a rly a d a ptive . Th e teeth of a n i m a ls we re clea rly related to t h e i r d iet, as were th e bea ks of b i rd s . Wh a l es, though m a m m a ls, were so a d a pted to l ife in the seas that they h a d fis h l i ke bodies. D a rw i n s u g g ested that these h a d come a bout by natura/ selection of favora b l e d iffere n ces i n a nces­ tra l o rg a n i s m s . Those best a d a pted to t h e i r e nviron­ ment wou l d s u rvive l o n g e r and so p ro d u c e m o re off­ spr i n g tha n those that were not. Th e offs p r i n g wou l d i n h erit th e i r p a re n ts ' favora b l e c h a racteristics . N e w species cou l d d evelop i n t h i s way. Th e vigorous d e bate that fol l owed p ubl ication of D a rwi n 's book saw g ra d u a l acce pta nce of h is views . D a rwi n m is u n d e rstood t h e m e c h a n is m of variation a n d i n h e rita nce, but h i s g e n e ra l theory h a s withstood th e test of ti m e . 25

THE

LAWS OF I N H E R I TANC E, w h ich h a d e l uded Da r­

w i n a n d Wa l l a ce , were d iscovered by Gregor Me n d el ( 1 8 2 2 - 1 8 8 4 ) , a n Au stri a n m o n k . Men d e l is con s i d e re d t o b e the fo u n d e r o f m o d ern g e n etics. His wor k w a s p u b l is h e d i n 1866, b u t re m a i n ed g e n e ra l ly u n k n own u n t i l it was i n d e p e n d ently " red iscovered " by t h ree biologists in 1900 . Men d e l d e c i d e d to stud y the i n h erita nce of o n e o r two re a d i ly recog n i z a b l e c h a ra cters i n t h e g a rd e n pea-t h e s i z e a n d fo rm o f t h e peas, t h e i r fl ower color, a n d so o n . He cross- pol l i n ated o n e fo rm with a n other and t h e n ca refu l ly recorded the resu lts of this over seve ra l g rowi n g seaso n s . ( p . 60 ) MEN DEL raised peas by cross­ pollinating those with smooth, round peas and those with shriveled, wrinkled peas. He discovered that they did not produce a blending of the par­ ent characters, as was g e n e r­ ally believed, but that all the new peas were smooth and round. He then used these seeds to p rodu c e a n ot h e r cro p ,

covered t h a t t h ree-quarters of t h e n ew g e n e ra t i o n were s m oo t h a nd ro u n d a n d o n e q u arte r w a s wr i n kled. Mendel ca l l ed c h ara cters t h a t c o u l d be m a s ked i n o n e g e n e ra ­ t i o n b u t a p pear i n a n ot h e r ( s u c h a s w r i n k l ed p e a s ) recessive; t h os e t h a t ove r s h adow t h e m ( s u c h a s t h e s m o o t h , ro u nd p e a f o r m ) ,

cross-pollinated them, and dis-

Me nde l co n c l uded t h a t t h i s de layed a p pe a ra n c e o f reces ­ s i ve c h a ra c t e r s m u st i m p l y t h a t e a c h c h ara c t e r i s g over n e d by a n i nde p e n d e n t factor ( w h i c h w e n o w c a l l a ge n e) a n d t h a t t h e se m u st b e pa ired in t h e pare n t b u t n o t i n t h e g a m et e s. Me nde l m a de t h ree m ajor d i s cove r i e s : (1) t h a t c h ara cters are g ove r n ed by p a i red , b u t i n di­ vidu a l "fa ctors," (2) that t h e s e factors m a y b e do m i n a n t o r re c e s ­ s ive, a n d (3) t h a t t h e s e f a c tors c o m b i n e , w i t h o u t b l e n di n g , to prod u c e c h ara cter i s t i c ra t i o s in the later g e n era t i o n s.

dom inant.

Hugo de Vries, the discoverer of mutations, and the evening prim­ rose that he used in his studies.

MUTATI ON. Me n d e l h a d shown t h a t i n he rita nce was

p a rticu l a r a n d p red i cta b l e . But if this was so, how cou l d a ny new features ever a rise? Th e a n swer was fou n d p a rtly i n t h e a ction of n atural sel ection a n d pa rtly i n the work o f a D utch bota nist. Hugo d e Vries {18 4 8 -193 5 ) was Professor of Bota ny at Am sterd a m . He stu died the m ec h a n i s m of i n h erita nce of c h a racters i n t h e eve n i ng p r i m rose and beca m e in­ creasi n g ly s u s picious of t h e t hen c u r rent view that d iffere n t p a re n t a l c h a ra cters a lways ble n d e d in the offs p r i n g a n d that all va riatio n s were s m a l l . H e stu d i e d ove r 5 0 , 0 0 0 p l a n ts, a n d out of t h e i r several h u n d red thousa n d fl owers, he d i s covered rare exa m p l e s that were " sports . " They h a d giant size o r dwa rf size, o r twice the n o r m a l n u m b e r of peta l s . W h e n b re d to­ gether, they p rod uced s i m i l a r offs p ri n g . S u c h new forms de Vries c a l l ed mutants, the c h a nges p rod u c i ng th e m mutations. I n searc h i n g the l iterature, d e Vries red iscovered t h e work of Me n d e l . These m utatio n s p rovided th e g e n u i nely n ew c h a racteristics u p o n which evol ution by n atura l selection was d e p e n d e nt. 27

T. H. Morgan, a pioneer American geneticist.

Walter S. Sutton, geneticist who identified role of chromosomes.

T H E N EW SYNTHESIS OF EVOLUTIONARY TH EORY

came i n th e early yea rs of the twentieth centu ry, m a rked by recog n ition of chromosomes, m i n ute th rea d ­ l i ke structu res i n t h e c e l l n u c l e u s, a s t h e c a r r i e rs of h e red ita ry c h a racte rs . Th is d iscove ry, w h i c h a l s o showed a l inka g e of c h a racters that Me n d e l h a d n o t suspected, w a s m a d e independently i n 1 902 b y W . S . Sutton and b y T. Bove ri. T. H . Morg a n ( 1 886- 1 9 4 5 ), experi m e n t i n g with t h e fru it fl y, Drosophila, d e m o n ­ strated t h a t t h e g enetic d eterm ina nts were p r e s e n t i n a d efinite l ine a r order i n t h e c h ro m os o m e s and cou l d be " mapped . " Many wo r k e rs b e ca m e co n v i n c e d t h a t i t was s u d d e n , sponta n eo u s , l a rg e-sca l e m utations t h a t w e r e t h e rea l b a s i s of evolut i o n rat h e r t h a n , a s D a rwi n h a d s u g ­ g este d , t h e m ino r va riations . But why, oth ers o b j ected, s h o u l d so m a ny c h a ra cters then b e a d a ptive s i n c e m a ny m utations proved t o b e l eth a l rather t h an b en­ efici al? T h e d i scove ry in 1 9 27 t h a t X- rays, te m pe rature c h a n g es, g a m m a rays, a n d var iou s c h e m i c a l s could i n d u ce m utati o n s proved that the g re a t m a jority of them were m i n u te in t h e i r effects and t h erefore were m o re l i kely to s u rvive . 28

Magnified Segment

C h rom osomes from the fruit fly Drosophila g reatly m a g nified. The map s h ow s loca t i o n of g e n e s a l o n g part of c h ro m o s o m e l e n g t h .

T h e s i m p l e M e n d e l i a n concept of i n d e p e n d ent, p a r­ ti cul ar g e n etic d ev e l o p m e n t h a s g iven way to ac­ ce ptan ce of a n i n d ivi d ua l re pre s e n t e d by a ge ne co m ­ plex i n wh ich g e n e s a re l i n ked a n d i nte ract to g eth e r T h e curre n t synthetic th eory of evo luti o n is b a s e d on rig orous sta tistic a l an a l ysi s , stud y of t h e fo ssil re cord , exp eri mental studies, an d o b s ervat i o n of n at­ ural popul a tio n s . I t ac c e p t s as the bas i s for evo l utio n i n d ivid ual variations, ari si n g fro m mutatio n a n d repro­ ductive reco mbin atio n , an d ac ted upo n , fi l tered , c o n ­ serve d , int e n sifi e d or eli mi n ated by n atural s e l ection. .

Ge netic variation i n Drosop hila expressed by striking d iffe re nces i n fo rm. The fl y at left is t h e normal wi l d type. vestigia l norm a l

wing s

strap

twisted abdomen

n o win g s

29

INDI C ATIONS O F E V O LU T ION

The p roof requ i red for a n y p a rticu l a r statem e n t va ries with the n a t u re of t h e state m e n t . To p rove t h a t 2 + 2 = 4 i nvolves a n a ppeal to reas o n a n d mathematical l o g i c . T o p rove t h at a n ath l ete ca n r u n a fou r- m i n ute m ile i nvolves an a p p e a l to experi m e n t-th e ru n n i n g of a ca refu l ly m ea s u red d i stance u n d e r s pecified con d i ­ tio n s a n d with a c c u rate ti m e keepi n g . B u t n o experi m e nt cou l d prove t h a t t he s a m e ath­ l ete ra n a fou r- m i n ute m i l e o n Ju n e 2 0t h a yea r a g o . P roof of t h a t wou l d i n vo lve a n a p pe a l to the record boo k s a n d to witnesses. No experi m e n t ca n p rovi de p roof of p a s t eve n t s . Other k i n d s of evi d e n ce a re n eeded, a l t h o u g h o b servatio n a n d experi m en ts of ex­ isti n g facts a n d p rocesses m ay s u p po rt the pro b a b i l ity of a p a rticu l a r p a st event . Often p roof i n volves an a p­ peal to everyd ay experi e n ce to p rovi d e t h e m o st eco­ n o m i c a l expl a n atio n . You cou l d n ot p rove, for ex­ a m p l e , that all of the s p a rrows livi n g tod a y desce n d e d f r o m those l ivi n g t h ree h u n d red. yea rs a g o , b u t t h e b a l ­ a n ce of e x p e r i e n c e wo u l d s u p po rt t h a t i n terpretati o n . "I wi l l b e l i eve i n evol ution , " Wi l l i a m Jen n i n g s B rya n re m a r k e d , " w h e n I c a n sit i n m y g a rd e n a n d see a n o n io n t u r n i nto a l i ly. " Cl ea rly, i f w e h a d to rely o n that k i n d o f i n sta n t experience, evol ution co u l d n o t b e p rove d . B u t n e it h e r c o u l d t h e g rowth of a n o n io n s e e d i nto a n o n io n be p roved i n sta ntly. I t, too, i s a s l o w s c a rcely p e rceptible eve n t . W e ca n, h owever, o b s e rve p o p u latio n s c h a n g i n g a nd c a n a l s o o b s e rve the m e c h ­ a n is m s by w h i c h s u c h c h a n g e c o m e s a b o u t . The p roof of evo l utio n a l so l ies in its u n ique position as the o n ly a d equate expl a n atio n .fo r t h e orig i n of t h e d iverse fea­ t u res s hown by l iv i n g th ing s . 30

Frog

tadpoles,

metamorphose

into frogs by the resorption of the tail, loss of gills, and growth of lungs and paired limbs. Such radical change within a few weeks makes it less difficult to visualize evolu­ tion over countless years.

CONT I N U I TY of l ivi n g thi n g s i s p rovi d e d b y r e p ro­

d u ct io n . I n d ivi d u a l s l ive , g row old, a n d d i e, but the i r k ind i s p e rpetua ted i n the i r offs p ri n g . W e k n ow o f n o evi d e n ce s u g g e stin g tha t l ivi n g o rg a n i s m s a ri s e i n a n y othe r w a y tha n fro m p a re n ts of the sa m e s p e c i e s . I t wo u l d be d iffi c u l t to p rove, fo r exa m p l e , tha t a l l f ro g s a l ive tod a y m u st have d e s c e n d e d f ro m fro g s tha t l ived 1 , 0 0 0 ye a rs a g o , but a l l of our e x p e r i e n c e s u g g ests that they ha ve . Bu t if fro g s a l ways g ive b i rth to fro g s a n d cam e l s t o ca m e l s , h ow d o n ew k i n ds ( s p e c i es) of an i m a l s eve r deve l op? Two fea tures of c o n t i n u ity s u g g e s t pos s i b l e a n sw e r s . Fi rstly, con ti n u ity betwe e n pa r e n t s a n d off­ s p r i n g i n volve s both b roa d res e m b l a n c e s a n d in divid­ ua l diffe re n c e s a n d varia tio n s . Wha teve r e x p l a n a t i o n we s e l e ct m u s t ex p l a i n both fea t u r e s . Se con dly, c o n tin uous c han g e wi thi n the l ifeti m e of a s i n g l e i n d ivi d ua l a n i m a l i s very g re a t. I f s uch cha n g e s c a n occur i n on e g e n e ration , it m ay w e l l b e t h a t on e s p e c i e s could d evelop i n to a n other. 31

U NITY OF LIFE i s s h o w n by t h e fact that, i n s p i te of t h e ir d iversity of fo r m a n d variety of h a b its , the n e a rl y 1 % m il l io n s p e c i e s of p l a nts a n d a n im a l s a l l s o l ve t h e b a s i c p ro b l e m s of l ivi n g i n m u c h the s a m e w a y . They rese m b l e o n e a n othe r i n co m positio n , c e l l u l a r stru ctu r e , l i fe p r o c e s s e s , a n d b a s ic patte r n s o f r e p ro d u cti o n , a d a pta b i l ity, a n d d eve l o p m e n t . They a l s o s h a re a co m ­ m o n u n ity in the e n d le s s interd e p e n d e n c e o f a l l l i vi n g t h in g s . I f e a ch s p e c ie s i s a n e n tirely s e p a rate creati o n , why d o a l l sha re thes e b a s i c c o m m o n p r o p e rti es?

CELLULAR STRUCTURE i s a ch ar­ acter ist i c of all living material, and the cells are made of pro­ toplasm. Most cells are only a few thousandths of an i nch in diameter, but a few are m uch larger. T he yol ks of b ird eggs are single cells.

In sp ite of some di fferences, p lant and ani mal cel l s do have a s i m ilar bas i c str u ct ure. Even t h e simplest cel l co n s i s ts of thousands of di fferent m ole­ cules t hat i nte ract toge t h er in coordination. A typ ical ce ll str u cture i s shown below.

A N I MAL CELL P ROTO PLASM

Oxyg e n Ca r bon Hyd rog e n N i trog e n P h o s p ho r u s Pota s s i u m Sulfur Chlorine

32

76.0% 1 0.5% 1 0 . 0% 2 . 5% 0 .3% 0 . 3% 0.2% 0 . 1%

P R O T O P LASM is sh ared by a ll l i v ing things. It is composed of a distincti ve comb ination of large m o lecules of no n l iv ing s u bstances, inc l u d ing carbohy­ drates, fats, proteins ( including enzy mes), a n d nucle i c a c i d s t h a t are organized into a co l l o idal m i x t u re in wate r. T he u n i q ue p ropert ies of t h is mater i a l form the bas is of l i fe.

META B OLISM i n c l u d e s t h e n u ­ trition , re s p i ra t i o n , s y n t h e s i s , a n d excretion t h a t i s c h ara c t e r i s t i c o f a l l l i v i n g t h i n g s . N o n - l i v i n g food materi a l s a re c o n ve rted i n to t h e org a n i s m ' s l i v i n g t i s s u e s , c e rta i n of w h i c h b rea k d o w n t o provide t h e e n e r g y t h a t i s v i t a l to t h e processes e s s e n t i a l to l i fe. Meta b ­ o l i s m i n vo l v e s a c o n s ta n t f l ow of e n ergy a n d m a t e r i a l s w it h i n a n d betwee n a n org a n i s m a n d i t s e n v iro n m e n t .

REPRO D U C T I O N of n e w d u p l i ­

cate i n d i v i d u a l s i s c h a racteristic o f a l l l i v i n g t h i n g s . The con­ t i n u i t y of form i n volved i n re­ prod u c t i o n i s contro l l e d by the activity

of

s e l f-du p l i ca t i n g

c h e m i c a l structu res c a l l e d g e n e s ( p . 56).

of newborn i n d i v i d ­ uals i s a co m m o n prope rty of a l l living things.

GRO WTH

of a l l living t h i n g s i nvolves c o n t i n u i n g a d ­ j u s t m e n t t o a c h a n g i n g e n v i ro n ­

ADAPTAT I O N

m e n t. I n d i v i du a l s po n s e s include

adaptive rea c t i o n

re­ to

sti m u l i , i rritab i l ity, p h ysiolog i c c h a n g e s , h ea l i n g of I n J U ri e s , a n d m ov e m e n t. Over l o n g pe­ riods, po p u l a ti o n s show m o re g e n e r a l a d a ptati o n s .

33

Sponges

Des pite t h ei r diversity, all livi n g t h i n g s s h a re co m m o n propertie s .

T H E NAT U R E OF L I FE i s u n de rs tood l a rg e l y in terms

of a s e r i e s of fu n d a m e n t a l pr o pe rties ( p p . 3 2 -3 3 ) . N o s i m p l e d e fi n ition o f " l ife " i s pos s i b l e , p a rtly beca u s e o f its co m p l exity a n d p a rtly b ec a u s e it is un i q u e . But we c a n d efi n e l ife in term s of s o m e of its s i m p l e r p r o p e rti e s . livi n g o rg a n i s m s co n s i st of u n i q u e a n d com p l e x co m b i n ati o n s o f c e rta i n n o n l ivi n g m ate­ r i a l s , a r ra n g e d in l a rg e m o l e c u l e s t h a t a re c a p a b l e of g rowt h , r e p rodu cti o n , a da ptatio n , a n d t h e g a t h e r i n g a n d u s i n g of exte r n a l foo d a n d e n ergy. So m e of these i n d ivi d u a l p ro p e rties of l i vi n g thi n g s a re a l s o p r e s e n t i n n o n l ivi n g th i n g s , b ut o n l y l iv­ i n g o rg a n i s m s e x h i b it t h e m a l l s i m u lta n e o u s ly . O t h e r k i n d s of d efi n it i o n s of l ife a re p o s s i b l e a n d a re e q ua l l y va l i d . Sc i e n tific d efi n itio n s o r stu d i e s a re l a rg e l y c o n c e r n e d wit h how l ife d eve l o p e d a n d how i t i s m a i n ta i n e d . P h i l o s o p h i c a l and re l i g i o u s d ef i n itio n s a re m o re c o n c e r n e d with why. T h e two k i n d s o f d efi­ n itio n s a re u s u a l l y co m p l e m e n ta ry, not c o m p etitive .

34

I NTERDEP E N DENCE is a c h a racte ristic of a l l l iv i n g thin g s . Eve ry in d ivid u a l exi sts a s p a rt o f a n i n terbreed­ ing po p u l atio n that co n s i sts of m a ny g e n et i c a l l y simi­ l a r in d ivi d u a l s . These po p u l ati o n s of i n d ivi d u a l s exist wi thin c o m m u n ities of m a ny s pecies that i n te r a ct with o n e a n othe r a s p rey a n d p re d ator, host a n d p a ra site, co n s u m e r a n d p rod u c e r, a n d co m petitors fo r s p a c e or food . The intera cti o n cuts a c ross the m a j o r divi s i o n s of p l a n ts a n d a n im a l s ; thus , trees she l t e r b i rd s , i n sects fe rtil i z e flowers, he r b ivo res co n s u m e g ra s s , fish s u p ­ port p a r a s ites, s e a a ne m o n e s shelter c l o w n fi s h , etc . i n teract with C O M M UN I T I ES t h e i r p h y s i c a l e n v i ro n m en t , c o n ­ stitu t i n g a n ecosyste m . C h a n g e s i n ra i n fo l l , t e m p e ra t u re ra n g e , s o i l type , e l e va t i o n , l a t i tude, depth of sea water, sedi m e n t i n strea m s , a nd cou n t l e s s o t h e r p h y s i c a l factors a l l i n f l u e n ce t h e

deve l o p m e n t of com m u n ities. Orga n i s m s i n t u r n m a y m odify t h e i r e n v i ro n m e n t, crea t i n g lo­ c a l s h ade in forests , m odifyi n g a n d e n r i c h i n g s o i l s , p reve n t i n g eros i o n , a nd i n m a n y o t h e r w a y s . T h i s i n te rde p e n de n c e p ro­ vides i m porta n t data .

O x y g e n a n d c a r b o n c y c l e s s h ow i n terde p e n d e n ce of a l l l i f e . CARBON- HYDROGEN­ OXYGEN CYCLE ( on land )

T H E S I M I LA R I T I E S t ha t exist betwee n l ivi n g o rg a n is m s

a t a l l leve l s h a v e c e rta i n i m p l icati o n s . Offs p r i n g o f t h e sa m e parents h a v e a m o r e o r l e s s close rese m b l a n ce to o n e a n oth e r a n d to th e i r p a re n t s . Althou g h e a c h i n d iv i d ­ u a l is u n i q u e , m e m b e rs o f the sa m e s p e c i e s s h a re " o b v i ­ o u s " co m m o n featu res t h a t a re c o n s e rved a n d p e rpetu­ ated i n re p rod uctio n . W e d o n ot ha ve tro u b l e recog n i z i n g a l io n , fo r exa m p l e - o r even a d o g , d e s p ite t h e m a n y va riati o n s tha t d o m estic b reed i n g h a s p rod uced i n d og s . o f rese m bl a nce a l so · exist a m o n g related species. Ocelots, p u m a s , bobcats, a n d dom estic cats, f o r exa m p l e , a l l have ce.rta i n b a s i c c h a ra cteris­ tics i n co m m o n , and these a re recog n ized in a n i ma l c l a ssifica­ tion (taxono my) by g rou p i n g t h e m a l l togeth er i n the s a m e g e n u s-Fe lis . B u t g e n e ra , too,

DEGREES

e x h i b i t d e g rees of rese m bl a n ce so that we c a n g rou p t h e m i n tq fa m i l ies of s i m i l a r m e m bers. S i m i l a r fa m i l ies a re g roup ed i n to ord e rs, orders i n to classes, a n d c l a sses i n to p hyla. Each " h i g h er" g ro u p thus i n c l u d e s m o r e forms, a n d these h a ve prog re ssively fewer features i n com m o n ( p . 1 1 ) .

The o rioles be low belong to a sing le g e n us, Icterus. They have dif­ ferent. colors a n d geog ra p h i c ra nges, b ut they share m a ny co m m o n features. T h e y a re m e m bers of t h e s a m e f a m i l y a s b l a c k b i r d s .

To rosa u r u s

Trice ra tops

Arrhin oceratops

T h e s e h o r ned c e ra to p s i a n d i no s a u r s s h ow how deg rees of rese m ­ , b l a n ce s u g g e s t evo l u t io n a ry re l a ti o n s h i ps . T h e g e o l o g i c prog re s ­ s i o n i s a r ra n g ed f r o m b o t t o m to t o p . ( After C o l bert. )

T h e m e a n i n g of t h e va r i o u s d e g re e s of res e m b l a n ce wa s a t fi rst t h o u g h t to l i e i n t h e i r a p p ro x i m a t i o n to the a rc h e t y p e o r i d e a l fo r m , u p o n w h i c h e a c h s p e ­ c i e s h a d b e e n " d e s i g n e d " o r p l a n n e d . But to l ater stu­ de n ts, t h es e c l u stered re lat i o n shi ps, often p i ctured a s t h e b r a n c h e s of a t r e e ( a s a b ove ) s u g g ested o n ly d e ­ g r ees of r e l a t i o n s h i p , a l tho u g h the c l a ssification itself wa s esta b l i s h e d before t h i s w a s recog n i zed ( p p . 1 0 1 1 ) . J ust a s t h e b r a n ches of a tree g row by conti n u ­ o u s d evel o p m e n t fro m a seed , e a c h b ra n c h bei n g fo r m e d b y s l o w a n d a l m o s t i m pe rcepti b l e m o d i fication of e a r l i e r b ra n ches fro m an i n iti a l ste m , so t h e b r a n c h ­ i n g pattern o f c l a s s i fi c a t i o n s u g g ested a co m m o n ori­ g i n . T h e b r a n c h e s r e p re s e n te d d e g re e s of r e l a t i o n s h i p t o t h e o rg a n i s m s o f t h e c e n t r a l a n cestra l ste m . 37

DEGREES OF S IM I LARITY between l i v i n g t h i n g s

a re

reflected by va r i o u s featu res . The overa l l form a n d structure ( m o rpho l o g y ) o f a l l creatu res s how va ryi n g d e g rees o f s i m i l a rity. W hen we s pea k o f a " d eer, " w e th i n k of a p a rt ic u l a r k i n d o f a n i m a l, b u t the d eer fa m ­ i ly c o n ta i n s 2 0 d ifferent g e n e r a a n d m a ny species . Al­ though they d i ffer in s iz e , a ntl ers, colo r, a n d g eo­ g r a p h i c d i st r i b u t i o n , all m e m bers of the deer fa m i ly sha re b a s i c features . Their s keleton s rese m b l e o n e a n ­ other, b o n e fo r b o n e ; their i nte r n a l o rg a n s a re si m i l a r; a n d t h ey d i s p l a y m a ny s i m i l a r b e h avio r a l c h a racteris­ t i c s . Thi s co m p rehe n s ive s i m i l a rity, s howi n g a u nity of b a s i c fo r m but a d ive rs ity of i n d ivid u a l pattern, sug­ g e sts the i r d e rivation f r o m a co m m on a n cesto r that pos­ s e s s e d these co m mo n featu res . DEVELOP­ EMBRYO N I C THE M E N T of m a n y s pecies s h ows

startl i n g s i m i larities, even in forms that h ave few rese m ­ b l a n ce s as a d u lts. T h u s a m a n , a p i g , a n d a c h ic ken have a g e n e ra l s i m i l a r i ty d u r i n g their develop m e n t. I f each s pecies is e n tirely d i s ti nct fro m every

o t h e r species, i t m a kes n o s e n s e t h a t they s h o u l d h ave s u c h e m ­ b ryo n i c rese m b l a n ces a n d t h e n l o s e t h e m i n a d u l t l i fe . A l t h o u g h t h i s e m b ryo n i c s i m i la rity i s less than was c l a i m e d by late n i ne­ tee n t h c e n t u ry zoologists, it is a n i n d ication and an i m print of their re mote k i n s h i p.

STRU CTU RES

HOMOLO G O U S S T R U CTURES i n

A N A LO G O U S

m a n y orga n i s m s s u g g est t h e i r derivation f ro m com m o n a n ces­ tors. The s ke l etons of cats, horses, whales, bats, m i ce, a n d m e n , for exa m p le, a l l have a n ess e n t i a l l y s i m i l a r for m . The structure of t h e ve rte brae a n d t h e fused bones of t h e s k u l l a re s i m i la r i n every verte brate, fro m fish to m e n . So a re the re­ lated n e rves, m uscles, a n d b lood vessels. In less c losely re lated species, h o mology i s less well m a rked, s u g g esti n g their m ore d ista n t com m u n ity of ori g i n .

s h ow a s i m i l a rity of f u n ction but not of d eta i l e d structu re . T h e w i n g s of a n i n sect perform the sa m e f u n ction as those of a b i rd , but they h ave a ve ry d if­ fere n t structure . S u c h d i ffer­ e n ces res u l t n o t from i n h eri­ t a n ce fro m a co m m o n a n cestor b u t from a d a ptation to s i m i l a r If e n v i ro n m e n t a l co n d i ti o n s . each s pecies h a d a s e p a ra te orig i n , t h e n a n a lo g o u s s truc­ t u res s h o u ld be m o re co m m o n t h a n h o m o l ogous s tructures, b u t t h e reverse i s true.

VESTIGIAL STRUCTUR E S deve l ­ o p w h e n a n org a n i s reta i n ed eve n thoug h its ori g i n a l f u n c ­ t i o n i s reduced or lost. S u c h structu res a re fou n d i n a l l a n i ­ m a l s . I n m a n , t h e e a r m u scles a re usually nonfu n ctional, b u t i n other a n i mals, s u c h a s t h e dog, these m u scles m ove t h e ears and d i rect them towa rd particu ­ l a r sou n d s . T h e h u m a n a ppen ­ d i x h a s n o obvious f u n ction a n d i s a n u i sa n ce, b u t in other a n i ma l s, t h e a p pen d i x i s m ore stro n g l y deve l o ped a n d serves a n

i m porta n t d i g estive f u n c t i o n . I n w h o l e s a n d i n s o m e s n a kes, ves­ t i g i a l h i n d l i m bs o re preserved, s u g g esti n g t h a t they o re t h e rem m o n ts of a n cestra l structures.

B I O C H EMI CAL S I M I LARITIES a lso e x i s t betwee n re­

l a t e d o rg a n is m s . The most stri k i n g feature of these si m i­ l a rities is the way they confirm i n d e p e n d e ntly the vari­ o u s g ro u p i n g s of p l a nts and a n i m a ls that were esta b ­ l i s h e d o n t h e b a sis of thei r overa l l for m . T h i s i m plies t hat t h e c l a ssification that h a s been d eveloped (pp. 3 63 7) is n ot w h o l l y a rtifi c i a l but reflects the a n cestra l - d e ­ scen d a n t ( p hylog e n etic) relatio nsh ips o f org a n is m s . Oth e r m o re g e n eral b i o c h e m i c a l si m i l a rities i m ply t h e co m m o n k i n s h i p of all o rg a n i s m s . These i n c l u d e the use of n u c l e i c acids as a g ents of heredity (p. 68), the use of a p a rticu l a r phosph ate, ATP, i n e n ergy tra n sfer, a n d t h e use by p l a n ts with c h l o ro p hyl l of this g reen pig me n t a s a cata lyst i n p h otosynthesis. BLOOD P I GMENTS d i ffer in d iffere n t a n i ma l g roups. I n ve rteb ra tes a n d some other a n i ­ m a l s , the blood has a r e d pig­ ment, he moglobi n , which h a s a res piratory f u n ction i n carrying blood from the lungs or g i l l s throug hout the body. I n a l l ar­ th ropods, the respi ratory pig­ ment i s a b l u e copper com-

pou nd, ca l l e d h e mocya n i n ; i n m a r i n e wo rms, a g reen i ron com pou n d , ca l l e d c h l o rocruo­ rine. These p i g ment s i m i l a r i­ ties con fi r m the re l a t i o n s h i ps between m e m bers of t h e g ro u p s esta b l i s h ed by o t h e r criteri a . Prote i n s o f e a c h s pecies, a l ­ thoug h d i s t i n ct, s h ow co m p ar­ a b le d e g rees of s i m i l a rity.

A serolog ical test (p. 4 1 ) i s made by m i x i n g seru m and a n ti seru m a n d record i n g t h e h i g h e st d i l ution of seru m t h a t wi l l s t i l l g i ve a wh ite r i n g of p rec i p itate. (After Boyd en.) HOMOlOGO U S TEST beef se r u m a n d a n ti - beef seru m

1 000

1 5 00

2000 3000 4000

6 0 0 0 8 000 con trol

TI D DU D E n d Point

HETEROlOGOU S TEST sheep serum a n d a n ti- beef s e r u m

40

SEROLOG I CA L S I M I LARIT I E S a re m e a s u re d b y i m m u n ity tests . If b l o o d fro m o n e s p e c i e s , s u ch as a cow, i s i n ­ jected i n to the b l o o d s t re a m o f a n othe r, s a y a g u i n e a p i g , the g u i n e a p i g p ro d uces a p re c i p i t a t e , a n a n t i ­ seru m , tha t i m m u n iz e s i t a g a i n st cow ' s b l o o d . Whe n this a n ti - cow s e r u m i s m i x e d with the b l o o d of o the r a n i m a l s , it p ro d uces p reci p i ta t e s of va ryi n g i n te n s i ty tha t corres p o n d to the n e a r n e s s of the o the r s p e c i e s i n the sche m e o f c l a s s i fi c a t i o n . Th u s , a n t i - cow s e r u m g i ves 1 0 0 p e rc e n t p re c i p i t a tio n w i th the b l o o d of a n ­ o t h e r cow, 48 p e rce n t w i th a she e p , a n d 2 4 p e rce n t w ith a p i g . Thi s b i o c h e m i ca l i n d i c a t i o n o f co m m o n a n ce s t ry i s a m ethod o f c l a s s i fi c a t i o n c o n fi r m i n g wha t w a s esta b l i she d i n d e pe n d e n t l y b y c o m p a ra t i ve a n a ­ tom ica l s t u d i e s . Serolog i c a l tests m a d e w i t h a n t i- h u m a n s e r u m g ive varyi n g per­ c e n t a g e s of p r e c i p i ta t i o n . T h i s refl ects t h e q u a n t i ta t i ve deg re e s of s i m i l a r ity betwee n m a n a n d ot h e r s pe c i e s .

G o ri l l a

O ra n g u ta n

K a n g a ro o

to the pa rti c u l a r e n v i ro n m e n ts i n w h i c h they l i ve a re s h own b y a l l l ivi n g creatu res. S o m e that a re so g e n eral t h ey may b e overlooked e a s i l y i n ­ c l u d e t h e won d e rfu l ly efficient b u t d isti n ct wi n g struc­ ture of i n sects, b a ts, and b i rd s ( a l so those of t h e ex­ tinct pterodactyls), the s h a pe a n d structure of fi s h , the speci a l i z e d ste m s of d esert cacti, a n d cou ntl ess oth e r s . Sti l l o t h e r a d a ptati o n s are m o re specific . O f the m a ny exa m p les a m o n g b i rds, those of the wood pec kers were fi rst d escri b e d by C h a rles D a rwi n . Ada ptation i s so w i d es p read i n both p l a nts a n d a n i m a l s th at, althoug h n ot a proof o f evo l ution, i t sugg ests that n atural sel ectio n i s a very p ro b a b l e ex­ p l a n ation fo r o rg a n i c d iversity.

ADAPTATI O N S

PROTECTIVE fo rm and colora­ tion a re adaptatio n s s h own by many a n i m als . T h e p u pa e of some i nsects rese m b le thorns or twigs. Others m i m i c less v u l n er­ able species by color rese m -

Wood pecker

42

b l a n ces. The color of some a n i ­ m a l s , s u c h a s t h e c h a meleon, changes with t h e color of the backgro u n d . Experi m e n ts h ave s h own the s u rviva l value of t h i s coloration ( p. 8 4 ) .

A wood pecker h a s two l a rg e toes d i rected backward so t h a t i t s f o o t forms a n a n chorlike h o l d . I t s stiff t a i l feath ers form a prop a s t h e b i rd chisels with its powerful beak. It extracts insects with its l o n g , barbed ton g u e . All of t h e 1 79 species of wood pec kers h ave esse n ­ t i a l ly s i m i l a r structure .

�··�·-=--- Salomon Islands

New G &� i nea ( 5 20 species

of b i rds)

•

. •:. .: '

Fiji

Island

( 1 26)

• •

1541

Henderson •

Island

(4)

TH E NUMBE�S OF ISLA N D S P E CI E S a n d t h e i r res e m b l a n c e to those o f t h e m a i n l a n d decrea s e wit h i n crea s i n g d i s ta n c e f ro m t h e la n d . T h e n u m be r of m a m m a l s p e c i e s s hows a s i m i l ar decrea se, s u g g e s t i n g t h a t t h e species were deri ved from t h o se o n t h e m a i n l a n d .

P R E S E N T LI M ITED D I S TR I BUT I O N O F MANY S P E C I ES, s u c h a s ta p i rs, can be i n terpreted o n ly on t h e a s s u m ption t h a t they a re descenda nts of m ore w i d e sprea d foss i l a n cestors, some of w h i c h hove bee n fou n d i n i n termed iate a reas.

�

T. indicus

llr.'-.... .. +

0

GEOGRAP H I C

PlPlieoiscene tocenespecispecies es

•. / �.

After De leer

D I STRIBUTIO N of many p lants a n d

a n i m a ls s h ows features t h a t c a n b e a ccounted fo r o n ly by s u p p os i n g t h a t t h ey a re t h e d escen d a nts of co m ­ m o n a n cesto rs . T h e fa u n a s o f t h e G a l a pagos a n d C a pe Verde I s l a n d s were m a jo r clues i n D a rwi n 's d evelop­ m e n t of an evo l utio n a ry theory (pp. 18- 19) . Alfred Russel Wa l l a ce noted t h a t l a rg e r g roups, s u c h as orders, h ave a w i d e r g e o g r a p h i c d i stri bution than do s m a l l e r g roups, s u c h a s fa m i lies o r g e n era . S pecies most s i m i l a r a re fou n d i n a d j a c e n t a re a s, s u g ­ gesting their evo l ution fro m co m m o n a nc esto r s. 43

..

LIV I NG SPEC I ES of p l a n t s a n d a n i m a l s a re c h a r a c ­

t e r i z e d by t h e i r con sta n cy of g e n e r a l fo rm a n d t h e i r g reat ran g e o f i n d ivi d u a l variatio n . Each s p e c i es b re e d s " true " a n d i s r e p ro d u ctively i s o l ated from o t h e r s pecies, even t h o s e t h a t are closely si m i l a r . Y e t n o two i n d ivi d u a l s of t h e s a m e species a re i d e ntica l . We n ow recog n i z e t h a t t h e i n h erited c h a racteristics of a l l l iv­ i n g t h i n g s a re contro l l e d by t h e i r g e n e s a n d c h ro m o­ s o m e s a n d t h a t t h e s e stru ctu res u n d e r g o s p o n ta n eo u s m u tatio n s ( p . 7 4 ) . T h i s i n pu t of n ew c h a racteristics m e a n s that ove r a long period of ti m e species a re not Axed e ntities a s o n c e s u p posed . Both in n a t u r e a n d in ca ptivity, we see evid e n c e of va riations with i n a spe­ cies, s u g g esti n g t h e i r evo l utio n a ry c a p a c ity. S E L E C T I V E B R E E D ING of do­ m e s t i c p l a n ts a nd a n i m a l s i ndi ­ cates t h e g reat v a r i a b i l ity of many s pe c i e s . I l l u stra ted h e re , a re dog s of t h e s a m e s pe c i e s .

Basset H o u n d

Dach s h u n d

Vendee Hound

-- �

Canis fomiliaris leineri

T h i s s u g g ested to D a rw i n t h a t natural selection might be ana­ l og o u s i n i t s a c t i o n t o ( a r t i f i c i a l ) do m e s t i c s e l e c t i o n a s a n a g e n t of c h a n g e.

Egyptian Grey h o u n d

S t . H u bert Hou n d

Sa l u k i

Beagle

Talbot H o u n d

Afg h a n H o u n d

Sleuth Hound

PO PULAT I O N S s h o w loca l v a ri­ a t i o n i n n a t u re . T h e s m a l l e s t u n i ts , ca l l e d d e m e s, a re o n ly partly i s o l a te d populations, with i n w h i c h t h e re i s c l o s e g e n ­ e t i c s i m i l a r ity. V a r i a t i o n b e ­ twe e n d e m e s i s ofte n ra ndo m , b u t betwee n s o m e i t i s n o n -

ra ndo m , f or m i n g g raded c l i n e s t h a t m a y s h ow corre la t i o n w i t h differe n t e c o l og i c c o ndit ions. Th u s loca l r a c e s o r s u b sp e cies d e v e l o p , e a c h a d a p t e d to t h e c o n di t i o n s of a part i c u l a r a rea a n d i n terg rad i n g w i t h o n e a n ­ o t h e r o n l y i n o v e r l a p p i n g a re a s .

P. major m a jor

P.

I

rnajqr

intermedius

•

P. major minor

• Z o n e of Overl ap Z o n e of Ove r l a p

Geog ra p h i c a l D i s t r i b u t i o n of t h e Great Ti t . Th e G re a t T i t of Europe a n d As i a s h ow s deve l o p m e n t o f typ­ ical geog ra p h i c r a c e s or va r i ­ e t i e s . T h e m o s t w i d e s p read race i s P o r us major major, e x t e n d i n g from B r i ta i n to Ea s t A s i a . P . c i n e r e u s a n d P. m inor h a ve m o re restri cted ra n g e s . T h e i n te r b reedi n g i n I ra n o f P . major w i t h t h e c e n t ra l As i a n a n d I n dia n v a r i ety P . cin ere us g ives a f o u r t h v a r i e ty-P. i n te r -

( Af t e r De B e e r )

medius. I nterbree d i n g of P. cinereus a n d P. m inor in I n d o ­ c h i n a g i v e s a fi ft h , P . com m ix ­ I u s . B u t P. major a nd P . c i n er­ eus occ u r tog e t h e r w i t h o u t i n ­

te r b ree d i n g in n o rt h c e n tra l A s i a , a s do P. m i n or a n d P . m a jor i n n o rt h e a stern C h i n a . R e p rodu ctive i s o l a t i o n b e t w e e n g e o g r a p h i c races s u g g e s ts o n e mecha n i s m for the formation o f n e w s pe c i e s ( p . 76 ) .

45

CHANGES IN SPECIES h a ve bee n observed eve n d u r­

i n g the l i m ited t i m e that accu rate observatio n s h a ve been reco rded . S o m e d isease- p ro d u c i n g bacteria h a ve been s uccessfu l l y treated with d ru g s, but o n e of th e s i de effects of this med icatio n h a s been the deve l o p ­ m e n t o f v a r i o u s dru g resista nt stra i n s of the bacte ri a . Escherichia coli i s a co m m o n bacte r i u m that h a s d e ­ ve l o p e d p o p u l a tio n s e n ti re l y resista n t to strepto m y ­ ci n . T h ese resista n t g ro u p s a rise fro m m u ta tio n s . W h e n t h ey a p p e a r, t h ey a re t h e o n l y g ro u p a b l e to s u rvive a n d m u l ti p l y .

A l t h o u g h m o s t evo l u ti o n i s pro b a b l y t h e res u l t o f s low, c u m u l a tive c h a n ge , by t h i s process of " prea d a ptatio n " i n w h i c h a m u ta ti o n " e n co u n ters " a favora b l e e n v i ro n m e n t, t h e w h o l e c h a racter of a popu l a ­ t i o n m a y c h a n g e very ra p i d ly. By a s i m i la r proces s , so m e s pe­ cies of destructive i n sects have d eveloped an i m m u n i ty to vari­ ous i nsecti c i d e s . Sca l e i n sects of t h e citrus reg i o n s of Ca l i fornia have beco m e i n crea s i n g ly re­ s i sta n t t o hyd rocya n i c a c i d , for exa m ple.

A

sca le

ra ntii.

i nsect,

A o n idella

a u­

MELA N I SM ha s been observed i n s cores o f spe­ cies of moths d u ring the past century. In i n d u strial a reas, many species have beco m e p ro­ gressively darker, o r even b l a c k , w h i l e m e m bers of t h e s a m e s p e ­ c i e s i n rura l a reas re m a i n l i g h t colored. T h i s d e m o n strates h ow plastic a n d c h a ngeable many species a re, eve n over short pe­ riods of t i m e . The m e c h a n i s m o f t h i s c h a n g e i s d iscussed o n p. 84 . I n t h e p h otogra p h b e ­ l o w , t h e l i c h e n - covered tree tru n k prov i d es concea l m e n t for Pep pered the l i g h t-colored Moth but m a kes the d a rker, i n ­ d u stri a l m e l a n i c form co n s pi c u ­ ous. The soot-covered tree tru n k from an i n d u s tri a l a re a concea l s t h e d a r k form of t h e Peppered M o t h b u t ma kes t h e l i g h t form c o n s p i cuous. I N DUSTRIAL

cred i t : H. B. D . Kettlewe l l

Peppered moth, Bis t o n b e t u la r ia, show i n g l i g h t a n d d a r k fo r m s o n two d i ffere n t b a c k g rou n d s .

FOSS I L S PEC I ES a re d ifficu l t to reco g n ize b ec a use

the

test of re pro d u ctive isolation, by w h i c h l ivi n g species a re d isti n g u i s h e d , c a n n ot be used . But we c a n reco g ­ n iz e i n fossils t h e s a m e d e g rees o f structu ral d iffere n c e a s betwe e n related l ivi n g species . We c a n a l s o recog ­ n iz e i n d e p e n d e ntly deve l o p i n g fossil g ro u ps, a n d t h ese, b y d efi n itio n , ca n be reg a rd e d a s s p e c i e s . T h e foss i l record a l l ows us to observ e c h a n g es ove r fa r l o n g e r periods of ti m e th a n a re eve r a va i l a b l e i n livi n g p o p u l atio n s . I n fossils, w e c a n reco g n i z e evo l u ­ tio n i n a ctio n . Althou g h they tel l us l ittl e a bout t h e de­ t a i l e d m ec h a n i s m s of c h a n g e, fossi l s d o p rovi de pow­ erfu l evi d e n ce t h a t evo l ution h a s occu rred .

FORAMI N I FERA a re m i c roscopic

protozoa n s , most of w h i c h se­ crete a s h e l l . I l l u strated i s a m a r i n e g e n u s , Textularia, stud­ ied i n rocks of Tert i a ry a g e (p. 98) i n New Zeala n d . When traced t h roug h a period repre-

s e n te d by the a cc u m u l a t i o n of 500 feet of strata, t h ere is a m a rked c h a n g e i n s h a pe for each p o p u l a tion . Two s pecies a re recog n ized. Horizo n t a l l i n e s represe n t s ta n d a rd d ev i a t i o n f o r eac h . ( After Ke n n ett. )

Fossil b iva lve cl a m s ( b e low) , fro m roc k s of P e n nsy l ­ va n ia n a n d P e r m i a n a g e ( p . 98 ) o f t h e m id c o n ti n e n t re­ g ion of t h e U n ited States, s h ow successive d evel o p m ent by desce n t of closely s i m i l a r s pecies of the g e n u s Mya­ /in a . Each of the n u m bers o n g ra p hs represents a d iffe r­ ent species, l isted b e l ow. Form Ratio of a n g les b/ a

Ratio of length

FIGURE A

to He i g h t

0.9

1 .0

1 .1

1 .2

Misso u r i a n

1.

M y a l i n a cop h a

2 . M . lepta 2 . M. iepta M. w y o m i n g e n s is

3.

Desmoinesian

4. M . m iope t i n a

5 . M . p l iap e t i n a

6. 7.

M . cop e i

9.

M. p e t i n a

M. arbo/a

Ato k a n

8 . M . g /oss idea

1 0 . M . a v i c u loides

F I GURE B

Fi g u re A is a plot of t h e form ratio of length to h e i g h t of the shells, p lotted a g a i n s t the ra tio of the a n g le b to the a n g le a (see d ia g ra m ) . T h e rig h t - h a n d l i n e represents i n - l i n e evol u t i o n , w h e r e new s pecies a rise by s u c­ cessive modification of earlier populations. T h e left- h a n d l i n e re p res e n ts speciation by bra n c h -

i n g or s p l i tt i n g ra t h e r t h a n by co n t i n uous c h a n ge. Fig ure B s h ow s i n ferred evo l u ­ tionary descent a n d re lation­ ships (phyloge ny) of species of Myali n a . N u m bers refer to the sa m e species a s t h os e i n Fig. A. T h e n a mes a re those of s u cces­ sive roc k d iv i s i o n s . (After New­ ell and Moore.)

49

H I G H E R TAXA ( g e ne ra , fa m i l ies, etc.) of a n i m a l s

a n d p l a nts a re fou n d i n t h e foss i l record a l so t o a ri s e by d e s c e n t w i t h s l ow mod ification from a n cestra l fo r m s . T h i s is evol utio n . The fos s i l record provi d e s repeate d evi d e n c e t h a t it i s t h e n o r m a l m ethod b y w h i c h n ew g ro u ps of o rg a n i s m s orig i n ate . C E R A T O PS I AN D I N OS A U RS (a l l dra w n t o s a m e s c a l e ) l i ved i n t h e Creta c e o u s Period ( p. 98), 7 0 m i l ­ l i o n years a g o. T h ey s h ow a n overa l l i n crea s e i n s i z e a n d i n t h e re l a t i ve di m e n s i o n s a n d co m ·

p l e x i ty o f t h e b o n y a r m or t h a t covered t h e i r h ead a n d n e c k , Triceratops rea c h ed a l e n g t h o f 2 4 f e e t a n d we i g h e d u p to e i g h t to n s. O n ly t h re e g e n e ra are s h own. (Afte r Co l b e rt.)

O l D WOR L D

Equus

Styfohipparion

Hipparion

Pfio hippus

Anchitherium

GRAZERS I

BROWS E R S

Orohippus

To R h i noceroses

1£)

Epihtppus

H ORS E S p rovide a c l a s s i c e x a m ­

p l e of t h e evo l u t i o n of n e w g e n ­ e r a f r o m e a r l i e r o n e s over a period of 70 m i l l i o n yea r s .

After S i m p s o refl e cted m odi fi c a t i o n s L a te r c h a n g e in d i e t from brow s i n g to g r a z i n g . R i g h t u p p e r m o l a r t o o t h s u rf a c e s a re s h o w n .

51

"MI S S I N G L I N KS," a s e v i de n ce t h a t o n e g ro u p de­

veloped from a n o t h e r, were often d e m a n d ed by o p po ­ n ents of evo l uti o n i n e a r l i e r y e a r s of t h e evoluti o n a ry d e bate. At t h e ti m e of t h e p u b l ication of On the Origin of Species, very few of these tra n sitio n a l fo r m s were k n own , b u t m a ny h ave since b e e n d iscove red . T h ey b r i d g e m a ny of t h e m a jo r g ro u p s of existi n g o rg a n is m s . I n th e ve rtebrates, for exa m p le, t h e re a re tra ns itio n a l fo r m s b etwe e n fi s h a n d a m p h i b i a , a m p h i b i a a n d rep­ tiles, reptiles and b i rds, and repti l e s and m a m m a l s . They i n d icate t h a t these m a jo r g rou ps, d isti nct a n d s epa rate i n l iv i n g forms, a rose fro m for m s th a t s howed s o m e c h a racters i nte rmed iate betwe e n two g ro u p s a n d oth e rs now restricted t o j u st o n e . ARC HAEO PTERYX, a n a n cestra l

fos s i l b i rd from t h e J u ras si c of Germa ny, had m a n y features of the rept i l i a n g rou p fro m w hi ch i t developed. Althou g h it had the feathers of a b i rd , i t h a d a repti l e l i ke too t h e d bea k a n d clawed w i n g s . I t h a d b i rd l i ke

feet b u t re pt i l i a n vertebra e a n d ta i l . I t h a d t h e w i s h b o n e of a bird b u t a repti l i a n b ra i n . A rchaeopteryx was i ndeed a m os a i c or j u m b l e of va riously d eve loped c h a racteristics that were s u bs e q u e n tly restricted to d i ffe rent g roups (p. 1 29).

Cynognath us

T H ER I O D O NTS " beast­ toothed ") were reptiles that l ived i n Perm i a n a n d Triassic times (p. 98). T h ey s h owed many m a m m a l ia n c h a racteristics. Cynognath u s was a typ i c a l m e m be r of t h e g ro u p . An active carn ivore, six feet l o n g , i t h a d a l o n g s k u l l w i t h m a m m a l- l i ke THE

d i fferentiation of the teeth i nto i n c isors, ca n i nes, a n d cheek teeth . I t had an " u pri g h t " m a m ­ m a l i a n postu re, a n d m a ny d e ­ ta i l s o f t h e s k u l l , vertebra e , h i ps, s h o u l d e rs, a n d l i m b s were a l s o m a m m a l - l i ke . Ma m ma l s a re b e l i eved to h ave deve loped from these o r s i m i la r re pti l e s .

P l a ty p u s FO S S I LS a re s u rviving representa tives of a n c i e n t fos s i l g ro u p s . T h e m o n otre m es-the duckbilled platypus and t h e sp i ny a n tea ters ( ec h i d n a s ) of Austra l i a-a re very p ri m i tive m a m m a l s t h a t reta i n m a n y typ i ­ cal repti l i a n c h a ra c ters i n t h e i r LIV I N G

s k e l e to n s . T h e y l a y l e a t h ery, l a rge-yo l ke d eggs a n d s ecrete milk fro m sweat m o d i fi e d g l a n d s . S u c h a n i m a l s pro b a b l y a rose fro m t ra n s i t i o n a l repti l ­ i a n - m a m m a l i a n form s . G i n kgos and a ra ucarias a re p l a n t l ivi n g fos s i l s .

53

T H E F OS S I L R E C O R D

s hows t h ree other g e n e ra l featu res w h i c h s u g g est that species a rose b y c o n ti n u ­ o u s evol utio n . I t d i s p l ays diversificati o n , enviro n m e n t ­ fi l l i n g , a n d co m pl e x a d a ptatio n a l c h a n g e . Th ese a re p recisely w h a t wou l d b e pred i cted, a priori, o n t h e b a s i s o f t h e t h eory of evol uti o n . THE

ADAPTAT I O N

OF

OR­

GA N I S M S to a g reater ra n ge of

envi ron m e n ts has d eveloped with t i m e . T h e o l dest orga n ­ is m s were confi n e d t o the seas, but fresh waters, the l a n d , a n d t h e a i r were successively colo­ n ized . Deta i l s a re given on p. 1 1 8 and t h e fol lowi n g pages.

Mammals

Modern Amphibians

AQUATIC

Cartilaginous fishes

54

T h e h istory of t h e verte b rates s h ows an i n cre a s i n g ra n g e of ada ptation. Birds and a few m a m ma l s and extinct re ptiles h ave colonized the a i r; others h ave " retu r n e d " to t h e a q uatic l ife a b a n d o n e d by t h e i r a n ces­ tors. Deta i l e d a d a ptati o n s have deve loped i n each enviro n m e n t .

_j

_

_

l_j__l_j

----.

c 0

�

;;;

c 0

.., � 0

1.

_ _

l

I

,

0

�

. .,

0 �

0

u

" �

T h e c l a sses of vertebrates s h ow an i n c re a s e i n co m p l e x ity a o d d i ­ vers i ty w i t h t i m e . T h e w i d t h of t h e co l u m n s i n d i ca t e s t h e re l a t i v e a b u nda n c e of e a c h g ro u p ; dotted l i n e s s h ow t h e pro ba b l e or i g i n o f e a c h g ro u p . ( After S i m p so n ) T H E TOTAL N U M B E R of s p e c i e s h a s s h own a steady i n c r e a s e th rou g h g e o l og i c t i m e. I n sects , for exa m p l e , c o n s t i t u t e m o re t h a n t h re e-qu a rters of a l l l i v­ i n g s pe c i e s , yet t h ey d i d not a p p e a r u n t i l the Devon i a n , 375 m i l l i o n yea rs ago ( p . 98). The c l a s s e s (a bove) s h ow s i n crea s i n g n u m b ers of s pe c i e s .

G R EATE R C O M P L E X I TY of o r g a ­ n i s m s w i t h t i m e h a s a c co m pa n i ed t h e i n va s i o n of n ew e n v i ro n m e n ts . "Co m p l e x i ty" i s a n a m b i g u o u s q u a l ity, b u t m o s t wo u l d a g ree that fish, a m p h i bians, reptiles, a n d m a m m a l s represe n t s u c h a s ca l e . Th i s i s a l s o t h e o r d e r of t h e ir a p pe a ra n c e in the f os s i l reco rd.

55

_ _

THE PROCESS OF EVOLUTION

I N H ERITAN CE p rovi d e s t h e c o n sta n cy of fo r m ( a l i o n i s a l i o n i s a l i o n ) a n d va ri a t i o n i n d e ta i l ( b l u e e y e s , g re e n e y e s , b ro w n e y e s , b l a c k eyes ) t h a t a re c h a ra c ­ teristic o f a l l l ivi n g p l a nts a n d a n i m a l s . lobsters p ro ­ d u ce o n l y l o b sters; h u m a n s p ro d uce h u m a n s . B ut n o two l o b sters a n d n o two h u m a n s a re eve r i d e n t ica l i n a l l c h a r a cte rist i c s . H o w a re t h e s e two, co n sta n cy a n d va riety, t ra n s m itted fro m p a re n ts to offs p ri n g ? GENES, a s demon strated by Greg o r M e n d e l (p. 26) and by s u bs e q u e n t stu d e n ts of g e n et i cs , are t h e reg u lators that g overn t h e deve l o p m e n t of c h a racteris­ tics i.n new i n d iv i d u a l s . Genes are m a d e of deoxyr i b o n u c l e i c acid-DNA (pp. 72-75)-a n d reproduce themselves exactly. G e n e s are i n corporated i n v i s i b l e structu res c a l led c h ro mosomes, e a c h of w h i c h conta i n s

m a n y g e n e s . Ea c h species h a s a defin ite type a n d n u m be r of c h ro m osomes. In all b u t t h e m o s t s i m p l e orga n i s m s , t h e c h romosomes a r e conta i n e d i n t h e ce l l n uc l e u s . C h a ra cteri stics of a n o rg a n i s m a re governed by parti c u l a r g e n e s ; b u t i n d i v i d u a l g e n e s m a y i n teract w i t h o n e a n ot h e r o r co m b i n e to prod uce c o n t i n uous v a r i a t i o n o f s o m e c h a racters .

T h e h u m a n c h ro m osomes s h own be low ca rry g e n e s that d eterm i n e i n d ividual c h a racteristics. X a n d Y a re the s e x c h romosomes.

11

2 3 ,.;,.

of c h romosomes i n order of s i z e

\

3< 6

\

KK 81

1

XI

II

8

7

'� - � I "'"' J'

:J-I 1-.•f �

��7":\ 'l

f

"'

lit

Kl 9

\ l

� 1"1 ��}�I' 1. �

_ .t t.

3

2

\

I

I1 3I

\

XX 10

IX 16

XK

\ 19

c h rom os o m e s i n s p e r m a tog o n i u m

J fi K& 4

5

��

i ii

a1 7x

1 1

20

Hl

A1 5ft 12

II

14

II

I

AM 21

18

11 5 22

I

I y

A

I

II

spermatogon i u m

;:

rr t p rmatocyte '

1 II

..

(f\ \�! ...

THE DEVELOPMENT OF

••

11 1 1

fi �st oocy t e

. JI" •• • • , . .,.,_ lfC

seco n d spermatocy te$

!

J

!

.J.

!

...

' P "m'

!

II \.

111 1

•.! • •

)(

I

J)

a�

,J.

\

a re the s peci a l ized re productive c e l l s of p l a n t s a n d a n i m a l s . Most s p e c i e s prod uce both male g a m etes (sperm) a n d fe m a l e g a m etes (egg s). Eac h g a mete c o n ta i n s o n ly one c h ro m oso m e from each p a i r

�" . .

.

fi rst ' � ; polar body

II·.,I' ...........

•

-\.ve pola

ootid

! �' / I II • •

GAMETES

_...,

second oocyte

••

\ sp � r m atl ds J

'<,

o ogoniu m

S PERMS A N D EGGS

� �:��

egg

fertil ized e g g (zygote)

of the o rg a n i s m 's d i p l o i d c e l l s . U n l i ke t h e body c e l l s , w h i c h h ave a d i p loid o r 2 n c h ro m o ­ so m e n u m ber, t h e tota l n u m ber of c h ro m os omes i n a g a m ete i s n . These ce l l s w i t h u n p a i red c h romosomes are c a l led h a p l o i d .

C H R O M O S O M E S of most species occu r in pa i rs so that e a c h c e l l c o n ta i n s two s i m i l a r ( ho m ol o g o u s) c h ro­ moso me s . Ea c h c h ro m o s o m e i n turn conta i n s h o m o l o ­ g o u s g e n e s . S u c h cel l s a re ca l l ed d i p l oi d . T h e d i p l o i d n u m b e r i n d icates t h e tota l n u m b e r o f c h ro m os o m es t h a t t h e s e c e l l s c on ta i n . I n m a n , fo r e x a m p l e , m ost c e l l s conta i n 2 3 chromo­ som es, h e n ce t h e u s u a l c h rom o so m e n u m be r (n) is 2 3 . But s i n ce t h e c h ro m os o m e s a re p a i red i n d i p l o i d c e l l s, the tota l d iploid n u m be r i n each c e l l is 2 n o r 46.

57

C E L L D IVIS ION-the consta nt m a n u fa ct u re of n ew

c e l l s-is essenti a l for t h e l ife a n d g rowth of a l l m u lti­ cel l u l a r p l a n ts a n d a n i m a l s . I n j u re d or worn out cells a re replaced by this p rocess, too . Th e new c e l l s a re exact replicas of th e orig i n a l cel ls, conta i n i n g t h e sa m e k i n d a n d n u m b e r of c h ro m oso m e s . Th e p rocess o f p ro d u c i n g n ew c e l l s by d ivision of the orig i n a l cel l is c a l l e d m itosis. In m ost c e l l d ivision , the p rocess is co m pl eted i n less t han two hours. Mi­ tos i s c a n pro d u ce n ew i n d ivid ua l s as wel l as n ew body cel l s . In m ost org a n is m s that reproduce asexually, thE> new c e l l is ide ntica l to the p a re n t c el l .

STAGES OF M I T O S I S i n ­ clude : Ia) PRO PHASE, fi rs t stage o f m i tosi s , i s m a rked b y thicken i n g o f c h ro m osomes i n n ucleus a n d s e p a ration o f ce n ­ trioles from cen tra l body ( ce n ­ tros o m e ) a bove n uc l e u s . Fibers from ce n trioles form a s ter. ( b ) S PI N D LE o f fi bers t h e n g rows between the two s e pa ra ted a s ­ ters. N uclear m e m b ra n e d i si n ­ teg rates , a n d· t h e t h icke n e d c h ro m o s o m e s divide l e n g t h w i s e i n to t w o c h ro m a ti d s . ( c ) I N METAPHASE, the d u p l i ca ted chromosomes leave t h e n u ­ cleus. They arra n g e t h e m s e lves i n to pa i rs across the e q u a tor of the e n l a rged s p i n d le. I d l I N A N A PHASE, o n e o f each o f t h e c h ro m os o m e s p a i rs , or c h rom a ­ tid, m i g rates to opposite poles of t h e s p i n d le . l e l TELO P H A S E , fi n a l stage, i s m a rked by loss of s p i n d le, division of centriole, and division of p a re n t ce l l i n to two identica l d a u g h te r ce l l s .

THE

58

MEIOSIS

2N

,

2N

OR

Division

I N

2N \ Division N

� 2N \ N

N

MEIOS I S is shown for a cell with a d i ploid n u mber (2N) of four c h ro m oso mes. The fl rst d i ­ vision i n volves t h e ra n d o m s h o r­ i n g of exactly h a l f the n u mber of c h ro mosomes betwee n the daughte r ce l l s . In this reduc­ tion d iv i s i o n , the s h o r i n g of t h e orig i n a l c h ro m osomes m a y pro­ d u c e e i t h e r of two co m b i nations i n the haploid d a u g hter c e l l s , ea c h of w h i c h h a s N c h ro m o­ somes-that i s , h a l f of the orig i ­ n a l n u m ber o f c h ro m o s o m e s .

._.. OR

� 2N \Division N

N

N

N

The s e c o n d d iv i s i o n occ u rs by m i tosis, in w h i c h each n ew d a u g h ter c e l l d iv i des into two f u rther h a p l o i d c e l l s , each a l s o with N c h ro m osomes. These form the g a m etes, or reprod u ctive ce l l s , ea c h of w h i c h h a s one c h romoso m e from each of the ori g i n a l pa i rs. The u n i o n of two ga metes, each of which has t h e haploid o r N c h ro m oso m e n u m ­ ber, g ives the , offs p r i n g the orig i n a l 2 N c h romoso m e n u m be r o f t h e parents.

S EXUA L R E P R O DUCT I O N i n vo lves a seco n d a n d d if­ fe re n t k i n d of c e l l d ivision , c a l l e d m e i o s i s . I n sexua l l y re p rod u c i n g o rg a n i s m s, m eiosis is c o n f i n e d to t h e tes ­ tis of t h e m a l e a n d to t h e ova ry of t h e fe m a l e . A l l t h e oth e r c e l l s o f t h e body i n c rease i n n u m b e r by m itos i s . Meiosis i n volves two d isti n ct sta g es of c e l l d ivisio n .

59

T H E PATTERNS OF I N H E RITANCE we re fi rst d e m o n ­

strated by Gregor Me n d e l ( p . 2 6 ) with seve n e a s i ly recog n i z e d c h a ra cters i n g a rd e n p e a s ( s i z e , s h a p e , col o r of flowers, a n d so o n ) . I n studyi n g t h e i r occu r­ r e n ce t h ro u g h s u ccessive g e n e rati o n s, he d i scovered t h a t c h a ra cters a re contro l l e d by factors ( n ow c a l l e d g e n es) t h a t d o n o t b l e n d i n t h e offs p ri n g . H e a l so d i s­ cove red t h a t i n t h e seco n d g e n e ration so m e c h a ra c ­ t e r s , s u c h a s s h o rt ste m s , yel low c o l o r , a n d w ri n k l e d p e a s , m ay b e m a s ked b y others, s u c h a s l o n g ste m s , g re e n c o l o r , a n d rou n d s h a p e . The m a s k e d features wi l l re a p p e a r i n the t h i rd g e n e ratio n . H e c a l l e d t h ese c h a racters dominant a n d recessive and c o n c l u d e d t h a t t h ey a re i n h e rited i n d e p e n d e ntly of o n e a noth e r . THE I N HER I TED RAT I O o f o n e

.

c h a racter to a no t h e r w a s s h ow n by Mendel to be c ons t a n t H e c ross-po l l i nated p u re b r e d ro u n d p e a p l a n ts (R) w i t h p u reb red w ri n kled pea p l a n ts ( r ) . T h e Ft (fi rs t fi l ia l ) g e n e r a tio n peas were a l l rou n d , but t h ose in the next g e n ehtt i on ( F2 ) i n cluded 25 per­ cent wri n k l e d , as s h own h e re .

(

RR

R

P

1 . Pure Domin a n t

60

ents

I

rr

Go m e t e s F,

Rr

ar

Offs p r i n g Rr

1 . Pure Recessive

Me ndel co n c l u d ed that eac h p l a n t h a d a p a i r of g e n e s for each c h a ra cter, b e c a use s o m e p l a n ts ( s u c h a s t h o s e of t h e Ft) w i t h a do m i n a n t c h a ra c t e r p ro­ du ced so m e offspr i n g w i t h t h e recessive c h a ra cter. F t pl a n t s a re s h ow n a s Rr o n t h e d i a ­ g ra m . P u rebred p l a n ts , a l so w i t h pa i red g e n e s , a re RR or r r

.

Me n d e l reaso n e d t h a t t h ese pai red g e n e s m u st s e p a rate to form g a m et e s. At f e rt i l i za t i o n , t h ey u n i t e ra n do m l y w i t h ot h e r g a m etes, p rodu c i n g a p redi ct­ a b l e ra t i o o f c h a racteri s t i c s i n t h e offs p r i n g. Ma n y f e a t u res m a y be i n f l u e n ced b y m o r e t h a n two k i nds of c o n t ra s t i n g c h a racters, cal l ed a l l e l e s , in e a c h gene. Con­ t i n uo u s l y varyi n g c h a ra c te r i s t i c s , l i k e h e i g h t, a r e a c co u n ted f o r p a r t l y by t h i s a nd p a rt l y b y t h e f a c t t h a t severa l d i f f e re n t pa i rs of g e n e s m a y c o n t r o l t h e sa m e c h a ra c t e r .

T h e gen oty p e of a n y o rg a n i s m i s its tota l c o m p l e m ent of g e n etic m a te ria l s . T h e p h ysica l c h a ra cte ristics of an o rg a n i s m a re called its phen otyp e . T h e g e n otype Rr p rod uces t h e p h e n otype of ro u n d p e a s . Beca u s e o f t h e d o m i n a n ce of some c h a ra cters, o rg a n i s m s with t h e same p h e n otype m ay h ave

TH R E E G E N OTYPES - R R , R r, a n a rr-a re s h ow n i n p l a nts o f t h e F 1 g e n e ra t i o n a n p . 6 0 . A p l a n t w i t h a pa i r of t h e sa m e g e n e s for a part i c u l a r c h a ra c t e r ( R R a n d rr) i s ca l l ed h omo z y g o us m ea n i n g "sa m e pa i r." O n e w i t h a pa i r of d i ffere n t g e n e s i n t h e g e n otype ( R r) i s h eterozygous, o r " d i ffe re n t pa i r." G e n e s of t h e sa m e g e n e pa i r - R a n d r, f o r e x a m p l e , a re a l le l e s .

g e n otyp e s . d iffere n t Ro u n d p e a s , f o r e x a m p l e , m a y h ave g e n otypes of e i t h e r RR or R. O n l y by s t u d y i n g t h e i r offs p r i n g c a n we d i st i n g u i s h b e twee n t h e m . H o m ozyg o u s ( R R) p l a nts wi l l b r e e d tru e , p ro d u c i n g a l l R offs p ri n g . H e te rozy g o u s p l a n ts ' p ro d u ce b o t h R a n d r offs p ri n g .

BLENDING OF SOME CHARAC­ T E RS m a y occ u r w h e n o n e a l l e l e i s n o t co m p l etely d o m i n a n t over the o t h er. In s h o r t h o r n c a t t l e , a red b u l l ( R R ) a n d a w h i t e cow (ww) p ro d u c e h e t e r o z y g o u s

ca lves t h a t a re roa n ( a m i x t ure of red a n d w h ite h a i rs ) . C ro s s ­ i n g a roa n b u l l ( Rw ) a n d a roa n cow ( R w ) g ives a p r o b ­ a b l e ratio of 1 w h i te , 2 roa n , a n d 1 red .

,

61

T H E LAWS O F I N H E R I TAN C E a re d e m o n strated below

b y the i nteractio n of two d ifferent c h a ra cters i n stud­ ies of a co m b i n ation of pea p l a n t size (T tall, do m i n a nt; and t dwa rf, recessive) a n d p e a s h a pe ( R = rou n d , d o m i n a nt; r = wri n kl ed , recessive) . C ross­ i n g a p u re rou n d -seeded, ta l l p l a n t ( RT) with a pure wri n k l e-s e e d e d , dwa rf p l a n t ( rt) g ives the results s h own . Men d e l ' s s i m p l e ratios were obta i n e d by avera g i n g t h e resu lts of c rossi n g l a r g e n u m bers o_f d iffe rent pla nts . =

=

R = Ro u n d ( do m i n a n t ) r = Wri n k led ( recessive ) T = To l l ( do m i n a n t ) t = S hort ( recessive )

PARENTS

r R IT

R rT t

MEN D E L I A N RAT I O 9 3 3 1

Round-To l l Rou n d-short w ri n k led-Ta l l w r i n k led-s hort

62

TA I LS

Prob a b i l ities a re of m a jo r i m p o rt a n ce i n u n de r ­ sta n d i n g g e n etic m ec h a n i s m s . Pro b a b i l ity is t h e li keli­ hood of a p a rticu l a r even t h a p p e n i n g . I f you toss a coi n , t h e re is a 50 percent c h a n ce that it wil l com e down h e a d s . W e ca n ca l c u l ate t h e p ro b a b i l ity o f t h i s . P is t h e p rob a b i l ity, f is t h e total n u m ber of ways i n w h i c h t h e eve n t m ay o c c u r, a n d u i s t h e n u m be r of ways in which so m e other u n favo ra b l e event m ay occur. f P = -­

f+

u .

If we toss a coi n 1 00 ti m es, it i s j u st a s li kely to com e down h e a d s a s it i s tai l s for a ny g iven th row. S o the p rob a b i l ity of t h rowi n g a h e a d ( PH) will b e : P H eo d

=

f( Heads }

�---

f( Heads } + U( rai ls )

50

50 + 5 0

50

1 00

=

0 5 •

I n g e n etics, g a m etes a re co m pa r a b l e to t h e coi n s , a n d t h e zygotes res u l t i n g fro m t h e u n io n of t h e g a m etes a re c o m p a ra b l e to t h e h e a d s a n d ta i l s . I n m a n, with 2 3 p a i rs of c h ro m oso m es, t h e re a re 223 o r 8 , 3 8 8 , 6 0 8 d i ffe rent co m b i n atio n s possi b l e i n t h e p ro­ d u ction of s p e r m s a n d e g g s . S i n ce any o n e of these s p e r m may ferti l i z e a ny of an e q u a l n u m be r of k i n d s o f e g g s fro m a fe m a le, it i s theoretic a l ly possi b l e for o n e pa i r of h u m a n s to prod u c e a s m a ny offs p ri n g a s t h e wo r l d p o p u l a t i o n wit h o u t a n y t w o b e i n g i d e n ti c a l . 63

I N H ERITAN CE PROBAB I LITI E S ca n be ca l c u l a te d j u s t a s

p r o b a b i l ities a r e c a l c u lated i n fl i pp i n g a coi n . With 0.5 means a 50/50 or a th e coi n , a va l u e of PH o n e out of two p roba b i l ity of obtai n i n g a head o n a n y o n e t h row . S i m i l a rly, t h e proba b i l ity ( PT) o f o b ­ ta i n i n g a t a i l i s a l so 0 . 5 . T h e s u m o f a n y d e p e n d e n t p ro b a b i l i ties is always 1 . I n the c a s e of t h e coi ns, t h i s 1 ca n b e exp ressed a s P H + PT Proba b i l ities ra n g e from 0 to 1 . 0 . P = 1 i m p l ies a pa rtic u l a r eve n t is ce rta i n to ta ke p l a ce . P = 0 m e a n s a n eve n t is i m possi b l e . T h e p ro b a b i l ity o f two o r m o re i n d e p e n d e n t events occu r r i n g s i m u lta n eously is g iven by t h e p ro d uct of their i n d ivi d u a l proba b i l ities, o r P = P 1 X P2 • I n t h i s fo r m u l a , P i s t h e p ro b a b i l ity o f t h e eve nts occu r r i n g s i m u lta n eously, P 1 is t h e p roba b i l ity of o n e eve n t, a n d P2 is the proba b i l ity of t h e other. S o m eti m e s a single eve n t ca n have m o re tha n o n e cause. B l a c k color , i n g u i nea p i g s b e l ow for exa m ple, may be p rod uced by a l l black o r by b l a c k and wh ite a l l e l es, if b l a c k is d o m i n a nt. T hen the p ro b a b i l ity of suc h a n eve n t ( 2 ) t h a t m a y a r ise i n m o re t h a n o n e wa y is the s u m of the p ro b a b i l ities fo r each of its ca u ses (p a n d q ) . T h i s can be c a l c u l a ted by P2 = P P + PQ . =

=

P u rebred parents

P u re bred parents

88 ( p u rebred b l a c k )

8b ( m i x e d b l a c k )

64

•

M i x e d parents

8 8 , Bb

or

bb ( p urebred w h i te )

C h e c k e rboard C a l c u l a t i o n of Possible G e n otypes

C H E CKE R B O A R D s h ows m et h o d f o r ca l c u l a t i n g t h e g e n otypes i n offs p r i n g of t w o g u i n e a p i g s t h a t a re h e t e rozyg o u s f o r b l a c k , s h o r t h a i r. ( B = b l a c k , b = b row n , S = s hort h a i r, a n d s = l o n g h a i r. ) T h e f o u r poss i b l e g a m etes f r o m e a c h a re a r­ ra n g e d a lo n g t h e s i d e s of t h e c h ec k e r b oa rd . T h e i r co m b i n a ­ t i o n s a re p l otted i n t h e s q u a re s . S i n ce B and S are d o m i n a nt, a

b l a c k s h ort p h e notype c a n be p ro d u c e d by BBSS, B b S s , B b S S , and BBSs. I n t h e c o m b i n a t i o n , t h e re w i l l b e 9 b l a c k s h orts ( 1 BBSS, 2 B b S s , 4 B b S s , a n d 2 BBSs), 3 b rown s ho rts ( 1 b b SS a n d 2 b bS s ) , 3 b l a c k l o n g ( 1 B B s s a n d 2 B b s s) , a n d 1 b ro w n l o n g ( b b s s ) . T h i s rat io of 9 : 3 : 3 : 1 correspo n d s to t h a t of t h e H a rdy-We i n b e rg P r i n c i p l e, b e lo w .

o r fe rtilized e g g s, a re f o r m e d by two i n d iv i d u a l g a m et e s , e a c h o f w h i c h c a n b e reg a r d e d a s a n i n d e p e n d e n t " eve n t . " I f p i s t h e p ro b a b i l i ty of a zygote w i t h a b l a c k c h ro m os o m e a n d q i s t h e p rob­ a b i l ity o f a zygote with a w h i te

t h e proba b i l i ty o f t h i s by ra n ­ dom mating i n a large popula­ t i o n f r o m t h e for m u l a t h a t g ives t h e p rod uct o f the male c o n t r i ­

ZYGOTES,

chro m oso m e , p + q = 1 . A zyg ote w i t h o n e b l a c k a n d o n e w h ite c h ro m os o m e m a y be p ro d u ced in two w ay s-e i t h e r pq o r q p , . e a c h g iv i n g t h e i d e n ­ t i c a l resu lt. W e c a n ca l c u l ate

bution ( p + q ) a n d the female co n t ri b ut i o n ( p + q ) X ( p + q) = ( p + q) 2 = p 2 + 2 pq + q 2 = 1 This

re l a t i o n s h i p ,

called

the

H a r dy-We i n berg Pri nciple, shows t h a t in a l a rge, s t a b l e p o p u l a ­ tio n , freq u e n c i e s of d i ffere n t g e n otypes a re predicta b l e a n d co n s e rva tive fro m o n e g e n era­ tio n to a n other.

65

T H E M E C H A N I S M O F I N H E R I TA N C E revea l e d by the

stu d ies of M e n d e l and l a t e r g e n eticists s h owed t h a t e a c h i n h erited c h a ra cte r, s u c h a s s e e d s h a p e o r col o r i n peas, is contro l l e d by a pa i r of g e n e s . O n e m e m b e r o f e a c h p a i r ( a n a l l e l e ) i s contrib uted b y e a c h p a re n t . The g e n e s a re l ()cated o n vis i b l e rod l i k e c h ro m os o m e s , w h o s e b e havior s h ows a c l o s e pa ra l l e l t o th a t p re­ d icted fo r g e n es (p. 6 0 ) . We h a ve a l rea d y s e e n how these structu res d ivid e i n e g g and sperm fo r m a ti o n a n d com b i n e i n fe rtil izati o n . B u t w h a t d ete r m i n es t h e sex of the n ew i n d ivid u a l ? SEX of the offs p r i n g is deter m i n e d by s p e c i a l ized pa i rs of sex c h romosomes. The fe m a l e parent h a s one p a i r o f s i m i l a r chromosomes (X c h ro m osomes, p. 56), and t h e m a l e parent has one pa i r of d i ssi m i la r c h ro mo­ somes (X a n d Y). T h e d i v i s i o n of the c h ro moso mes w h e n e g g s a n d s p e r m s a re produced g ives the eggs all X c h ro m osomes a n d

THE

t h e sperm h a l f X a n d h a l f Y c h ro m o s o m e s . T h e ra n do m p a i r­ i n g i n ferti l i z a t i o n res u l ts i n either X X (fe m a le) o r X Y ( m a le). O t h e r g e n e s , i n c l u d i n g those t h a t produce h e m o p h i l i a a n d some k i n ds o f color b l i n d n e s s , a re l i n k ed to sex c h ro m osomes. These sex- l i n ked c h a racters c a n be tra n s m i tted to t h e offs p r i n g of a p parently " n o r m a l " parents.

S e x of a n i m a l s i s d e term i n ed by t h e sperm . c h r o m o s o m e t h e offspri n g w i l l be m a l e . ,

66

I f i t co n ta i n s a Y

G ENES c o n t ro l i n h e rita nce, b u t w h a t c o n t ro l s g e n es?

How do they t ra n s m it their c o m plex g e n et i c i nfo r m ation from one g e n e ra t i o n to a n o t h e r ? C h e m ica l a n a lys is of c h ro m o so m e s h a s s h ow n t h a t they co n s i st of fou r b a s i c co m pou n d s : two p rote i n s , one with a re l a tively low m o l ec u l a r we i g h t a n d t h e other m u c h h i g h e r, p l u s two n uc l e i c a c i d s, d eoxyribo­ n ucleic a c i d ( D N A ) and r i b o n u c l e i c a c i d ( R NA ) . The co m b i n a t i on of t h e s e fo u r m o l e c u l es fo r m s chromatin , the " s tuff" of w h i c h c h ro m osom es a re m a d e . DNA ca rries and c o n t ro l s g e n etic i n fo r m atio n . DNA was f i rst s u s p ected t o be t h e " b a s i c co m po n e n t" of g e n e s w h e n i t w a s d i scovered t h a t t h e n u c l e i of b o t h re p ro d u c t i v e a n d body c e l l s of a n y p a rt i c u l a r s p e ­ c i e s c o n ta i n e d i d e n t i c a l a m o u n ts of D N A for ea c h c h ro m o so m e set . O n e stra i n o f t h e bacte­ r i u m P n e u m ococc us cou l d be tra n sfo r m e d i n to a n o t h e r by i n ­ fec t i n g i t w i t h a p u r i f i e d e x t ra c t

from d e a d cel l s of t h e s e c o n d stra i n . T h e " t ra n sf o r m e d " stra i n b r e d t r u e , a n d t h e ex t r a c t was l a t e r i d e n t i f i e d a s DNA. T h i s n o n l i v i n g D NA f r o m a s e c o n d stra i n t h e re f o r e h a d t h e a b i l ity to t r a n s f o r m the h e re d ita ry m e c h ­ a n i s m of b a cte r i a c e l l s . S i m i l a r tra n sfor m a t i o n s i n d u c e d i n ot h e r bacte r i a s h o w t h a t g e n e s a r e co m posed o f D N A.

T h e ge netic tra nsfo rmation of Pneumococcus Typ e I I cel l s i nto Type I l l c e l ls by the addition of an extract from d e a d Type I l l cel ls. EXPER I MENTAL C U LT U R E

nutrie nt, p l u s Type I I cells

sterile nutrient

type I l l cells a d d e d

type I I c e l l s tra n sfo r m to t y p e I l l

t y p e I l l cells a d d e d

no colonies

CONTROL C U LT U R E

67

( de ­ o xyri bonucleic a c i d ) , w h i c h controls hered ity b y reg u­ l a ti n g i n structi o n s of g rowth and form from cell to cell a n d fro m parent to offs p ri n g . DNA i s present in all l iv­ i n g creatu res. In structure, the DNA molecule is a double helix, re­ sem b l i n g a l a d d e r that has been repeatedly twiste d . Ea c h r u n g of t h i s m olecu l a r l a d d e r is m a d e u p o f a pai r o r two fro m fou r c h e m i c a l bases-a d e n i n e , thy­ m i n e, g ua n i n e, a n d cystosi n e . The size a n d structure of these bases is such that each " ru n g p a i r " always con ­ sists o f either a d e n i n e a n d thym i n e o r o f g u a n i n e a n d cystosi n e . It i s the seq u e n ce o f t h e pairs i n the ru n g s that provi d es t h e code b y w h i c h g rowth i n stru ctio n s a re tra n s m itted . From these fou r basic code substa n ces, a n a l m ost i n fi n ite va riety of seq uences can d evelop.

T H E CH EMICAL KEY TO INHERITAN CE IS D NA

T h e portion of a DNA molecular m o d e l h e re s h ows t h a t each ru n g of t h e h e l i x ladder i s b u i l t of a pa i r of c h e m ical bases.

adenine

cystosine

guanine

Model of

DNA Molecu le

thymine

T h e s p l itt i n g of a DNA ladder a n d t h e reco m b i n ation of t h e un­ z i pped r u n g s with a p p ropriately fitti n g n ew base u n its, i d e ntical to t h e i r origi n a l partners o n the run g , p rod uces two n ew m ol e­ c u les. H a l f of each new m olecule is derived from t h e orig i n a l pare n t molecule. "

"

DNA con t ro ls g rowth by h a v i n g the u n i q u e a bi l it y

to u n z i p d own the m i d d l e of t h e m o l e c u l a r l a d d e r, thus freei n g the p a i red e n d s of each c h e m i c a l run g . The exposed com pou n d s then l i n k u p with si m i l a r n ew u n its that a re d e rived from the o r g a n is m ' s food sup­ ply. As t h e s p l it of t h e m o l ec u l a r l a d d e r conti n u es, each half b u i l d s an exact rep l i ca of itself by this l i n k­ a g e . The rig i d seq u e n ce of pairi n g i s preserved by t h e fit o f the o ri g i n a l structure. I n t hi s way the DNA mole­ cu le c a n repro d u c e itself over and over a g ai n . 69

PROT E I N MAN UFACTURE . RNA ( ri b o n u c l eic a ci d ) h a s

a structu re m u c h l i ke that of D NA . U n l i ke D NA, h ow­ ever, it is not c o n fi n e d to the c e l l n uc l e u s . It is t h e es­ se ntial m esse n g e r in the m a n ufacture of p rote i n s by cells. Prote i n s a re i nvolved i n every body p rocess . They fo rm the basic m ate rial of l iv i n g o rg a n i s m s . They a re very l a rg e m o l e c u l es, m a d e u p of various co m b i n atio n s of 20 essential k i n d s o f a m i no a c i d s . R N A i s m a n ufactured fro m D NA, fro m w h i c h i t dif­ fers i n h avi n g o n e m o re oxyg en atom i n its sug a r ( ri bose) a n d i n havi n g u racil i n stead o f thy m i n e i n t h e m o l e c u l a r stru ctu re. S l i g htly d ifferen t " m esse n g e r " R N A m o l e c u l e s c o n t r o l t h e m a n ufa ctu re of e a c h of the m a ny k i n d s of protei n . Each is i m pri nted with a c o d e d tem ­ pl ate o f its structure. Selected a m i n o a c i d s a re t h e n tagged by sti l l o t h e r d i stin ctive " tr a n sfe r " RNA m o l e ­ c u l e s , e a c h i m pri n te d w i t h a structure t h a t e n a b l es i t t o b r i n g t h e p a rti c u l a r a m i n o ac id t o t h e m esse n g e r te m p l ate . W h e n these a m i n o a c i d s a re a rra n g ed a n d c o m b i n e d i n correct seq u e n ce, R N A m a y s e p a rate fro m newly p rod uced p rote i n a n d perfo rm its ta s k a g a i n . T h e m e s se n g e r RNA bar carries t h e cod e , w h i l e each of t h ree s m a l l e r co m po n e n t s of t h e RNA u n i t is s t r u c t u red to a c c e p t a pa r­ t i c u l a r a m i n o a c i d s h a p e . I n t h e bac k g ro u n d is a DNA h e l i x .

Tra n s f e r R N A u n its locate ( Le f t ) a n d tra n s port t h e i r ( Ri g h t ) vari­ ous a m i n o acids toward t h e coded RNA te m p l a t e bar.

Tra nsfer RNA u n its u n ite with a messe n g e r u n it op posite t h e i r pa i red p a r t n e r c o m p o u n d s , s o arra n ged as to b u i l d a n a m i n o a c i d s e q u e n c e i n to a part i c u l a r p rote i n m o l e c u l e .

T h e new prote i n m o l e c u l e cons tructed f r o m a seq u e n ce o f a m i n o a c i d s , separates f r o m t h e R N A . M o s t p rote i n s consist o f h u n d reds of a m i n o a c i d u n its that are a rra n g e d tog ether.

71

WHAT ARE THE S O U RCES OF VARIABILITY? If g e netic

e q u i l i b ri u m i n any p o p u l ation is so conservative, h ow does c h a n ge eve r take p l a c e ? How c o u l d evo l utio n i n p l a n ts a n d a n i m a l s occur? W hat a re the so u rces of variati o n s in p l a n ts a n d a n i m a l s ? No two i n d ivid u a l s a re eve r exactly a l i k e . T h e s e i n d ivid u a l d i fferen ces a re a s recog n i z a b l e i n d o g s o r i n g o l d fi s h a s t h ey a re i n m a n . S o m e d i ffere n ces a re acq u i re d d u r i n g life th roug h b e h avio r o r fro m t h e e n ­ vi ro n m e n t . D i eti n g , o r l a c k of i t , i nfl u e n ces wei g ht . R e s ­ p i rato ry d iseases m ay be pro d u ced by s m o ki n g o r b y l ivi n g i n a p o l l uted a t m o s p h e r e . But m a ny i n d ivi d u a l c h a racteristics a re h ered ita ry, n o t enviro n m en ta l . These i n c l u d e s u c h phys i c a l features a s eye c o l o r, co m p l ex­ i o n , color b l i n d n ess, a n d h e i g h t . How do t h ese d if­ feren ces a rise? Variations of this kind a re pro d u ced in two ways : by a s h uffl i n g a n d red i stri buti o n of g e n e s d u ri n g re­ ( reco m b i n atio n } a n d by sponta n eo u s pro d u ction c h a n ges i n g e n e structure l e a d i n g t o n ew c h a racteris­ tics ( m utatio n } . Crowd sce n e i l l u s trates vari a b i l i t y of o u r ow n speci es.

RECOMB I NAT I O N i s the ch ief so u rce of v a r i a tion . I t

m ay occu r e i t h e r b y a s i m p le reco m b i n at i o n o f the c h romosomes w h e n ferti l i zati o n between p a rents ta kes place o r by a c ross i n g -over that i n volves an exc h a n g e o f g e n etic m ateri a l between c h ro m osomes . Reco m bi n a ­ tion i n volves t h e fo r m a t i o n o f n e w g e n otypes by the res h u ffl i n g of e x i s t i n g o n es ( p. 6 4 ) . I n o rg a n i s m s t h a t reprod uce a s e x u a l ly, m uc h less pote n t i a l va riati o n is poss i b l e t h a n in those that reprod uce sexu a l ly. Th is has put an evo l u ti o n a ry " p re m i u m " on sex u a l rep ro­ d uctio n . Sex u a l re p ro d uct i o n i s the m a j o r so u rce of the wide va r i a t i o n in p l a n t a n d a n i m a l s pecies . of chromo­ somes s o m e t i m e s takes p l a c e duri n g m e iosis, t h e formation of sex c e l l s . This results from c h ro moso m e stra n d s stic k i n g to­ g e t h e r so that an e x c h a n g e of genes ta kes p l a ce betwee n c h ro m oso m e pairs. Where these c h ro m osomes a re joined and the CRO S S I N G-OVER

exchanges

occur

a re

called

chiasmata. Ea c h cross-ove r dou­

b l e s t h e n u m ber of k i n d s of g a m etes and modifies the l i n k­ a g e of g e n es , t h u s prov i d i n g a n i m porta n t sou rce o f varia b i l i ty. I n the i l l u stration of cross i n g ­ over below, d iffere n t colors rep­ resent d ifferent g e n e s .

CROSSING-OVER

73

Horned Hereford

Polled H e reford

Po l led H ereford cattle l a c k horns beca u s e of t h e effects of m u ta t i o n of a g e n e w h i c h reg u l a tes horn s ecretio n .

MUTATIONS a re c h a n g e s i n g e n e t i c materi a l that

pro d u ce n ew c h a ra cte risti cs. The c h a n g e m ay b e o bvi­ ous (as exa m pl es, wi n g for m in fl i es, horn l ess pol l e d He refo rd cattle, s h ort- l eg g e d A n co n s h eep, o r double flowers), o r it m ay be a n i n co n spicuous and subtle c h e m i c a l a n d p hysiological d iffe re n ce . Particu l a r g e n es m ay m utate i n m o re t han o n e way, a n d m a ny can re­ ve rse the d i rection of c h a n g e .. Each g e n e has a d is­ ti n ctive m utation rate . Some wi l l m utate o n ly once per m i l l ion g a m etes, for exa m ple, while others u n dergo m utati o n s m o re than 500 ti m es as freq uently. The rate of m utation ca n be i nfl u e n ce d by va rious factors, such as te m p erature c h a n g es, ra d iation, a n d c h e m ical sti m ­ u l a n ts . C o n s p i c uous m utation s a re m o re ofte n h a rmfu l to t h e org a n i s m t h a n a re t h e s m a l l e r o n es . Mutation see m s t o be caused b y s l i g ht i m perfection s i n the self-copyi n g c h e m ical structu res o f the D NA molecules that m a ke u p the g e n e s . They m ay affect the n u m ber and structures of the DNA molecules of the genes. They m a y a lso affect the n u m be r a n d structure of c h ro m osomes. Occasion a l l y the c h ro m oso m es d ivide but the cell does n ot, resu lti n g i n a whole new set of c h ro m osomes ( polyploidy). The n ew m uta n ts b reed true and do not revert to the orig i n a l typ e . 74

DRIFT was discove red by t h e Ame rica n gen eticist Sewa l l Wrig ht, who studied the math e m atics of population g e n etics. The g e n e pool-that is, the tota l gene contrib ution of a particul a r popul atio,n to its offspri n g-is g reatly i n fl u e n ced by the size of t h e pop­ u l atio n . In very s m a l l populations, such a s those iso­ lated from their parent g ro u p, cha nce pl ays relatively a m uch g reater rol e i n p rod ucing g e n etic c h a nge, som eti m e s lea d i n g to n o n - a d a ptive c h a n g e s . Although the real ity of g e n etic d rift has been confirmed i n lab­ oratory experi m en ts, its role i n evol ution i s sti l l not clear. It m ay b e of some i m porta nce i n small popula­ tions that l ater i n c rease i n size and m ay acco u n t for some of the puzzl ing, pers iste nt n o n - a d a ptive o r neu­ tral c h a n g es observed i n d iversified wild populatio n s . Va ri a b i l ity t h u s a rises fro m fo u r possi b l e s o u rces­ reco m b i n ation, crossi n g -over, m utation, a n d g e n eti c d rift. Of these, reco m b i n ation is by fa r t h e most im­ porta n t . Once it was thought that variabil ity a l o n e cou l d p ro d u ce evoluti o n a ry c h a n g e, without a i d from a d d itio n a l factors . Statistical a n a lysis showed , how­ ever, that o n l y natural sel ection cou l d a ccount for t h e perpetuation a n d refi n e m ent o f t h e e n d less a d a pta­ tions shown by livi n g t h i n g s . Varia b i lity is not the whole of t h e evol utionary reci pe, but i t is t h e essenti a l raw m aterial o n which everything else d epen d s . GENETI C

75

I S O LATION of g e n e pool s d ist i n g u i s h e s s pecies from

races a n d d e m es, w h i c h a re p o p u l ations ca p a b l e of i n terb re e d i n g when t h ey co m e i n to conta ct . The de­ ve l o p m e n t of isolation between o n ce i n te rb reed i n g g ro u p s i s a n i m po rta n t fa cto r i n evo l utio n . O n c e i so­ l ated, e a c h p o p u l ation wi l l u n d ergo i n d e p e n d e n t, g r a d u a l g e n etic c h a n g e u n t i l it i s n o l o n g e r co m p ati b l e w i t h t h e g ro u p with w h i c h it o n ce i n terbre d . S i n ce c l i ­ m a tic a n d e c o l o g i c c o n d itio ns u n d e rg o s l ow c h a n g e s , th e re is a d yn a m ic i n tera.ction betwe e n th e m a n d s p e ­ c i e s d istri b u tio n . Ma ny now- d ivi d e d ra n g e s of s p e c ies reflect i so l atio n of orig i n a l ly w i d e s p re a d p o p u l a t io n s . I s o l a t i o n m a y a r i s e i n d i ffe r e n t w a y s . I t m a y b e g e o ­

1 9) .

g r a p h ic, a s i n s o m e i s l a n d p o p u l a ti o n s (p.

R e l a ted

species t h a t d o not ove r l a p i n territory a re c a l l e d a l ­ lopatric;

those that do,

sy m p a t r i c .

G e n et i c

isolation

m a y a r i s e b etwe e n e i t h e r k i n d of p o p u l a t i o n . I t m a y i n vo l v e e c o l o g i c a l , b e h a v i o r a l , m o r p h o l o g i c a l , o r p h y s ­ i o l o g i c a l d i ffe r e n c e s , a n y of w h i c h m a y p r eve n t m a t i n g . Eve n i f m a t i n g d o e s t a k e p l a c e , v a r i o u s i n t e r n a l c e l l u ­ l a r o r d ev e l o p m e n ta l b a r r i e r s m a y p r e ve n t f e r t i l i z a t i o n o r p r o d u ce n o n vi a b l e , w e a k o r s t e r i l e h y b r i d s . is I S OLAT I O N GEOGRAPH I C s h ow n b e l ow i n s c h e m a t i c for m : (A) w i d e s p r e a d s pe c i e s w i t h n o g eog ra p h i c v a r i a t i o n , (B) d i f fer­ ent popu l a t i o n s d ev e l o p in ex­ t re m e s o f ra n g e s, ( C ) p a rt i a l barrier develops, g eog ra p h i c

(D) b a r r i e r bec o m e s a tota l pre­ ve n tative to i n t e r bree d i n g of two p o p u l a t ion s ( E) cu m u lative g e n et i c d i ffere n c e s b e co m e so g reat that g e n et i c s e pa r a t i o n prod uces two species, eve n i f t h e b a r r i e r i s e l i m i n a ted. ,

M I GRAT I ON of po p u l a tions i n to new a re a s may l ea d

to perm a n e n t co l o n i zatio n . I n so m e cases, d i fferent a reas may h ave very closely s i m i l a r c l i m a tes a n d ter­ ra i n s b u t b e s e p a rated by such b a rri ers a s oceans, deserts, o r m o u n ta i n s . Over long periods, a few i n ­ d ivid u a l s m a y c ross these ba rriers a n d beco m e esta b­ l ished i n th e i r new e nv i ro n m e nt. The n ew c o l o n y will i nterbreed in isolation fro m t h e parent population a n d m a y i n t i m e d evelop i nto a n e w s p e ci e s. S o m e of t h e g reatest tra n sfo rmations i n the h istory of l ife h ave a ri s e n by m i g ratio n . These i n c l u d e t he col­ o n i zation of t h e l a n d by p l a nts and a n i m a l s . M I GRA­ I N TERC O N T I N E NTAL T I O N of some Plei s tocen e m a m­

m a l s is s h ow n h ere. Asia a n d North A m erica were periodi ­ c a l l y joined b y t h e Beri n g

Straits. T h e " fi l ter" effect of these i s l a n d co n n ecti o n s co n ­ fi n ed s o m e m a m m a l s ( ci rcled ) to t hei r ori g i n a l co n ti n e n ts . ( After Kay a n d Co l bert. )

N AT U R A L S E L E C T I O N is the seco n d m a j or co m po n e n t

o f t h e evoluti o n a ry p rocess. Left t o t h e m selves, l a rg e a n d ra n d o m l y i n t e r b reed i n g p o p u l atio n s w i l l m ove to­ wa rd g e n etic e q u i l i b r i u m (p. 6 5 ) . N a t u ra l s e l ectio n is t h e n e t re s u l t of a l t t h e p hysica l a n d b io l o g i c a l e n v i ro n ­ m e n ta l factors t h a t t e n d t o d i sturb t h i s e q u i l i b r i u m a n d c h a n g e a p o p u l a tio n ' s g e n e poo l . N a t u r a l s e l ecti o n d o e s not i t s e l f c re a te n ew va riatio n s ; it sel ects , w i n n ows, a n d p r e s e rves e x i s ti n g va riati o n s . The g e n et ic co m po n e nts of a p o p u l ation dete r m i n e what i t m a y b e co m e ; s e l e c t i o n d ete r m i n e s w h a t it w i l l becom e . I n he rita n c e p rovi d e s i t with its potent i a l ; sel ection tra n sfo r m s pote ntia l to a ctu a l ity. THE ACTI O N of n atural se l e c­

tion depends on t h e tende ncy of a l l species to p rod uce more offs pri n g than can norma l ly sur­ vive (p. 24) in a n environ ment of l i m ited capacity. Better ada pted org a n i s m s w i l l l ive longer tha n the l ess wel l ada pt­ ed, hence prod uce a g reater n u m ber of descenda nts. The descendant popu lation will i n­ c l u d e a n i n c reas i n g ly large pro­ portion of i n d ividua l s that have features favora b l e inherited from their parents. Natura l selection is rarely a "stru g g l e for existe n c e " be-

tween two c o m pet i n g i n divid­ uals. Predation and direct com ­ petition are on l y two of

many factors i nvolved, includ­ i n g such other t h i n g s as mo­ bil ity, physiological and struc­ tura l efficiency, resista nce to d isease, a n d sexual vigor. Natura l selection produces d ifferential rep roductio n . It is the "su rviva l of the fittest" o n ly in th e sense that the fittest produce more offspring. In ef­ fect, it not o n ly e l i m i nates the less we l l a d a pted characteris­ tics but also p rod uces positive res u lts.

Stro n g b u l l-m a n y offs pri n g

W e a k b u l l-no offspri n g

SELECTIO N

PRESSURE varies from season to season and from place to p lace. It is influenced by such fl uctuating factors as cli mate, population size, food supply, a n d m i g ration. Selection genera l ly does not lead to long­ term stability because so many of these and other factors un­ dergo more or less continuous change a n d are re i n forced by such major changes as colon iza­ tion of a new environ ment, iso­ IQtion, or change in the existi ng geogra phy of a n a rea. Natural selection does not operate as a stea m-roUer effect on one c h a racter, but as a subtle series of i n teracti ng " com promises" t h a t affect t h e

whole organism a n d may influ­ ence many differe n t c h a racteris­ tics. Selection is a c u m u lative process. Even over a s h ort pe­ riod, its effects can be s ubsta n ­ tial ( p. 8 4 ) . Over l o n g periods its effects can be enormous. The increasing ada ptation of a population to its environ ment is the res u l t of natural selection. Each n iche is genera l ly fi l led by on ly a single species, but a n u m ber of s pecies may share the sa me area by their va rious adaptations to speciflc food suppl ies or to habitats. The bet­ ter adapted leave more off­ spring, g radually cha nging the character of the desce ndant popu lation. Con trasti n g effects of m utation ( B ) a n d n a tura l selection ( C) are s h own on a " norm a l " dis­ tribution curve ( A ) of a char­ acter in a large populatio n . ( After Hardi n . )

Carn ivore

79

PRO OF OF NATURAL SELECTION ca me long after

D a rwin presented t h e theory i n The Origin of Spe­ cies. D a rwin c o m p a red it with a rtificial d o m estic selectio n and d e m o n strated its p roba b l e causes a n d a p p a re n t effects, but the p rocess itself h a d not b e e n d e m o n strated . M a n y l a b o ratory stud ies h ave s i n c e b e e n c o n d ucted t h a t i l l u strate both the operatio n a n d the effects o f natural selection . I n these, the biolog ist atte m pts to d u p l icate and to isolate ce rta i n natural e n ­ viro n m e nts a nd t o a n a lyze their i nte racti n g p rocesse s . M I C E a n c:l o the r wi l d a n i· m a l s , color c h a n g es a re ofte n closely associated w i t h t h e pre­ d o m i n a n t color of the b a c k ­ g rou n d so i l a n d vegetat i on w h e re t h e a n i m a l s l ive. I n Florida, for e x a m p l e , t h e deer mouse (Peromyscus m a n iculatusJ s h ows a co n t i n uous color tra n si­ tion from very light o n t h e cora l s a n d of isla n d reefs to dark i n i n l a n d a re a s of d a rker s o i l . I s t h i s t h e res u l t of n a tura l s e lectio n ? Research ers worked w i t h two color va rieties-g ray a n d b uff. Two c on t ra s t i n g b a c k g rou n d s IN

were set u p i n a large d a r k­ e n e d cage. T h e s o i l i n t h e cages m o re o r l ess matched t h e color of e a c h of the s h a d e s of m ic e . E i g h t m i ce, four of e a c h color, were e x posed to a barn owl for 1 5 m i n u tes. T h e b a c k g rou n d s were a l ternated with ea c h e x ­ experi­ p e r i m e nt. After 8 8 ments," it w a s fou n d that t h e o w l h a d t aken 1 07 of t h e " n o n ­ m a tc h i n g " m ic e a n d on ly 62 o f t h e bette r c o n cea led ones. T h i s s h ows how powerful a se lective factor p rotective backg rou n d coloration ca n b e , eve n over a s hort period.

6 2 M a tc h i n g

1 07 N o n - m a tc h i n g

1 1 20

1 100 I I 80 I I 60 I I 40 I I 20 I

A

c

D

G

I

0

V i a b i l ity of seven stra i n s (A-G) of h o m ozygous Drosoph ila fruit fl ies with d e leterious g e nes, before and a fte r t h e e x peri m e n t described below. Th e via b i l ity is shown a s % o f n o r m a l viabil ity ( 1 00 % ). B l a c k col u m n s i n d i cate t h e v i a b i l ity before t h e expe r i m e n t; the red col u m n s , v i a b i l ity of u n treated stocks after 50 g e n e ra­ tions; t h e yellow col u m n s , of stocks i r ra d iated with X- rays after 50 g e nerati o n s . (After Dobz h a n s ky and Spass ky.) POORLY

ADAPTE D

A N IMALS

are rarely fou n d in nature, b u t t h e effect of natura l selection upon such poorly a d a pted form s ca n be observed u n d e r experi m e n t a l c o n d i t i o n s . Dobz­ h a n s ky and S p a s s ky ra i s e d sa m ­ p l e s o f t h e fruit fl y D ro s ophila that were h o m ozyg o u s for d i ffere n t c h ro m oso m e seven co m b i nations (A-G). Ea c h was as soc iated with some a b nor­ m a l ity s u c h a s reta rded deve l ­ op me n t or defor m e d w i n g s , l e g s , a n d a b d o m e n s-t h a t re­ s u lted in a low via b i l ity. Ea c h of t h ese stocks was ra ised for fi fty g e n e ra t i o n s a n d kept i n c rowd ed c u l t u re bott l e s t o i n crease s e l ection pressure.

The populations were s a m p l e d a t reg u l a r i n te rv a l s to s t u d y t h e c h ro m os o m e s of repre s e n tative i n d iv i d u a l s . In most cases, th ere wa s a ra pid a n d m a rked de­ c l i n e in the a b u n da n ce of t h e deleterious co m b i n a t i o n s . The experi m e n t i n c l u d e d e x pos u re of o n e s a m p l e of each sto c k to X - ra y ra d i a ti o n , b u t th ese pro­ duced n o s ig n ifi c a n t d iffere n ces. O f t h e 14 stocks, 1 1 s h owed m a rked i n crea s e in v i a b i l i ty, 2 decl i n e d i n v i a b i l ity, a n d 1 re­ m a i n e d u n c h a n g e d . Of the con­ trol g roups, u n m i xed w i t h the stro n g e r n a t u ra l pop u l a t i o n s , 8 s h owed a m a r k e d deterioratio n , 6 we re u n ch a n g e d or s h owe d only s l i g h t i m p rove m e nts.

81

NATURAL POPU LATI ONS a re a l so stu d ie d to dete r­

m i n e the effects of n atura l s e l ectio n . One exa m p l e is how n atura l s e l e ct i on m i g ht h ave p ro d u ce d the .d iffer­ e n ces that exist b etween the va rious k i n d s of D a rwi n ' s fr n c h e s o n t h e Ga l a p a g o s I s l a n d s ( p . 1 8 ) . Ty p i c a l fr n c h es , s u c h a s s p a rro w s a n d c a rd i n a l s , h a ve s t ro n g c o n ica l be a k s t h a t a re u s e d t o c r u s h se e d s . B ut t h e G a ­ l a p a g os fi n c h es s h o w a g reat va riety of b e a k s . S o m e a re sto u t a n d co n i c a l , o t h e rs s l e n d e r, a n d sti l l ot h e rs a d a pted fo r q u ite d iffe re n t d iets . T h i s d ivers i ty wa s probably p rod uced by the presence of m a ny vaca n t ecolog ical n i c h e s w h e n t h e fi n c h e s fi rst beca m e esta b l is h e d on t h e Gala pagos.

I n c reas i n g n u mbers of b i rds feed i n g o n a l i m ited seed s u p ­ p l y provi d e d t h e sel ective pres­ s u re tha·t favored those that coul d e x i st o n new food .

DARW I N 'S FI NCH ES ( GA L A PAGOS I S LANDS )

82

The g rou n d - l ov i n g fi n ches re­ ta ined t h e i r seed-eating h a bits, a lthough i n d iv i d u a l s pecies show m a rked variation i n beak proportions t h a t see m related to parti c u l a r types of seed prefere nces . Other fi n ches turned to k i n ds of food a n d hab its t h a t a re c h a racteristic o f species o f oth e r fa m i l ies o n t h e conti n e n ts . S o m e beca m e tree-dwe l l ers, i n c l u d i n g types that a re w a r b le r l i k e i n se ctivores and oth ers t h a t a re pa rrot l i ke veg eta r i a n s . Sti l l oth e rs beca me special ized for feed i n g on cacti. An insectivore developed a strong bea k t h a t it. uses to bore i nto bark. Lacking t h e long ton g u e of true wood peckers ( p . 42), it uses a c a c t u s s p i n e t o extract the i n sects fro m t h e b u r­ rows. All fi n c hes are rat h e r s i m i la r i n t h e i r g e n e ra l s i z e a n d h ave d u l l p l u m a g e , probably because of t h e d a r k volca n i c roc ks that outcrop over m u c h of their ter­ ritory. The Ga l a pagos fi n c hes prob­ ably a rose fro m an a n ce stra l South A m erican f or m , b u t they n ow d iffer so g reatly from a l l e x i s t i n g m a i n l a n d fi n c h e s that their a n cestry c a n not be recog­ n i zed. This m a r ked d i ffe re n ce s u g g ests the that fi n ches rea c h e d t h e i s l a n d s earl i e r than oth e r s p e c ies, which are m u c h c l o s e r t o t h e m a i n la n d for m s . T h e a n cestra l fi n c h e s pres u m ­ a b l y rea c h e d t h e i s l a n d s b y be­ ing " a c c i d e n ta l l y " carried t h e re , perh a ps a i d e d by ocea n i c cur­ re n ts . It s ee m s most u n l i ke l y t h a t t h e Ga l a p a g os I s l a n d s w e re ever j o i n e d to t h e m a i n la � d .

I n contrast t o t h e diversity of t h e Ga l a pagos fi n c h es , Davi d Lack has s h own t h a t i n the n e i g h bori n g Cocos I s l a n d t h ere is only a s i n g l e species of fi n c h , a l t h o u g h t h e i s l a n d prov i d e s v a r i e d h a b itats a n d l a c k s m a ny o t h e r typi ca l s pecies o f co n ­ t i n e ntal b i rd s . Th i s see m s t o re­ s u l t fro m the s i n g l e i s l a n d of Cocos l a c k i n g the n u m bers of s m a l l , isolated e n v i ro n m e n ts t h a t a re provi ded by t h e a rc h i pela g o n a t u re of t h e Galapagos I s ­ l a n d s . ( See m a p p. 1 9 )

Tool-u s i n g Finch

( Amarhnyncluis pallid us)

N AT U R A L S E L E C T I O N I N A C T I O N is s h ow n b y t h e

p e p p e red m ot h ( p . 47), w h i c h i s a n exa m p l e of a s p e c i e s t h a t h a s s h own a stri k i n g c h a n g e i n t h e fre­ q u e n cy of a d a r k ( m e l a n i c ) fo rm in the last c e n t u ry . T h i s m o t h wa s we l l k n own to t h e m a n y a m a t e u r e n to m o lo g i sts i n B r i ta i n i n t h e ea rly yea rs of t h e 1 9t h centu ry . U n t i l 1 8 4 5 , it was k n own o n l y i n t h e " p e p ­ p e r e d " fo r m , w h i c h h a s d a r k m a rk i n g s o n a l i g h t w i n g b a c k g ro u n d . I n t h a t yea r, a d a r k fo r m w a s d i s covered i n t h e i n d u stria l c ity of Ma n c heste r. At that ti m e t h e d a rk fo r m m a d e u p l e ss t h a n 1 p e r c e n t o f t h e tota l p o p u l a ti o n . With i n fifty yea rs i t m a d e u p 9 9 p e r c e n t of t h e p o p u l a tion i n t h e Ma n c h e ste r a re a . T h e b l a c k m o t h i s n ow t h e p re d o m i n a n t fo rm ove r m u c h o f E n g l a n d , a n d t h e o ri g i n a l p e p p e red fo r m i s a b u n d a n t o n l y i n n o n - i n d ustri a l a re a s w h e re p o l l u t i o n h a s n ot b l a c k e n ed t h e trees o n w h i c h t h e m o t h l ive s . I n s o m e of t h e n o n ­ i n d u s t r i a l ea ste r n a re a s o f t h e cou n t ry w h e re heavy s m o g is ca r r ied i n , t h e b l a c k fo r m a l s o d o m i n a tes t h e ni oth p o p u l a ti o n . Rece n t s t u d i e s i n M a n c h ester and other i n d u s t ri a l cities w h e re stri n g e n t a n t i - p o l l ution o rd i ­ n a nces h ave b e e n e n force d , h ave s h ow n a revers a l of t h e tre n d tow a r d s d a rker for m s , a n d a s l o w but m a rked i n ­ crease i n a b u n d a n ce o f t h e l i g h ter peppered form s . P E P P E R ED MOTH D I ST R I BU T I O N I N B R I TA I N • ( red ) M a j o r i n d u s tr i a l cities o ( w h i te ) l i g h t form pred o m i ­ nant • ( b l a c k ) d a r k form pred o m i ­ nant • ( g ray ) i n te r m e d i a t e popu­ lation

84

I N DUSTRIAL

MELAN I SM

(the

p red o m i n a n ce of d a r k varieties) i s a l so s hown by a bout 70 oth e r species of m ot h s in Europe. In the U n i ted Sta tes, the Pittsburg h reg io n s h ows a com­ para b l e predo m i n a n ce of once rare black forms in a l m ost 1 00 species of m oths. T h e c a u s e of i n d u strial m e l ­ a n i s m l ies i n t h e i n teraction o f a d o m i n a n t g e n e , p ro d u c i n g t h e b l a c k m u tatio n , a n d n a t u r a l s e ­ lection. H . B. D . Kettlewe l l h a s d e m o n strated t h e i m porta n c e of natura l selection by studyi n g rates of b i rd predation o n t h e two form s of t h e p e p pered moth . H e released k n own n u m ­ bers of m a rked moths of e a c h f o r m i n t w o a re a s a n d a n ­ a lyzed t h e n u m b e r of e a c h f o r m that h e recaptu red by a t ­ tra c t i n g t h e m to a l i g h t a t

n i g ht. I n t h e i n d ustria l Birm i n g ­ h a m a rea, w h e re t h e l oca l pop­ u lation has 90 percent black forms, h e released 477 bla ck a n d 1 37 l i ght i n d iv i d u a l s . He reca ptured 40 p e rcent of t h e b l a c k for m s b u t o n ly 1 9 percent of the p e p pered. In the u n poll uted coa sta l a rea of Dorset, from a l most 1 ,000 of t h e two colors of m o t h s re­ leased, 6 perce n t of the b l a c k a n d 1 2 . 5 perce n t of t h e l i g h t form s w e re reca pture d . So i n p o l l uted a r e a s , t h e b l a c k - w h i te reca pture ratio w a s 2 : 1 . I n u n ­ pol l u ted a re a s i t w a s exactly reversed-1 : 2 . I n both a reas, caref u l ob­ servation and fi l m i n g of b i rd s eati n g t h e m ot h s from tree tru n ks con fi r m ed that these ratios were t h e res u lts of rela­ tive predation.

85

FOS S I LS a l s o d e m o n st r a te the effects of natura l se­

l ectio n . B . K u rten h a s s hown its effects i n the E u ro p e a n cave bea r (Ursus spelaeus) t h a t i n h a b ited north e r n E u ­ rope d u r i n g t h e P l e istoce n e . Ku rte n coll ected fos s i l s from caves i n t h e Od essa reg i o n o f the U . S . S . R . B y c o m p a r i n g these with s keleto ns of th e c l osely related l ivi ng b e a r (Ursus arctos), h e wa s able to d iv i d e each of the fossil popul atio ns i nto g rowth sta g e s . The fos s i l s fro m a l l l o c a l ities s h owed stro n g s e p a ratio n i nto s i m i l a r, yea rly g rowth stages, prob a b l y because t h e caves i n wh i c h they were collected were i n h a bited o n l y d u ri n g a n n u a l " h i b e r n atio n . " The a n a lysis o f these g rowth stages d e m o nstrates the effects of natural sel ectio n . Y O U N GEST STAGES of t h e cave

bear eq u a l in size to n ewborn cubs of l i vi n g bears . a re rare, proba b l y beca use of the f ra g i l ­ i ty of t h e b o n e s . T h e m o s t a b u n d a n t g rowth s t a g e h a s a l l t h e per m a n e n t teeth d ev e l o p e d , eq uiva l e n t to a b o u t 4 o r 5 m o n t h s i n l ivi n g bears, s u g gest­ ing birth d u r i n g t h e period of w i n ter d o r m a n cy, but a h i g h m o rta l i ty o f c u b s toward t h e

end of this time. The next g rowth s t a g e i s 1 . 4 y e a r s , i n ­ dicati n g t h e n e x t a n n ua l " h i ­ ber n a t io n . " A t 4 yea rs, bears were f u l l y g rown . Life e x pecta ­ tion a t bi r t h w a s a b o u t 3 . 5 ye a r s ; m a x i m u m a g e , 1 8 years. Divi d i n g the tota l n u m be r of i n d ivi d ua ls i n a g iven age g ro u p by the s u m of these a n d a l l o l d e r i n d i v i d u a l s g i ves t h e m o rta l i ty i n d ex f o r t h e g ro u p .

P l ei stoce n e Cave bear

86

1 000

- Ma n Cave bear

�------� o

1 00

1

•

- 1 00

0

� . 1 00 2 0 0 300 4000

Age in % deviation from meon longevity

Survivors h i p c u rves from b i rth for cave bea rs and m a n , s h ow­ ing the s i m i l a rity of t h e c u rves . ( After K u rten . )

24

26

28

30

32

Mola r M ' Paracone I n d e x

Selecti o n pres s u re o n form of paraco n e of seco n d molar teeth of cave bear i s i n d i c a ted by red u c t i o n of v a r i a t i o n a n d de­ ve l o p m e n t of a rela tive l y s m a l l ­ e r paraco n e w i t h i n creas i n g a g e . ( After Kurte n . )

Mola r M' of Cave b e a r

TEETH of fossi l c a ve bea rs a re ve ry se n sitive to n a t u ra l selection, because s u rviva l d e p e n d s o n s uccessful feed­ ing p rio r to h i bernatio n . K u rten stu d ied the form of t h e seco n d molar teeth (M2) . The ratio 1 O O x the l e n g t h of the l a rg est cusp ( pa ra co n e ) to the tota l l e n gt h o f the seco n d m o l a r s h owed a sig n ifi c a n t c h a n g e , b e ­ c o m i n g s m a l l er with i n c reasi n g a g e . The you n g e r a g e g ro ups were f a r m o re va ria b l e th a n th e o l d e r, b u t fo r m s w i t h less wel l - a d a pted teeth we re e l i m i n ated . O l d e r tooth m e a s u re m e n ts g ave c o m p a r a b l e resu lts. S a m ples fro m oth e r caves g ave s i m i l a r b u t not i d e n t i c a l resu l ts, s u g g esti n g d iffe rences i n s e l e ctive p ress u re betwee n d iffere nt l o c a l e nviro n m e ntal n ic h es i n w h i c h the b e a rs l ived .

87

ADAPTATION

i s t h e con t i n u i n g resu l t of n a t u ral s e l ection . Ma n y o rg a n is m s a re very p recisely a d a pted to p a rti cu l a r n i c h e s o r to p a rticu l a r ways of l ife . The b i r d s of Hawaii p rovi d e a c l a s s i c i l l ustration of how a n a ncestral g ro u p beco m es a d a pted t o s p e c i a l n i c h e s i n a n e w e nviro n m e nt. Hawa i i cons ists o f a g ro u p o f iso­ l a te d vol ca n ic isl a n d s situated i n the m id - Pa c ifi c . L i k e m ost ocea n i c is l a n d s, the n u m b e r o f l a n d b i rd s is s m a l l . Th e h i g h d e g re e o f specia l i zation o f s o m e of t h e b i rd s m a kes t h e m v u l n e r a b l e t o e nv i ro n m e n t a l c h a n g e .

Ko uoi okepo

LOBELIA

Hawaii akialoa (Hemignalhus obscurus ohscurus)

o r drepa n i d i d s , a re a fa m i ly of b i rds fou n d only i n Hawa i i . L i ke D a rwi n 's fi n ches i n t h e Ga l a p a g os I s l a n d s (p. 8 3 ) , t h ey h ave a d a pted t h e m ­ se lves to a wide d i v e r s i ty of conditions. The ori g i n a l for m s se e m to h ave f e d o n the i n ­ sects a n d n e ctar of s h ortbe l l e d flowers. T h ey p roba b ly h a d

S I CKLEB I LLS,

88

s h ort, s l e n d e r beaks. O t h e r fo r m s s how d i versity of a d a pta­ t i o n s to d i ffere n t d iets, i n c l u d­ i n g s o m e w i t h re m a r k a b l y l o n g , curved b e a k s f o r feed i n g o n the l o n g , t u b u l a r fl owers of t h e Hawa i i a n l o b e l i a . Some of the various for m s of s i c k l e b i l l s a re i l l ustrated . N i n e of t h e 22 k n own species a re n o w extinct.

Pseu donestor

Psittirostra kona

H e m i g n a thus o b s c uru s procerus

Hemignathus lucidus

P ro b a b l e evo l ut io n of o n e g ro u p of d re p a n i d i d s from a n a n cestor, p o s s i b l y s i m i l a r to t h e H o n eyea t e r ( p . 8 8 ) i s s h own a b ove . D repa n i d i d bea k s s h ow a d o p ti o n to va r i o u s d i ets . H e mign a t h us o b scurus u s e s its e l o n g a ted b e a k c h iefly to p ro b e fo r i n sects in b a r k caviti e s , a l ­ t h o u g h o t h e r s p eci e s u s e t h e i r l o n g " s i c k l e - b i l l s " c h i efly to p r o b e l o b e l i a f l owers for n e cta r. The n ow ext i n ct H . lucidus had a s h o rt l ow e r m a n d i b l e , w h i c h i s r e d uced even fu rth e r in H . wilso n i, which uses it, wood p e c k e r­ fa s h io n , a s a c h i s e l . T h e l o n g tu b u l a r to n g u e s of i n sect­ eati n g s pe c i e s refl ect t h e i r d eve l o p m e n t f ro m n e cta r­ fee d i n g fo r m s . Pseudon estor has a p a r rot l i k e b i l l , w h i l e seed-eati n g s p e c i e s , s u c h a s Psittirostra k o n a , h a v e h e a vy fi n c h - l i ke bea k s . 89

MIMICRY i s rel atively com mon

i n i nsects a n d in so m e fl owers . In i n s ects it see m s to h ave a risen c h i efly fo r defense; in flowers, for po l l i n ation . The i n fl u e n ce of n atura l selection in the deve l o p m e n t of m i m icry is s h own by the d i stribution of m i m i c s pecies o n ly i n a reas where t h e i r models a re co m m o n . Where the m o d e l s a re a b u n d a nt, the m i m ics s h ow g reater vari­ a b i l ity, w h i c h c a n be exp l a i n e d o n ly by assu m i n g th a t t h e l owe r predato r fa m i l i a rity with t h e m o d e l s i n s u c h a reas h a s p ro d uced l ower selection p ressure o n t h e m i m ics. Develop m e nt o f m i m ics d e p e n d s n o t o n " acci­ d e n ta l " p a ra l l e l m utatio ns but on a series of i n teracti ng ge nes that h ave u ndergone strong sel ection .

THE S L I P P E R O R C H I D o f Eu rope

and the M i d d l e East e m its a perf u m e that attracts a wasp wh i c h pol l i nates it. Other re­ lated Ophrys orc h i d s have bri g h t, i n sectlike flowers t h a t aid i n a ttracti n g m a l e wasps.

I N S E CTS h ave developed v a r­ ious protective s h apes a n d col­ o rs . The h a r m less b u m bl ebee m o t h , for exa m p le, i s a m i m i c of t h e s ti n g i n g b u m b l ebee. The tree hoppers below look l i ke thorns.

Tree h opper on rose stem

Ophrys orchid

a n d pol li n ating

wasp

90

S EX UAL S ELECT I ON i s t h e s u m of va rious cha ra cters

by wh ich o n e p a rtner, gen erally the m a l e , of a spe­ cies attracts a m ate a n d repels riva l s of the sa m e sex. S i n ce it i n creases the i n d ivi d u a l ' s repro d u ctive produc­ tivity, it con stitutes a d i sti n ct ki n d of sel ecti o n . Ela bo­ rate m a l e p l u m a g e a n d song in b i rds, courts h i p d i s­ pl ays, m o re i n ten se m a l e coloratio n, g reater s i z e a n d a n tlers i n various a n i m a ls-these a re so m e o f t h e c h a r­ a cteristics t h a t m ay resu lt. Th e rel ative i m porta n ce of sexua l selectio n a s a component of natural selection is sti l l not clea r . D a rw i n was convi n ced it was of m a jor i m po rta n ce, but su bseq u e n t writers h ave been less sure.

MALE S U PERB LYREBI RD of A u s ­

tra l i a sta n ds w i t h i t s e l a bora te ta i l feathers exte n ded a n d q u iv­ ering, as it perform s its cou rt­ s h i p d i s p l ay. T h e f e m a l e re­ s e m b l e s the m a l e i n color but l a c k s the special ized tai l feath­ ers. You n g male b i rds are l i ke the fe m a les. T h ey do not de­ velop the ta i l feathers u nt i l t h ey a re a bout t h ree yea rs old.

Each male esta b l i s h es a l a rg e territory, a n d t h ro u g h o u t t h i s a rea, h e b u i l d s a series o f m o u n d s of ea rth , each a bout a ya rd i n d i a m eter. H e patro l s h i s territo ry by v i s i t i n g each mou n d and pe rfor m i n g his d i s­ play on it. T h e d i s play beg i n s w i t h a voca l perform a n c e that i n cludes a series of m i m i c k i n g so u n d s a n d c a l l s .

91

N AT U RAL S ELECT I O N I N MA N i s re d uced i n i ts o b ­

vious effects by t h e stru ctu res of society a n d b y the e l a b o rate patte r n s of fa m i ly c a r e . Even so, m a n y body fe atu res show t h e i n fl ue n ce of n a t u r a l selectio n . S o m e g e n e c h a n g es a l so d e m o n strate i t s effects. S i c k l e- c e l l a n e m i a , a d isease c a u s e d by a s i n g l e g e n e , is m a rk e d b y d i sto rtion of t h e b lood c e l l s fro m t h e i r n o r m a l d i s c s h a p e t o s i c k l e s h a pe w h i c h b l o c k s t h e fl ow of b l o o d i n c a pi l l a ries a n d t h u s cau ses a n e m i a . S o m eti m es d eath results when sufferers a re s h o rt of b reath o r wor k at h i g h a ltitu d e s . In West and Central Afri c a , w h e re m a ­ l a ri a i s e n d e m i c , po pulations with a s i c k l e- g e n e e q u i ­ l i b ri u m freq u e n cy of u p t o 2 0 perce n t a re com m o n . A m o n g desce n d a nts of th ese people i n the U n ited States, freq u e n cy of sickle-cell a n e m ia has d ropped to 9 perce n t in two ce nturies. How has this co m e a bout? T h e fre q u e n cy of a sickle-ce l l g e n e i n Africa i s p lotted be low a s p e rce n t of pop u l a t i o n . H ig h freq u e n c i e s a re re stricted t o e q u a tor ia l a re a s i n w h i c h tertia n ma laria is a n i m porta n t c a u s e of death . N o r t h a n d s o u t h of t h i s b e l t , m a la ria is less c o m m o n a n d i s b e n i g n . S i m i l a r h i g h f r e q u e n c i e s occur i n m a l a r i a l a reas of S i c i ly, G reece, T u r key, a n d I n d i a . (Afte r All ison.)

Eq u a tor 1 0- 1 5% 5 - 1 0% 0-5%

92

S I CKLE G E N E w a s demon ­ strated b y b iologist Ver n o n I ng ra m to h ave t h e a b i l ity t o change o n e of t h e t h re e h u n ­ d red a m i n o a c i d u n its i n t h e h e m o g l ob i n m o l e c u l e s t h a t m a k e u p t h e r e d b lood c e l l s . A t o n e point i n t h e c h a i n of n i n e te e n d i ffere n t a m i n o a c i d s t h a t m a ke u p the prote i n , va l i n e is s u b ­ stituted for g l u ta m i c a c i d . T h e res u l t i s t h a t n or m a l red b l ood corpuscles beco m e s i c k l e -s h a p e d . THE

s i c k le genes have c o m pa ra t i v e l y l i tt l e a d ­ verse effect u n d e r n o r m a l co n ­ d i t i ons, b u t t h e h o m ozyg o u s co n d i tion has far m o r e seriou s effects, ofte n ca u s i n g blood c l ott i n g or p re m a t u re deat h . Why d o s i c k l e g e n e s sti l l per­ s i st i f n a t u ra l s e l e c t i o n i s effec­ tive? I t so h a p p e n s t h a t s i c k l e ­ s h a pe d h e mog l o b i n i s resista n t t o i n fection b y t h e m a l a r i a l parasite Plasmodium, which feeds o n r e d b l ood c e l l s . I n m a l a r i a l a reas, t h e refore, a se­ lective e q u i l i b r i u m e x i sts, with t h e heterozyg o u s s i c k l e - ce l l i n ­ d iv i d u a l s re s i sta n t a n d th u s favore d . T h e h o m ozyg o u s i n ­ d i v i d u a l s (w i t h 2 s i c k l e - ce l l H E TEROZY G O U S

Amino acids in normal hemoglobin

Amino acids i n s i c k l e cell hemoglobin

g e n e s) often d i e. A n e q u i l i b r i u m tends to occur betwe e n t h e n u m be r of peo p l e w i t h n o s i c k le - c e l l g e n e ( t h e s e a re t h e " n orma l " i n d i v i d u a l s) w h o d i e of m a la r i a a n d t h o s e w i t h two s i c k l e - ce l l g e n e s w h o d i e of a n e m i a o r b l ood c l o t t i n g . I n t h i s c a s e , a n a p pa re n t l y h a r m ­ f u l g e n e t i c effect m a y be pre­ served beca u s e of side b e n e fi t s . I n m a l a ri a - f re e U . S . , t h e co n ­ d i t i o n h a s n o s u rv i v a l v a l u e . T h u s , n a t u ra l s e l ec t i o n repre­ se nts not a b l i n d , c r u s h i n g wave of exti n c t i o n but the reso l u t i o n of confl icti n g e n v i ro n m e n t a l d e ­ mands on a population. It in­ volves s u b t l e i n te r p l a y of i n ­ tern a l a n d e x ter n a l i n fl u e n c e s .

93

( p . 5 2 ) c o n fi r m t h e a ction of n at u ra l s e l ectio n a n d d e m o n st ra te t h e way i n w h i c h i t o p e r ­ a tes . O n ce i t was a rg ued t h a t n a t u ra l sel ecti o n co u l d n o t poss i b l y h ave p rod u ced t h e e l a b o rate co m p l e x of c h a nges requ i red to b r i n g a bout s u c h m a j o r evo l utio n ­ a ry c h a n ges as the d evelop m e n t of a m p hi b i a n s from fish o r b i r d s from reptiles. Tra nsitio n a l fossil for m s, o r " m issing l i n ks, " s h ow h o w the p rocess took p l a c e .

M I S SI N G LI NKS

A Devo n i a n lobef i n crossopte­ ryg i a n fish c l i m bs a s hore.

Labyri nthodont a m phibian, a de­ s ce n d a n t, goes a s h ore c l u m s i ly.

RANDOM

52-53) s how that c h a n g e from one m a j o r g ro u p to a n other was a piece m e a l process. T h e re was not one big j u m p or a syn ­ c h ro n ized deve l o p m e n t of a l l t h e va rious c h a racters i nvolved . I n A rchaeopteryx, some featu res, s u c h as t h e bra i n , c l aws, teeth , a n d ster n u m , were p r i m itive a n d repti l i a n ; others, such a s w i n g s a n d fea t h e rs a n d t h e g e n era l body s h a pe, were a l ready f u l l y b i rd l i ke . T h i s j u m b l e d mosaic­ evo l u t i o n i s exactly what we should ex pect i f natural se lec­ tion h a s bee n an effective a g e n t of change.

NATURAL

S E LEC­

T I O N , the critics o n ce a rg u ed,

cou ld n ot have produced t h e ela borate series of variations that g ra d u a l ly t ra n sfor m e d rep­ t i l i a n a r m s i nto b i r d w i n g s a n d gave b i r d s a l i g h t a n d mod ified skeleto n , feathers , and s i m i la r fea tures . E a c h of t h e s e e n d less m i nor mod ifications, i t was thought, m i g h t offe r n o special advantage-not e n o u g h to bring a bout the va r ious s u pposedly i n terd e p e n d e n t a n d coord i n a ted c h a n g e s that were i nvolve d . B u t Archaeopteryx a n d other tra n sitio n a l fos s i l forms (pp.

94

MOSAIC EVO LU T I O N is shown

wel l i n the i c h t hyosteg ids, pri m­ itive a m p h i b i a n s of t h e D e ­ von ia n (p. 98). T h ey h a d many fi s h l i ke f e a t u re s . T h e c h a rt g i ves

!

8

"'

75 1 00 1 00 1 00 1 00

a b reakdown of t h e i r a m p h i b ­ i a n vers us fi s h l i ke c h a racteri s ­ tics, with 1 00 poi n ts f o r f u l ly developed a m p h i b i a n form a n d 0 for fu l ly fi s h l i ke for m .

�a li

.c

...

Skull tlructure Eye._titlon of

limbs Shoulder and hlp girdle• Skull articulation Rlbt-form of

lchthyoste ga, a pri m itive De­ vo n i a n a m p h i b i a n from East Gree n l a n d . About 3 feet lo n g . A reco n s tructi o n .

Skel eton of lch thyostega prob· a bly rese m b l es c l osely that of the a n cestors from w h i c h p h i b i a n s evolved.

D iplove rteb r o n, a l a byri nthodont a m p h i b i a n of t h e Carbo n i ferous, with many rese m b l a n ces to the i c h thyostegids.

Eusth enopteron, a Devo n i a n lobefi n crosso pteryg i a n fi s h 2 feet l o n g , h a s m a n y rese m ­ bla nces t o t h e ichthyostegids.

95

is the fi n a l co m po n e n t of the evo l u t io n a ry recipe . Ea rly oppon ents of evolution conte n d e d t h a t the age o f the ea rth, t h e n reg a rded a s l e s s t h a n 4 0 m i l l io n yea rs, d i d n o t a l low enou g h t i m e fo r t h e s l ow process of c h a n g e that evol ution i nvolve d . These es­ t i m ates were based o n t h e rate of coo l i n g of a s u p ­ pose d l y o n ce m o lten e a rth . But t h e d eve l o p m e n t of othe r m et h o d s of dati n g s u g g ested a m u c h g reater a g e, a n d t h e use of ra d io a ctivity n ow i n d i cates that t h e e a rt h i s p ro b a b ly about 5 b i l l i o n yea rs o l d . This is a m pl e ti m e fo r evo l ution to have taken p l a c e . T h e d e­ ve l o p m e nt of a geologic t i m e sca l e puts the fos s i l rec­ ord i n to perspective, a n d reveals the order a n d se­ q ue n ce of t h e va riou s fo r m s of l ife . TIME

THE GEOLOGIC CLOCK S h ows last 600 m i l l ion years of earth ' s h i s tory, each hour representing 5 0 m i l lion years.

u s to a p p re­ ciate the e n o r m o u s i n terva l of t i m e t h i s represe n ts . S u p pose a cos m ic h i storia n wri t i n g a h i s · tory o f t h e e a r t h beg a n t o write a t t h e creation of t h e e a r t h a n d wrote o n e l i n e every t h o u s a n d years. If the boo ks h e p rodu ced were t h e s i z e o f t h i s o n e y o u a re rea d i n g , h e wou l d b y now h ave pro d u c e d 94,000 boo ks.

ANALO G I E S h e l p

96

Most Fossils

ELEME NTS, such as u ra n i u m a n d rad i u m , have u nstable a to m i c n u c l e i that u n ­ dergo s p o n ta n e o u s breakdow n at a consta n t, m e a s urable rate to form oth e r, more sta b l e e l e ­ m e n ts . U ra n i u m , for e xa m p le , p rod uces a s e r i e s of " d a u g h ­ t e r " e l e m e nts a n d fi n a l ly y i e l d s l e a d a n d h e l i u m . O n e g ra m of u ra n i u m prod uces 1 / 7000 g ra m s of lead every m i l l i o n yea rs. T h i s r a t e i s u n affected by any c h a n ge s in heat o r pres s ure. M e a s u re m e n t o f t h e ra tio o f " o l d " u ra n i u m t o " n ew" RADIOACTIVE

lead i n u ra n i u m m i n e ra l s pro­ vides an i n d ication o f the age of the rocks in w h i c h t h ey are fou n d . O t h e r r adi oa c t i ve e l e m e n ts used in age m e a s u re m e n ts i n ­ c l u d e lead-thori u m , pota s s i u m ­ a rgon , ru b i d i u m -stro n t i u m , a n d carbon . S t u d i e s o f m eteorites, w h i c h seem to b e " left over" fra g m e n t s fro m the d evelop­ ment of th e solar syste m , a n d t h e rate of ex pa n s i o n of t h e u n iverse te n d to co n f i r m a f i g u re of a bo u t 4.5-5.0 b i l l i on years for t h e ag e of the e a rt h . URAN I U M T O LEAD B R E A K DOWN

Oldest u n d isp uted fossil

SCALI N G D O W N EARTH'S H I S­ TORY i n to a model s i n g l e ca l­

e n d a r year w i t h t h e orig i n of t h e e a rt h on J a n u a ry 1 st a n d t h e p res e n t day o n Dece m be r 3 1 st wou l d m a ke e a c h second e q u i v a l e n t to 1 67 years a n d e a c h m i n ute to 1 0,000 years.

The o l d e s t u n d i s p u te d fos­ s i l s wou l d a p pe a r about J u ly 1 ; t h e o l d e s t a b u n d a n t fos s i l s , a b o u t Nove m b e r 1 8 . Ma n wo u l d a p p e a r a t a b o u t 1 1 :50 p . m . o n Dece m be r 3 1 . A l l record e d h i s ­ tory wo u l d f a l l i n t h e fi n a l 4 0 seco n d s o f t h e ye a r.

97

"'

�

PERIOD

w

Q UATERN A R Y

u

2

0

;t

u

63

62

T ERTI ARY

C R ET A C E O U S

1 36

71

J U RASSI C

1 90

54

225

35

TRIASSIC

PERMIAN

280

55

325

45

345

20

P E N N S Y LV A N I A N

D EV O N I A N

u

0 N

395

50

�

430

35

8

M I SS I S S I P P I AN

SILU RIAN

O RDOVICIAN

500

70

CAMBRIAN

570 z <( "' "'

�0.. u �

70

P R EC A M B R I AN

4,030

THE G EOLOG I C TIME S C A L E

H E G E O LO G I C TIME S CALE d e ­

r-�� �··�r-

r-

f--

f--

f--

f--

f--

t-

+-

---

---

---

--

J

--

--

--

--

--

-­

--

----

_____.....___. . ....____. . ________

ope d fro m a study of stratified rocks a n d their fossils, is d ivided i n to fou r e ras, fos s i l s b e i n g a b u n­ d a n t o n ly i n t h e l a st th ree . The n a m es of m ost of t h e periods a re d e rived fro m p l a ce s w h e re rocks of t h a t a g e were fi rst stud ied . Thus, we spea k of Devo n i a n fi s h just as we spe a k of Rom a n a rc h itectu re . Wh e n t h e i r rela tive s e q u e n c e is k n own, both ca n t h e n b e fitted i n to a n u m e r i c a l ti m e sca l e . Preca m ­ bria n t i m e covers a l m os t 9 I 1 Oth s of a I I e a rth h i story .

RATES OF EVO LUTIO N v a ry e n o r m o u s l y fro m o n e species t o a nother. The s m a l l b r a c h i pod Lingula, a n i n ­

h a b itc;mt o f wa r m , s h a l low seas, h a s sca rcely c h a n ged i n t h e l a st 400 m i l l io n yea rs. But t h e s pecta c u l a r d i ­ versification o f m a m m a l s has ta ken p l a ce with i n t h e l a st 60 m i l l i o n yea r s . Develo p m e n t of a g eolog i c ti m e scale a l l ows u s to a n a lyze a n d i n terp ret t h ese d ifferi n g rates, wh ich m a y b e expressed i n various ways . IN STR U CTU R ES, rates of c h a n g e can b e m e a s u red for some foss i l g ro u p s . In the evo­ lution of early Tertiary h orses, c h a n g e s i n d i m e n s i o ns of molar teeth were o n ly 0. 1 5 mm per m i l l ion years. T h is is about equal to t h e d i a meter of a h u m a n hai r . S i n g l e populations ofte n conta i n e d u p to twe n ty t i m es as m u c h variation i n tooth d i m e n s i o n s . S u c h s l ow tra n s for­ m ation ca n be observed o n ly i n foss i l s .

TRA N S FO RMATI O N O F S P E C I E S

E C O L O G I C A L R E PLAC EMENT o f

R A P I D D I V E R S I FI C A T I O N of Mioce ne horses f ro m b rows i n g to g razi n g correspo n ded t o t h e w i despread c h a n g e fro m lowland forests to upla nd pra iries i n North A m erica a n d t h e f i rst appeara n c e of fos s i l g rasses. T h i s i n teraction see m s t o conf i r m t h e i m po rta n ce of n a t u ra l selection (p. 1 0 1 ).

s o m e ext i n ct g rou ps b y oth e rs of s i m i la r e n v i ro n m e ntal h a b i ts s u g g ests that com pet ition be­ twe e n the two g roups may have been a factor in some cases of exti n ct i o n . T h e width of each co l u m n in t h e d ia g ra m i s p ro­ portional to the d iversity of t h e group.

1 00

fro m one i n to a n other for Ter­ t i a ry m a m m a l s (p. 1 3 2) h a s b e e n calcu lated to be a bout 500,000 yea rs. With such s low rates, it is not surpri s i n g that few exa m p les of t h e develop­ ment of n ew species can be observed i n l i v i n g populations. S u c h rates also a l low a m pl e t i m e f o r t h e operation of " s low" evo l u t i o n ary mech a ­ n i s m s , s u c h a s t h e selection o f s m a l l va riations.

\

BROWSI N G TEETH ( low.crowned- no ceme n t )

G R A Z I N G TEETH ( H i g h crown ed-cemen t )

H Y PO H I P P U S

P A RA H I P P U S

ANCHITHERI UM

EVO LUTION O F H O RSES ( Teeth drawn to scale-After S i m pson )

PATTE RNS O F C H AN G E beco m e e v i d e n t i n s o m e g ro u p s w h e n t h e time factor is considered. The ra pid d i ve rs i f i cation of h orses i n M i o c e n e t i m e s represe n ted a c h a n g e i n fee d i n g h a b i t f ro m brows i n g to g ra z i n g , correspo n d ­ i n g to t h e w i d e spread c h a n g e f ro m l ow l a n d forests to u p l a n d pra i ries i n North A m e rica a n d t h e f i rst a p pe a ra n ce of fo s s i l g ra s s es. T h i s see m s to be a n i n ­ te raction co n f i r m i n g t h e i m por­ ta n c e of n a t u ra l selection ( p . 5 1 ).

1 01

s h owi n g t h e i n te r p l a y o f t h e va rious fa cto rs i n volve d , c a n b e s u m m e d u p fo r a ny popu l a tion a s d e m o nstrated i n the d ia g ra m below. Such a si m pl e recipe does not m ea n tha t evo l ution i s itself s i m p l e o r t h a t it fol lows a c l e a r ly predicta b l e p a t­ tern . The reverse is the case. The i n teraction of t h ese va rious p rocesses pro d u ces an eno rm o u sl y c o m plex dy­ n a m ic syste m . Both the co m p l exity a n d the potenti a l ity fo r n ovelty of the evo l utio n a ry process a re i n d i cated by t h e g reat d iversity of l ivi n g thi n g s .

A R E C I PE FOR EVOLUT I O N

I SOLATION :

VARIATION : Genetic Reco m b i nation

+

Over-Production Limited Food Supply NEW SPEC I ES

EVOLU T I O N S T I LL C O N T I N U E S .

Many g eo g ra p h i c races a re po­ te ntia l ly new s pecies i n t h e m a k i n g . Co n t i n ued i s o l a t i o n o f r a c e s of t h e g o l d e n w h i s t l e r i n

1 02

t h e Solo m o n I s l a n d s l o n g i n g to a s i n g l e

a l l be­ s pecies, Pachycephala pe ctoralis, wo u l d pro b a b l y c o n v e r t t h e m i n to re ­ prod u c t i v e l y i s o l a te d s pecies .

MUTAT I O N S a re i m po rta n t i n t h e evo l u t i o n a ry process, eve n though so m a ny i n l i v i n g popu­ lations see m to be h a r m f u l . I n a population w e l l -a d a pted to a part i c u l a r e n v i ro n m e n t , the most b e n efi c i a l m u tations have ofte n a l ready bee n i n corporat­ ed. In a d d i t i o n , m u tations with a re effects visible l i m ited known to be co m mo n .

N o t a l l h a r m f u l tra its a re e l i m i nated from t h e g e n e pool of a population . Selection is a l ways a c o m p ro m i se . Even h a r m f u l c h a racteristics may have s o m e be n e fi c i a l s i d e effects (p. 93). Prese rvation of such g e n e s provides a reservo i r of pote n ­ t i a l c h a n g e that may be o f critica l i m porta n c e if e n v i ro n ­ m e n ta l co n d itions c h a n g e .

COLOR CHANGES I N M I N K o r e t h e res u l t o f m utations.

FACTORS A C CO U N T I N G F O R N E W S P E C I E S orig i n s e e m a d e ­

q u a t e to a ccou n t a l so for a l l evol u t i o n a ry change. So m e writers re.f er t o m i c ro-evol u t i o n . a n d m a c ro-evol u t i o n , b u t t h e s e are not f u n d a m e n ta l l y d i ffere n t. C u m u l a t i ve devel o p m e n t of new

- 5

species leads to w h a t we later classify a s n ew g e n era a n d h i g h e r g ro u p s . Sepa ra t i o n of desce n d a n ts from a n ce s t ra l ·po p u la.ti o n s by ti m e re i n forces g e o g ra p h i c d i f ­ fere n ces be tween co n te m pora ry race s , as s h o w n below.

-

4 2

3 II IV

ARAB I C N O ' S . ( S PEC I ES)

l i ne-Evol ution of o Species

ROMAN NO ' S .

C o n t i n u e s to E xist Becomes Extinct

(GENERA)

1 03

T H E C O U RS E OF EVOLUTION

Pre-org a n i c evo l ution t h a t occu rred before t h e a p pe a r­ a n ce of life o n e a rth left l ittle d i rect evi d e n ce of the p rocesses i nvolved . Much s m a l l e r m o l ecu l es were i n ­ vo lved t h a n i n o rg a n ic evo l u ti o n , h owever, a n d the process resu lted i n bod ies of m u c h g reater d i m en sions, s u c h a s stars and g a l axies. The o ri g i n a l m ateri a l p ro b ­ a b ly co n s i sted of su b-ato m i c p a rticles, s u c h a s n e u ­ tro n s , p roto n s, a n d el ectrons, that l a te r p rod uced hy­ d ro g e n . Much of the m ate rial in the vi s i b l e u n ive rse see m s to be hydrogen, which cons ists of a s i n g l e p ro­ to n . Heavier e l e m e nts were pro b a b l y produced from the l i g hter hyd rog e n by a p rocess of neutron ca pture. Each neutron t h a t was a d d ed yiel d e d a new isoto pe. The p rocess by which hyd rog e n a g g reg ated to fo rm bodies s u c h a s stars, i n which e l e m ent- b u i l d i n g too k p l a ce, is sti l l t h e s u b j ect of speculation . It m ay be a conti n u o u s p rocess i n w h i c h n ew hydrogen i s b e i n g created con sta ntly. Altern atively the a p p a re n t exp a n ­ sion of t h e u n iverse m ay result fro m t h e " b ig b a n g " o f a s i n g l e e p i s o d e of creation about 5 t o 1 0 b i l l io n years a go. The u n iverse m ig h t u n d er go p u l s a t i n g m ove m e nts, so t h a t t h e p rese n t e x p a n si o n wou l d b e fo l l owe d by co n t ra ctio n . The " b ig b a n g " h y p o t h eses te n d s to be t h e c u rre n t fa vo rite . a n d t h e re s t of t h e s o l a r system pro b a b l y o r ig i n a ted by the a g g r e g a t i o n of a c l o u d of cos m i c d u s t . Earth ' s s t r u c t u res s u g ­ g e s t t h a t i t formed fro m c o l d ra ther t h a n fro m m o l te n m a teria l . E A RTH

THE PR I M IT IVE EARTH o n wh ich l ife o ri g i n a ted was

an enviro n m en t very d i fferent fro m a n y o n ea rth today. Th ree l i n es of evi d e n c e s u g g est that the e a rt h ' s p r i m i ­ tive a t m os p h e re p r o b a b l y co n s i sted of hyd rog e n , he­ l i u m , m et h a n e, a n d a m m o n i a . The present a t m o s p h ere of n itro g e n , ca rbon d ioxi de, a n d oxyg en ca m e l ater. METEOR ITES a pp e a r t o be ma­ terial " l eft over" f ro m the orig i n of t h e solar syste m . A n a lys is o f their composition p rovides a n i n d ication o f t h e poss i b l e " b u l k co m pos i t i o n " o f t h e earth. Most m eteorites a re m a d e of i ron­ n i ckel or "stony" m ateri a l . A few (the carbo n a ceous c h o n ­ dr ites) conta i n c a r b o n co m ­ pou n d s of extrate rrestrial orig i n .

3 4 - to n m e teori te f r o m G r e e n Ia nd

A NALYS I S of l i g h t f ro m other p l a nets s h ows that the s i x " ba s i c " e l e m e n ts of l i v i n g t h i n g s a re w i d e l y d i strib­ uted. Hydrog e n , oxyg e n , carbo n , a n d n i trog e n a re a m o n g t h e m o s t a b u n d a n t e l e m e nts i n the solar syste m . Sulfur i s n i nt h ;

SPECTRO S C O P I C

p h o s p h o rou s , s i x teen t h . T h i s i m ­ p l i e s t h ey w e r e a l s o proba b l y presen t in t h e p r i m i t i ve e a r t h .

Dark l i n es i n s pectru m a n a lysis a re prod u ced by a bsorption by coo ler g a s i n f ro n t of g lo w i n g sou r c e . TH E ATMOS PH E R ES of p l an ets m ost d i stan t f r o m t h e s u n h a v e p rob a b l y c h a n g ed t h e l ea st . T h e i r a t m os p h e res i n c l ude h ydrog en , h e l i u m , m et h a n e ( C H 4 ) and, in J u p i ter and S a t u rn , a m m on i a ( N H 3 ). Proba b l e c h an g e s in t h e eart h ' s a t m o s p h e re (water o m i t­ ted) a re b a s ed p a r t l y on a n a l o g y w i t h t h e s e.

PA L EO Z O I C .... Cl)

z 0 ""' <(

:;

j

�

�

o..

T H E O RI G I N OF LI FE m u st h a ve i n vo l ve d t h e deve l o p ­

m e n t o f protei n s fro m thei r a m i no-acid co m po n e n t s . An experi m e n t b y Sta n l ey Mi l l e r a n d H a ro l d U rey d e m ­ on stra tes o n e poss i b l e way i n w h i c h t h i s m i g ht h ave ta ken p l a ce o n the p r i m itive ea rth . AMMO N I A, meth a n e, h y d rogen,

and steam were m i xed together i n a closed c i r c u l a t i n g system · and then s u b j ected to a n elec­ trica l d i s c h a rg e . Afte r severa l days t h e c o n d e n s e d water was fou n d to conta i n a m i xtu re of a m i n o a c i ds. The effects of l i g h t n i n g o n t h e p r i m itive at­ mosph ere may have prod u ced a s i m i l a r n o n - biological synt h e s i s o f orga n i c m o l e c u l e s . _

M i l ler- U rey a p paratus, show n i n d i a g ra m matic for m .

1 06

I n orga n i c synthesis of other molecules, s u c h a s carbohydrates and n uc l e i c a c i d s , h a s also been d e m o nstrated. va rious These co m pou n d s were pro b a b l y pre­ served o n the early e a rth by t h e a bs e n ce of oxyg e n and o f other l iv i n g t h i n g s .

A LL LIVI N G T H I N GS tod ay de­

pend, d i rectly or i n d i rectly, u po n g ree n p l a n ts for t h e i r foo d . T h e e a r l i e s t orga n i s m s pro b a b l y " fed " b y s o m e fer­ m e n tatio n - l i ke proces s u po n t h e o rga nic " b roth" from wh ich they a rose, b u t t h i s food s o u rce was l i m i te d . C h a n g es i n t h e earth ' s a t m o s p h ere ca used both by s o l a r radiation and by t h e effects of t h e e a r l i e s t or­ g a n is m s produced a n e n v i ro n ­ m e n t w i t h i n creas i n g q u a n t ities of n i trog e n and carbon dioxide. T h i s proba b l y e n co u raged the a l tern ative of develop m e n t fee d i n g m ec h a n i s m s , i n vo l vi n g fi rs t t h e s y n t h e s i s of m o re co m ­ p l e x m o l e c u l e s . T h e m ore s o ­ p h i s ticated process of p h oto s y n ­ thesis in which s u n light sup­ p l i e s t h e e n e r g y f o r t h e co n ­ vers i o n o f a t m o s p h eric carbo n carbo h y d rates i n to dioxide ca m e l a ter. P h otosy n t h es i s re ­ l e a s e s o x yg e n , s o t h e e a r l y a t ­ m o s p h e re u n d e rw e n t tra n sfor­ m a tion from a red u ci n g to a n oxyge n a t i n g e n v i ro n m e n t .

s o l a r ra d i a t i o n w a s i n crea s i n g l y f i l tered o u t a s r e l e a s e of f ree oxyg e n p r o d u ced o I o ye r of at­ m o s p h e r i c ozo n e . C ol o n i za t i o n of s u rface waters , a n d l a t e r o f t h e l a n d , d e p e n d e d o n t h e g rowi n g effective n e s s ozone the of scree n . T h e " s u d d e n " a p pea r­ a n ce of h a rd - bod i e d i n vertebrate a n i m a l s in Early C a m br i a n t i m e s m a y reflect t h e d ev e l o p m e n t o f t h e ozo n e I o y e r , w h i c h p rovi d ed a p rotec t i ve e n v i ro n m e n t.

R E PROD U C T I O N a rose by d u ­ p l i cation of I o r g e m o l e c u l a r a g ­ g re g a t i o n s by a u tocata l y s i s , i n w h i c h e l e c t r i c a l l y a ct i ve co m ­ poun d s , s u c h a s p rote i n s , cou l d prec i p i ta te d ro p l e ts of col l o i d a l a g g r e g a t e s t h a t were c a p a b l e of d eve l o p i n g i n to o s u rface m e m b ra n e . W h e n i n c rea s i n g oxyg e n i n t h e a t m o s p h ere r e a c h e d a l evel to a l low d e ve l o p m e n t o f res­ s p i ro t i o n , harmful u l tra v i o l et

bi I l i o n s of years

EVOLUT I O N OF L I F E V

0

befo re pres e n t

-T__________________.______,

N

-0-

0

"' w z

< -+------� :r: ...

z 0

.... ;j ......

0

> w ......

EVOLUT I O N O F EARTH'S ATMOSPHER E & HYDROSPHER E

• E a rl i es t a n i mals

known

foss i I

m a r i ne

-1 -

* Fo s s i l g reen a l g a e ( m u l ti - ce l l e d , sex u a l reprod u ct i o n )

* Foss i l

bacte r i a

and

b l ue - g reen

a lgae

-

2-

F ree oxyg e n s u ffici e n t res p i r a t i o n

for a n i m a l

F ree oxygen i nv a d es a t m osphere; starts t o form oz o n e I oyer w h i c h screen s out u l trav i o l et rays • Wea th e r i n g of rock beg i n s

<

u

"' 0 ...... 0 ""

-

_,

-< u

�z

oo

_, _

Q l­

- ;j .., _,

u:. o

<>< > I>. W

* Ea r l i est known foss i l s ( s i n g l e ­ ce l l protists ( ba c t er i a ) a n d s i m p l e p l a n ts ( b l ue- g reen a l g ae ) . Deve l ­ o p m e n t o f ce l l m e m b r a n e

F ree oxygen i n h ydrosph ere creases; o x i d izes i ro n

-3-

Protei n and m a c ro m o l e c u l e s A m i n o acids S i m p l est c o m p o u n d s of and N

Deve l o p m e n t of p h otosyn thesis; re­ l eases free oxygen to hydrosphere Fermentation (?); adds d i o x i d e to h ydrosphere

Co l l o i d a l CO!Jcervates

C, H , 0,

-4 -

in­

c a r bon

Pri m i t i ve a t m osphere proba b l y H 2 , H 2 0 , CH4 a n d N H 3 ; n o free oxygen . I ntense u l traviolet radi­ ation

FO RMAT I O N O F P L A N ET EARTH

Crit i c a l events i n t h e ea r l y h i story of l i v i n g m a tter a n d i n t h e deve l o p m e n t of a t m o s p h e re and hydrosphere . • denote foss i l s , the earl ier part of the h istory being hypothetica l ( A fte r F l i n t and ot h e r s )

THE FOSS I L RECOR D i s the basis of o u r u n de rsta nd ­

i n g o f the h istory o f l ife a n d t h e course o f evol utio n . Fossils a re t h e re m a i n s o f or i n d ications o f prehisto ric a n i m a l s a n d p l a nts preserved i n the roc ks of the e a rt h ' s crust. Fossi ls a re o f m a ny k i n d s a n d a r e fo rmed by va rious p rocesses, but the c h a n ces of a n y o rg a n i s m be­ co m i n g foss i l ized a re s m a l l . Thus, the fossil record is a very i n c o m p l ete a n d rath e r b i a sed record of t h e h istory of l ife . Recog n ition o f t h i s is i m po rta nt i n i nter­ preti n g the fossil reco r d . Orga n i s m s l a c k i n g h a rd pa rts, fo r exa m pl e, a re ra rely fou n d a s fossi l s . For t h is reason records of the ea rly d evelo p m e n t of l ife a re p a rtic u l a rly m e a g e r . W H O L E A N I MALS A N D PLA NTS

are very ra rely preserved in t h e fos s i l record . Wooll y m a m moths, up to 1 0 feet ta l l , fou n d i n S i ­ ber ia a n d A l a s ka are e x a m ples of such preservation by a deep­ fre eze process.

O UTLI N E of soft parts of org a n i s m s b u ried in fi n e m u d i s s o m eti m e s preserved a s a carbon fi l m , t h e m o re vo l a t i l e com p o n e n ts d i s t i l led off b y h e a t a n d pres s u re i n rock s . Exa m p les a re t h e tri lobi tes and l eaves.

AN

Woo l l y m a m moth preserved i n froze n gro u n d

Ca m b r i a n tri l o b i tes

Fo s s i l p l a n t leaves fro m Pe n n s y l va n i a n

Growth ri n g s a re preserved i n s i l ica i n petrified wood. This cla m s h e l l i s a l m ost u n a l ­ tered except f o r leach i n g . MOST FOSS I LS consist of o n l y

the h a rd pa rts of a n i m a l s a n d pla n ts , s u c h a s s h e l l s , bones, teeth, and wood. In a few cases these are a l most u na ltered, but usua l ly they a re leached a n d partly o r wholly replaced by oth e r m i nerals, e s pecia l ly s i l ica (Si0 2) and calcite (CaC0 3) . The re p l a c i n g m i n e ra l s may some­ ti mes preserve t h e orig i n a l m i­ c rostru cture, a s i n s o m e s i l ici­ fied wood, but this i s u n u su a l .

A m m o n ite, c a l ci u m carbo n a te , replaced b y pyri te ( FeS2 ) .

i m p res s i o n or m o l d

IMPRESS I O N S A N D CASTS of a n i m a l s a n d p l a n t s may be formed in porous roc ks, s u c h as san dstone, w h e n a l l the o ri g i n a l com ponents a re d issolved away. T h i s leaves a cavity, w h i c h may later be ti l led by new m i nera l s , c a r r i e d i n sol u t i o n , to g ive o cast of t h e orig i n a l o u t l i n e . B U R R O W S , TRA I LS A N D TRACKS

may be preserved in s ed i m e n ts t h a t a re l a ter co n so l i d a ted i n to rocks. STO N E ARTI FACTS a re the most

D i n o s a u r t r a c k s i n s a n d s to n e

c o m m o n re m a i n s of p re h i storic man. They represent various types of tools and wea pons. P re h i s toric h a n d a x e

1 09

THE OLDEST FOSS I LS fou n d i n rocks est i m a te d t o b e . a bo u t 2 . 7 b i l l io n y e a r s o l d , co n si st of si m pl e p l a n ts, , i n c l u�d i n g l i m e-sec reti n g a l g ae, b a cteri a , a n d fu n g i . Various o rg a n i c a m i no-acid res i d u es a re a lso k n own frol?l these very a n cient rocks. Wel l - p reserved a n i m a l fo is i l s fi rst a p pea r i n roc ks 6 0 0 m i l l i o n yea rs o l d . Microsco p i c colo n i e s o f a l g a e f r o m 1 .6 b i l l i o n years ago. .Gu n f l i n t Formation, O n tario

�x200)

Preca m b r i a n Fo s s i l s

O L D E S T PLANTS a re p re­ served i n c h erts from North A m erica, Africa, and A u s t ra l i a a n d ra n g e i n a g e from 2 to a b o u t 3 b i l l i o n years. They i n ­ c l u de fi l a m e n tous a n d s p h e r i c a l al gae a n d bacte ria a n d ot h e r m i c rosco p i c structures t h a t a re n o t eas i l y classified. S o m e a re c l osely s i m i l a r to l i v i n g for m s . O t h e r m o r e w i d e s pread Preca m ­ bria n foss i l s i n c l u d e o p t i ca l l y active orga n i c co m po u n d s o f s u pposed org a n i c ori g i- n . Stromatolites, w i d e ly d i stri b­ uted in rocks of Preca m b r i a n age, a re m ou n d l i ke, la m i n a ted stru ctures, a few feet in d i a m ­ eter, fou n d i n calcareous roc ks. They represent the deposits of l i m e-secret i n g b l ue-g reen a l g a e. THE

OLDEST A N IMALS a re known from Ed iacara, South Austra l i a , in Preca m br i a n rocks that lie o n l y 500 feet b e l ow t h e Ca m b r i a n . T h ey a re soft- bod ied a n i mal s, i n c l u d i n g j e l lyfi s h , seg­ m e nted wor m s, sea p ens, and some a n i ma l s of u n k nown affi n i ­ t i es. I n con trast t o t h e oldest pla nts, which a re pri m i t ive, t h e o l d e s t a n i ma l s a re relatively ad­ va nced type s, s u g g e s t i n g a l o n g earl ier h istory.

THE

Seg m e n ted w o r m fl o un d e ri (Afte r Glaessner) Spriggina

1 .5

in.

Jel lyfish Medusian mawsoni

about 1 i n .

(Afte r Glaessner)

Ob olella, lower Ca m b r i a n brach iopod A b o u t 0.2 i n . .

0/ene//us, a lowe r C a m b r i a n trilobite. Le ngth to 9 i n .

T h e " s u d d e n " a p p e a ra n ce o f fossi l a n i m a l s a b o ut 6 0 0 m i l l i o n yea rs a g o i s o n e of t h e m a jo r evo l utio n a ry pro b l e m s . I t h a s been variously s u g g ested t h a t : ( a ) n o Preca m b ri a n a n i m a l s existe d ; ( b ) Preca m b r i a n a n i m a l s d i d exist but l a c k e d h a rd p a rts a n d were n ot fossi l i zed ; ( c ) Preca m b ri a n a n i m a l foss i l s have be e n d estroyed by erosion a n d meta m o r p hi s m ; a n d ( d ) Preca m bria n a n i m a l s were co n fi n e d to isolated, oxyg e n - ri c h a re a s a n d a r e a s y e t u n d i s cove red o r u n exposed a s fos si l s . N o n e o f t h e s e expl a n ati o n s i s n ecessarily u ntru e . E a r l y C a m b ri a n d iversification o f a n i m a l s exte n d e d over 3 0 m i l l i o n years, h e n ce w a s n o t rea l ly " su d d e n . " I t seems prob a b l e that a n i m a l s d i d not orig i n a te i n l ate Preca m b r i a n ti m es, t h a t t h e i r ea rli est representa­ tives were soft- b o d i e d fo r m s of restricted d istri b ution , a n d t h a t t h e later w i d e s p r e a d a p p e a ra n ce o f h a rd ­ bod ied fo r m s i n C a m b r i a n ti m es m a y m a r k a res p o n s e t o s o m e e n v i ro n m e ntal c h a n g e, s u c h a s t h e atmo­ s p h e r i c co m position o r cut-off of u ltravi o l et ra d i ation (p. 1 0 6 ) . I t w a s pro b a b l y r a p i d because of t h e rel a ­ tive " e m pti n e s s " o f m a n y enviro n m ents t o a n i m a l l ife, a n d t h e stro n g sel ective pressure that t h e d evelo p m e n t o f h a rd pa rts by a ny o n e g ro u p wou l d exert o n others . 111

M i.d d l e Ca m bri a n sea based on s peci m e n s f ro m B u rgess S h a l e of Bri t i s h Co l u m bi a : ( 1 ) j e l l yfi s h , ( 2 ) s p o n g e , ( 3 ) trilobite, ( 4 ) worm , ( 5 ) brach iopod, ( 6 ) x e n o pod a r t h ropod

I NVERTEBRATES were t h e m ost disti n ctive a n i m a l s of C a m b ri a n , Ordovi c i a n , a n d S i l u r i a n ti m es­ a period of so m e 200 m i l lion yea r s . Althou g h verte­ b rate fra g m e n ts a re fou n d in Ordovic i a n rocks, they were ra re u n t i l D evo n i a n t i m e s . The ea rliest i n ve rte­ b rates ( p . 1 1 0) i n cl u d e d jel lyfish, sea pens, a n d seg­ m e nted wo r m s , b u t C a m b r i a n fa u n as were d o m i n ated by trilob ites, riow extinct a rth ropods . Spong es, s n a ils, e c h i n o d e r m s, and s m a l l horny b ivalved brach iopods were a b u n d a n t i n s h a l low seas. In the Ordovician, cora l s , b ryozo a n s ( m oss a n i m a l s ) , a n d m a n y n ew k i n d s of b ra c h i o p o d s a n d t ri l o bites a p pe a re d . P ro ­ tozoa n s we re ra re. S q u i d l i k e cepha lopods, so m e 1 5 feet l o n g , d eveloped . I n t h e S i l u ri a n , e u rypte r i d s, a r­ t h ropods to 6 feet l o n g , l ived i n del ta s a n d estua ries. MAR I N E

1 12

Represe n t a tives of a l l t h e m a jo r l ivi n g i n verte ­ brate p h y l a a n d n ea rly a l l the c l a sses were esta b l i shed by Ordovi ci a n ti m e s . S i n ce t h e n , the m a j o r patter n s of inverteb rate l ife i n the seas h ave c h a n g e d l ittl e . A few m a jor g roups h ave becom e exti nct, d iffe re nt g eo­ g ra p h i c a r e a s a n d d iffere nt e nviro n m e nts h ave sup­ ported d iffere n t fa u n a s, and g e n e ra and spec ies h a ve shown va ried p a tterns of mod ificatio n a n d exti n ctio n . EARLY

PALEOZO I C

A N IMALS

s h owed many a d a ptations to d ifferi n g modes of l ife. They i n ­ c l uded fixed benthic for m s , such as cora l s a n d brac h iopod s ; va­ g rant benth i c types, s u c h as starfi s h and s n a i l s ; free-swi m ­ m i ng for m s , s u c h a s ce p h a l o ­ p o d s a n d e u rypterids; a n d free­ fl oating forms, such a s j e l lyfi s h . Comparable d ivers ity existed in feed i n g h a b its. The p hyto­ plan kton , on w h i c h m a ny l i v i n g marine i nve rte b rates f e e d , h ave s i l iceous a n d calcareous hard parts. These types a re u n k n ow n

i n the Ea rly Pa leozoic, perhaps beca use their foreru n n e rs were soft- bodi ed. T h e h a rd parts of Early Pa leozoic i nve rteb rates a re com posed of various m i nerals. Ca m brian for m s consist c h i efly of phosphatic, s i l iceou s , a n d ch iti nous m a te ria l s , but calci u m carbonate beca m e t h e pred o m i ­ n a n t s h e l l s u bsta nce i n O rdov i ­ c i a n t i m es . Little i s y e t k n own of the s i g n ifica nce of this b io­ c h e m ical evolution. O r i g i na l s h e l l com position i s ofte n mod i · fled b y s u bseq u e n t alteration duri n g fos s i lization ( p. 1 0 8 ) .

A Devo n i a n cora l reef : ( 1 ) trilobite, (2) cephalo pod, (3) bryozoan, (4) brac h i opod , (5) cora l , (6) cora l , (7) cora l ------

THE O LDEST VERT EBRATES a re fra g me nts of a rm o re d

fi s h fou n d i n roc k s of Ordovici a n a g e i n Wyo m i n g a n d el sewhere. Fish re m a i n ra re a s fossils unti l l ate S i l ur­ i a n ti m e s . They b e co m e d iversifi e d a n d a b u n d a nt d u r­ i n g t h e Devo n ia n . The o r i g i n o f ve rtebrates i s obscure. Th ey belong to th e Phyl u m C h o r d ata, conta i n i n g so m e m e m b e rs t h a t l a c k a vertebral col u m n ( a corn wo rms, sea s q u i rts, l a n celets, a n d t h e i r k i n ) tho u g h t h ey do h ave a sup­ porti n g n otochord and other featu res s h a red with the " h i g h e r " vertebrates . Larva l acorn wor m s show stri k­ i n g s i m i l a rities to l a rva l ech i n o d e r m s, sugg esti n g that both g roups may h ave a rise n fro m a com mo n b u t un­ known a n cestra l stock. primitive g roup of fi s h , a re represente d today o n ly b y t h e h a g fi s h a n d l a m preys. Agnatha l a c k the true jaws a n d paired fins typical of most l i v i n g fish. Ma ny of these oldest a n d most d iversified o f the early fi s h had a bony arm or. Cal led ostra­ coderms (" bony s k i n "), they

AG NATHA, t h e most

ra rely exceeded a foot i n l e n g t h . T h ey l ived ch iefly in stre a m s and estu a ries w h ere pre s u m a bly they fed on botto m m ud s or on suspended mate ria l . They a re n ot k n own i n roc ks you n g e r than the D evon ian, per­ h a p s beca u se th ey were soft­ bod ied-l ike their l iv i n g repre­ sen tatives. Pteraspis

U p p e r S i l u r i a n to De­ von i a n . About 6 i n .

B irkenia

A n S i l u ri a n a g n a t h a n f i s h a bo u t 4 i n . l o n g .

Drep a n aspis

1 14

lower Devon i a n .

T o 1 ft.

Climatius, U pper S i l u r i a n to De­ vonian, was a spiny a c a nth o d i a n "shark" with rhomboid s c a les, 2 spines on bac k , a n d 5 pa i rs of ve ntra l fins. L e n g t h 3 i n .

w a s a j o i n ted­ necked m a r i n e a rt h rod ire to 30 ft. lon g . It was the largest verte­ brate of Devon i a n l i m es. D u n k l e os t e u s

( late S i l urian to Per m i a n ) are the o n ly verte brate class to have beco m e extinct. They reached their peak i n the Devo n i a n a n d a re rare in you nger Pa l eozoi c ro'cks. Placo­ derms d i ffer fro m Agnatha i n havi ng pa i re d fi n s a n d pri m i tive jaws, i m porta n t fea t u res i n later d iversification o f verte­ brate s . bot h i n cluded Placoder m s freshwater a n d m a r i n e form s, such as t h e 30-fool, joi nted­ necked arlhrod ires; s m a l l fresh ­ water s p i ny a c a n t h o d i a n s ; a n d mass ively a rmored , strong-finned a n t ia rch s . PLACODERM$

shark jaws and teeth

a s h a rk f r o m the U p per Devonian, with a stream­ li ned naked body. To 4 ft.

Clacloselache,

Teeth of C arch a r o clo n, a 40- to 50-foot shark from the Mi o cene S H A R K S A N D RAYS belong to

the C h o n d richthyes, a class of predaceous, cart i la g i nous, open­ g i l led fish. Sharks s h ow m a ny ada ptations to life i n t h e open ocea n s , i n c l u d i n g strea m l i n i n g , wel l - developed teet h , a n d spiny skin scales. S o m e l i v i n g s harks rea c h a l e n g t h of 50 feet. The earliest m e m bers of t h e g roup, which ap peared in Devo n i a n t i m es , l ived i n f r e s h water.

Skates and rays, fl a tte n ed for botto m - d we l l i n g existe n c e , h ave fl a tte n e d teeth for cru s h i n g s h e l ls. Loss of t h e bony a r mor, de­ velop m e n t of efficient jaw sus­ pension, and more flexible fins pro v ided both s h arks and bony fi s h with an advanta g e over t h e i r placod e r m a n cestors. I so­ lated teeth a n d spines a re the m ost co m mo n fos s i ls.

1 15

Rece n t

I

EVO L U T I O N O F F I S H

Te rti a ry Bon y R a y - Fi n n e d Fi s h

Cretaceo u s ) J u ra s s i c

L u n g fi s h

Triassic Perm i a n P l acoder m s

::::::�·:1, �= Ord ov ici a n

Ag n a t h a

Crosso p teryg 1 a 1

Carti l a g i n o u s F i s h

?

....... . . . . . . ..

-

-

- -

- - - -· -

-

--- ·········· ··· ···········

· · · ··

'

.

...... . . .

.

BONY F I S H (Oste i c h t hyes) i n ­ c l u d e nearly a l l l i vi n g fres h ­ water a n d m a r i n e species. They have stro n g , but flex i b le, bony s ke l eto n s and either sca les or plates. Most k i n ds have a n a i r bla dder. Bony f i s h l i ve i n every k i n d of a q u a t i c e n v i ro n m e n t (eve n caves), a n d t h e y outn u m ­ be r a l l other vertebrates co m ­ bi ned, both i n n u m bers of spe­ cies a n d of i n d iv i d ua l s . The

oldest m e m bers w e r e fres hwater for m s from the M i d d l e Devo­ n i a n . T h ey i n c l u d e f i s h with two types of f i n s . The ray-f i n ned f i s h were a rare, f reshwater g roup i n the Pal eozo ic, b u t t h ey beca m e the do m i n a n t g r o u p i n t h e Mesozoi c a n d Cenozo i c . T h e i r scales be­ ca m e t h i n n e r, a n d t h e i r jaws and s keleto n s s h owed progres­ sive i m prove m e n t.

a Mi d d le Devo n i a n fi n ned fish. L e n g t h a b o ut 1 1 i n .

ray·

Cheirolepis,

I , ,,

Deta i l of ray fi n , w i t h typica l s u pporti n g b o n e s .

1 16

EVO L U T I O N O F AM P H I B I A N S

U ro d e l e s Ste reos po n d y l e

To Repti les

..

... .

L a b y r i n t h o d o n ts

). .l e-f)o fp·Q"� d y I e

. . . . . . . . . . . . . . . . . . .. . . .

.

Am p h i bi a n s

A I R-BREAT H I N G

BONY

FISH

( C h oa n i c h thyes), a s m a l l e r g rou p t h a n t h e ray-fi n n ed fi s h , have i n te r n a l n os tr i l s that o p e n i n to t h e m o u t h , a s do t h ose i n l a n d ­ l iv i n g vert e b r a t e s . L i vi n g fo r m s i n c l u d e t h e l u n g fi s h ; t h ree g e n ­

era a re k n own, o n e fro m each of t h e s o u t h e r n co n t i n e n t s . T h e y have p owe rf u l fi n s , s u p ported n ot b y a fan of s l e n d e r bones as i n t h e ray-fi n n e d fish but by

a strong bony a x i s . T h e y u s e t h e s e s t o u t fi n s to " wa l k " f r o m

p o o l t o p o o l d u r i n g t h e d ry sea­ son. Lobef i n s, t h e othe r m a j o r g r o u p , i n c l u d es t h e l i v i n g m a ­ r i n e coelaca n t h s a n d t h e i r more g e n era l i zed, freshwater, ca r n i v­ o rous, Devo n i a n crossopteryg i a n forebea rs. I t wa s t h ese t h a t g a ve rise to the terrest r i a l verte b rates ( p p . 94-95).

-

Oste ole pis, from M i d d le Devo­ nian, with t h ick, rhomboid sca les a n d short, lobed fi n s. To 9 i n .

fr i n ge

Lobe fi n , show i n g t h e strong s u p porting bo nes from wh ich feet deve loped.

1 17

c o mp a r a ti v e l y late de­ ve l o p m e n t . life p ro b a b l y o ri g i n ated in the s h a l l ow seas, where t h e m a jo rity of i n vertebrate g ro u p s a re sti l l restri cte d . Life o n the l a n d i nvolved m ajor c h a n g es fo r these c reatu res that ori g i n ated a n d lived i n t h e ocea n s . T h e m o d ificatio n s i n cl u d e d c h a n ges n ecess a ry fo r p rotection a g a i n st d ryi n g u p , n ew m ethods of s u p ­ port i n a i r a s o p posed t o the m o re buoya nt water, b reat h i n g oxyg en as o p posed to extra cti n g it from the water , n ew sou rces of food a n d water, a n d n ew repro­ d u ctive m e c h a n i s m s to assure ferti lization i n the ab­ sence of water. Colon ization of rivers a n d l a kes was only slig htly less for m i d a ble, for it involved d evelop­ ment of m ec h a n is m s to p reve nt d i l utio n of body fl u id s

L I F E ON TH E LA N D w a s a

A RTH R O PODS h a v e exceeded all o t h e r g rou ps in the d i ver· sity and n u m be r o f t h e i r terres­ tri a l and flyi n g re p r e s e n ta t i ve s . T h ey g a i n e d a " f l y i n g start" b y t h e i r tou g h , f l e x i b l e outer coveri n g and by t h e i r s t ro n g a p p e n d a g e s . T h e o l d est l a n d a rt h ro pods a re l a te S i l u r i a n

m i l l i pedel i ke f or m s t h a t m a y h a ve b e e n p a r t l y a q u a t i c . I n ­ sects f i rst a p peared i n t h e D e ­ von i a n . By C a r bo n i ferous t i m e s , a va r i ety o f a rt h ro p o d s , i n c l u d ­ i n g p r i m i t ive w i n g ed i n sects, coc k roa c h e s , s p i d e rs , and scor­ pions, had a p pea red . Most g ro u p s a rose in t h e Mesozo i c .

A RT H ROPODS

that, in a l l a n i m a ls, conta i n d issolved salts p re cisely adjusted to the osmotic balance of sea water. land dwe l l i n g , i n spite of its problems, offered all the a dva ntag es of an em pty envi ro n m ent. Beca use of the deli cate i nterdependence of a l l livi n g t h i n g s, it is not surprisi n g that both pla nts and a n i m a l s seem to have colo n i zed t h e l a n d at about the s a m e ti m e d u ring the Silu ria n a n d Devo n ia n . Th e i nvasion of the l a n d a l m ost certa i n l y i nvolved t h e earlier i nvasion o f fresh waters. Ma ny l iving g rou ps, wh ich a re essentially m a ­ rine, conta i n a few freshwate r colon ists (cla m s a n d crustacea ns, fo r exa m ple), but o n ly t h e p l a nts a n d three m a j o r g ro u p s o f a n i m a l s (snails, a rthropods, a n d vertebrates) h ave beco m e fully established o n the l a n d .

h a v e established them selves o n the land with varying degrees of s u cces s . Most a m ph i b i a n s a re l i m ited to a reas near e nough to water to a l low them to return to it to repro­ d uce. Most reptiles a re restricted to a reas from the tropical to the te m perate zones. Ma m ma l s and bird s a re more widely d i stri b­ uted a n d ada pted. Some verte­ brates, i n c l u d i n g turtles a n d other ext i n ct rept i l e s , porpoises, wha l es, and pe n g u i n s , have u n ­ dergone a secondary adaptation to m a ri n e l ife (p. 1 2 8).

VERTEBRATE S

S N A I LS have i nvaded fresh waters a n d t h e l a n d . Some h ave reta i ned the protective s h e l l , but ot h ers (slugs) a re na ked . Land for m s m ove a n d feed b y b rows i n g , m u c h l i ke aquatic for m s. T h ey have de­ veloped lungs for brea t h i n g .

Skeleton o f Perm i a n a m p h i b i a n , Eryops. L e n g t h a b o u t 5 feet.

La nd Snail, Helix

1 19

Cycads

600 Psilophytes

terids S p h e nophylls

g re e n a l g ae, which now exist i n both the seas and i n fresh waters. Like a n i m a ls, d ifferen t g roups of pla nts s how va ryi n g deg rees o f a d a ptatio n t o l a n d life.

LAND

PLANTS p rob a b l y a rose fro m

BRYO PHYTES ( m osses a n d l iver­ worts ) need w a ter in repro d u c ­ tion and f o r protection from desiccation . T h e i r parti a l a d a p ­ tation to land life is a n a logous t o t h a t of t h e a m p h i b i a n s . S m a l l p l a n ts, w i t h leaves a n d stems, they l a c k woody t i s s u e s for s u p port a n d c i rc u l a t io n .

1 20

' '

w h i c h incl udes the a lgae, f u n g i , a n d bacteria, lack t h e roots, stems, leaves, and va s c u l a r s u p port i n g and c i r­ cu l a t i n g syste m typ ical of h ig h ­ e r p l a nts. T h ey a re e i t h e r u n i­ cel l u la r or consist of loosely or­ g a n ized g roups of c e l l s . li m i ted to d a m p e n vi ro n m e n ts .

THALLOPHYTES,

A n g iosperms

G i n kgo

Corda ites

C o n ifers

VASCULAR PLANTS ( Tra c h e o p h ytes ) i n c l u de t h e m a ­

jority o f l ivi n g p l a nts. A l l have specia l i z ed vasc u l a r sys­ te m s of con d ucti n g tissues that tra n s port water a n d n utrie n ts fro m t h e s o i l t h rou g h t h e roots t o t h e other pa rts of the p l a nt. T h i s syste m a l so provi d e s s u p po rt, a l l owi n g so m e of these p l a n ts to g row to g reat sizes. They als o h ave a n outer l ayer (cuticle) that p revents d esiccation . The e a r l i est vascu l a r p l a nts were seed l ess kinds, such a s those shown i n Devo n i a n forest b elow. Devo n i a n f o r e s t sce ne. S h o w n a re : (1) a prim itive lycopod (P ro to· lepidode n dro n ), (2) tree fe r n (Eosperm atopte ris), a n d (3) s c o u r i n g r u s h (Cala m op hyton). g ro u n d .

Psi l opsids a re

low g rowi ng pla nts

in fore·

PSILOPSI DS, wh ich include the

e a r l i e s t k n own vasc u l a r pla nts, lack roots . Th e y h ave either pri m itive l eaves or are leafless. T h o u g h w ide s p rea d i n Devo n i a n times, they re m a i n e d s mal l i n s ize. O n ly two g e nera survive.

include the liv­ i ng scou r i n g rus h e s a n d s i m i l a r Paleozoic p l a nts that g rew to 40 feet ta l l . T h ey h ave roots and long, seg m e n ted, ribbed, c o ne be ari n g stems with circlets of l e av es at the nodes. S P H E N O PS I D S

-

S E E DL E S S VAS C U L A R PLANTS i n cl u d e psilopsids, lyco­

pods, ferns, a n d sp henops i d s . The a d u l t p l a n t prod u ces spores that d evelop i n to s m a l l speci a l i z e d l eafless p l a nts ( g a m etop h ytes ) . T h ese l ater p r o d u ce g a m etes, or sex ce l l s . Because sperm req u i re wa ter to re a c h t h e eggs, these seedl ess pla nts a re restricted to · d a m p envi ro n m ents . Widespread i n t h e P a l eozoic, they de­ clined a s seed-bearing p l a nts expa n d e d in Mesozoic. i nclude the living club mosses and g ia n t represen­ tatives fro m t h e Pe n n sylva n i a n coa l forests . LYCOPODS

1 22

which sti l l survive in la rge n u m bers, a re spore- bear­ ing p l a n ts. Some foss i l a n d l iv­ i n g for m s g rew to 50 feet ta l l .

FER N S,

SEED-BEARING PLANTS a re of two ba sic kinds: non ­ flowerin g a n d floweri n g . I n the non-flowe ri n g g roups (gym nosper m s ) , t h e seed i s n ot p rotected ; it i s " n a ked" -as i n pine cones. In fl oweri n g pla nts ( a n g iosperms), the seeds a re p rotected. In both ki n d s , resista nt pol len a n d e g g s a re p ro d u ced d i rectly from pa rent p l a nts. Pollen fertil izes the egg, wh ich d evelops i n to a seed wh ich is p rotected from d ryi n g . As a result, seed-bear­ i n g pla nts h ave colonized a g reat variety of l a nd a reas and a re the d o m i n a nt l ivin g g roup of pla nts. GYM N O S PERMS i n cl u d e ( 1 ) e x ­ t i n ct seed ferns, perhaps a n ces­ tral to other g ro u ps; ( 2 ) cycads

a n d their exti n ct relatives, which were a b u n da n t i n the Mesozoic; (3) exti nct cordaites, perha ps a ncestra l to conife rs, (4) the living ginkgos; and (5) the widespread, a b u n d a nt con ifers. FLOWER I N G PLANTS ( a ngio­ sperms) a re re pre sented today by over 250,000 species. They appeared in the Mesozoic a n d ra p i d l y d is p l a ced t h e gym no­ sperms, which we re then d o m i ­ na nt. Their flowe rs a re repro ­ d uctive structu res, m a ny of t h e m speci a l ly developed t o a ttract i n sects that ca rry the m a le pol­ len to ferti lize t h e fe m a le fl owers . Enclosure of the seed i n a protective covering a l so rep resents an adva nce over the gym nospe r m s . Floweri n g pla nts s how n u merous a d a ptations to d iffere n t e n v i ro n m ents, ra n g i n g fro m desert c a c t i to tropical swa m p trees a n d f l owers. C h a n g e s i n s o m e a n i m a l g ro u ps a ppear related to c h a nges i n vegeta ti o n ( p . 5 1 a n d 1 0 1 ).

Early cycad

F lowe ring Pla nts of the C retaceous

A M P H I B I A N S were t h e fi rst te r r e s tr i a l

v e r te b r a tes ,

b u t they a re o n ly p a rtly a d a pted to life o n l a n d . T h ey n eed to retu rn to water to lay t h e i r eggs, a n d t h e i r you n g d evelop i n water. Most k i n d s a re co n fi n e d to d a m p envi ro n m ents as ad ults. The o l d est a m p h i b i a n s, the i c h t hyosteg i d s fro m the U p pe r D evo n i a n , a rose from the crosso pte ryg i a n lobe­ fi n n e d fi sh , possi bly i n respo nse to popul ation pressu re in the pool s where the latter l ived ( p . 9 5 ) . The stout bony axis a n d m uscles of the fi n s a n d the p rese n ce of l u n g s a d a pted lobefl ns idea l l y for m ig ration from sta g ­ n a nt a n d seaso nal p o n d s . Life o n the l a n d p rovided u n l i m ited oxyge n suppl ies, the poss ib il ity of a d d itio n a l food sou rces, escape fro m pred a tors, a nd t h e m e a n s of reaching oth er bod ies of water. LATE PALEOZO I C AMPH I B I A N S

showed g reat d ivers i ty. Their adaptive rad iation onto land was ra pid, a n d some for m s u n ­ derwent a secondary return to : h e w a ter. Some l a byri n t h o ­ d o n ts were 1 5 f e e t i n l e n g t h . A m p h i b i a n s d o m i n a ted t h e l a n d

f o r over 1 0 0 m i l l i o n years. They d ec l i n e d in t h e early Mesozoic, per h a p s a s a res u l t o f competition w i t h t h e i r better­ a d a pted reptili a n desce n d a n ts . i n c l ude a m phi bia n s Livi n g n ewts, s a l a m a n d ers, frog s , toa d s , a n d caeci l i a n s .

Late Pa l eozo i c coo l -for m i n g swa m p w i t h la byri n t h od o n t a m ph i b i a n s .

Early re ptiles d i ffered f ro m t h e i r a m p h i b i a n a n cestors i n o n l y m i­ n or ways. They u n d e rwe n t ra p i d d iversification i n Perm i a n ti mes.

THE RISE OF T H E · REPTI LES m a rked a new stage of

a d a ptation to l ife on l a n d . Repti les d evelop fro m a n eg g with a tou g h outer coveri n g , provi d i n g a b u i lt- i n food s u p p l y a n d a sea l e d , l i q u i d -fi l l e d c a p s u l e for t h e deve l o p i n g e m b ryo . The i nfa nt repti les e m erge fro m the eg g m o re o r less fu l l y for m e d . R e p ti l e s were t h u s a b l e t o c o l o n i ze the l a n d a reas fa r removed fro m strea m s a n d l a kes . Repti l i a n s k i n is scaly o r c o r n i fi e d , a p rotection a g a i nst d ryi n g u p ; the l i m b s a n d c i rc u l a ­ tory system s o f reptiles a re g e n e ra l ly s u perior to those of a m p h ib i a n s . Reptiles u n d e rwe nt g re a t d ive rs ifica­ tion i n Mesozoic t i m es, d o m i n ati n g l ife not only on the land but a lso i n th e seas and i n t h e air. Th e i r de­ c l in e, sti l l not fully u n d e rstood, was m a rked by t h e ex­ p a n si o n of t h e i r d esce n d a n ts, the birds a n d m a m m a l s . 1 25

R h y ncoceph a l i a n s

Turtles .-

1it·y <

,,.,__ _ ,

',

��·

��.rd Plesiosaurs

Ichthyosaurs

J U RA S S I C

AQUATIC R E PT I LE S we re a b u n ­

d a n t i n the Mesozoic, as rep­ tiles m astered every major e n ­ v i ro n m e n t . Some were fi s h l i ke, oth ers rese m bled the later sea l s , and s ti l l oth ers were ser­ pentlike.

1 26

R E PTI LES had l i g h t , stro n g s kel eton a n d w i n g s , s u p ­ ported by a n e l o n g a ted fi n g er. So m e were s m a l l ; oth ers had 2 0 -foot wing s p a n s . T h ey were co n te m pora ries, b u t n o t a n ces­ tors, of early bird s . FLY I N G

lizards

��� -

Birds

Crocodiles

::::... Sauropods

d o m i nated l a n d D I N O SA U R S l ife for t h e 1 40 m i l l i o n years o f the Mesozoic. Aris i n g from the­ codont a n cestors, t h ey i n c luded two g roups with d istinct h i p structures : rept i l e - l i k e sauris­ chians and b i rd l i ke ornithis­ chians. Worldwide i n d i s tribu-

tion, t h ey were ada pted to m a ny d iffere n t enviro n m e n ts. They included herbivores a n d carn ivores a n d a l s o t h e l a rg est, most heavily a rmored l a n d a n i­ m a l s t h a t h a ve ever l ived. Reasons for their extin ction in the late Mesozoic are obscure.

1 27

ADAPTIVE R A D I AT I O N of repti les i n to fo rms a d a pted

to l ife i n d ifferent envi ro n m ents was p a r a l l e l e d by birds a n d m a m m a l s afte r the repti l es beca m e exti n ct. Adaptive rad iation occurs i n the early h istory of m a ny g roups, u s u a l ly fol lowed by m o re special ized a d a pta­ tions to niches with i n the wirler e nviro n m e nts . OCEA N S

FRESHWATER

EVO LUTI O N ARY C O N V E RG E N C E

i n form betwee n g e n etica l l y u n ­ re l a ted pe n g ui n s , d o l p h i n s , ich­ thyos a urs and s h arks res ults fro m a d a ptation to s i m i l a r e n ­ viro n m e n ta l co n d i t i o n s . I t i s a l so pres e n t i n m a n y other

1 28

-<

LAN D

g ro ups. A l t h o u g h e a c h of t h e mammalian tetra pod c l a s ses repre s e n ts a n e w or d i s ti n ctive ado ption to life in various e n ­ viron m e n ts , t h e t h ree " h i g h es t " c l a s s e s h av e each s u ccess f u l l y a d a pted to a l l e n v i ro n m e n t s .

C r o n e s, Rails, a nd A l l ies

['"•' "�/1 4-

A�

�-'' '"''

�

Loo n s

�

�- I't .{;
Grebes Ralites

Pe r c h i n g B irds

Climbing Birds

o n d R o l l e rs

B I R DS a re ra re a s fossils, beca use of t h e i r fra g i l e ske letons a n d because m a ny b i rd s l ive i n a re a s where b u r i a l cond ition s that l e a d to p reservation a re u n co m ­ m on . B i rd s s h a re t h e e g g - layi n g c h a racteristics o f rep­ tiles from wh ich they a rose, but the i m po rta nt, d isti n c ­ tive featu res i n t h e i r d iversification a n d su rviva l a re t h e i r s u p e rb a d a ptation to flyi n g , t h e i r ca re of the you n g , t h e i r fea t h e r cove r i n g , and their wa rm - blooded­ n ess . The histo ry of so m e i m po rta n t g ro u p s of b i rds is s hown a b ove .

1 29

EVOLUTION OF MAMMALS (T r i a s s i c to Rece n t) from t h e m a m m a l - l i ke therapsid repti l e s ( p . 1 24) i s well doc­

u m e nted i n the fossil record . S o m e fossi l s a re so tran sitio n a l i n c h a racter between the two g roups that there is doubt w h i c h they rep resent. Ma m ma l s a re typ i ca l ly cove red with h a i r o r fur, h ave d ifferenti ated teeth, a re wa r m - blooded, and h ave h i g hly d evel o p e d senses. Nea rly a l l m a m m a l s give b i rt h t o t h e i r you n g o n l y a fter a l o n g period o f protective e m b ryon i c d e­ velo p m en t with i n the moth e r ' s body, a n d then the mother fee 9 s the you n g m i l k secreted fro m her m a m ­ m a ry g l a n d s . These featu res a n d t h e h i g h l y d eveloped b r a i n s of m ost m a m m a l s m ust h ave been of m a j o r i m ­ porta n ce i n t h e evol utio n a ry success o f t h e g ro u p . Ma m m a l s ' reg ul ated b o d y tem pe rature e n a b l e them to survive i n a m u c h g reater e nvi ro n m ental ran g e th a n d i d the re pti l e s . THE

OLDEST

MAMMALS

were

s h rew-sized c reatures, a few of which reached about one foot in length. They a re kno�n from t h e i r tiny foss i l bones. These m a m m a l s re m a i n ed i n ­ conspicuous t h roughout the Mesozoic. A n cestra l fos s i l m a m m a l

Ec h i d n a

MO N O TR E M E S , l i ke the echidna (s piny a n teater) and platypus (p. 53), lay eggs and secrete sweat milk from modified g lands. T h e i r pri m itive repti l i a n c h a racteristics s uggest t h a t t h e y are a n a n c i e n t g roup, a n d they a re l i m ited to t h e Austra l ia area.

Pla typus

1 30

n u rs i n g you n g

SOUTH A M E R I CA

7�, -- ·· -rf •

'"�· . ....�\'1

-

�, �· .

L

t·.

NO RTH AM E R I CA

·

�­ !'

Mars u p i a l Carnivore

%: .

·r;r' t:�

Ca m e l - l i ke L i toptern

'(t�:�( .. \

Horse - l i k e Litoptern

Toxod o n t

j��� .

i1' "'

H o m a l o d o t h ere

MARS UPIAL MAMMALS (Creta­

ceous to Recent), s u c h as kan­ garoos and opossu m s , g ive b i rt h to very i m mature you ng that a re s h e ltered and fed i n s i d e t h e i r mother's pou c h . Ma rs u pials were widespread in South A m e rica i n the Ce nozoic, s h ow­ i n g many exa m p l e s of conver­ g e n ce (p. 1 28 ) w i t h placental m a m m a l s a s i l l u strated a bove . T h e j o i n i n g of N o rth a n d P LAC E N TAL MAMMALS i n c l ude

m ost l iving m a m m a l s . Because t h e e m b ryo is nourished a n d susta i ned b y the p l a c e n ta with i n t h e m o t h e r ' s wo m b, t h e n ew-

South A m e rica by t h e e m er­ g e n c e of the I st h m u s of Pa n a m a i n t h e l a t e Cenozoic e n d e d t h e isolation i n w h i c h t h e s e m a rsu­ pials h a d developed. Com peti­ tion with the better a d a pted North A merica n placentals re­ s u lted in the extinction of m ost m a rs u p ials. Ma ny have s u rvived in Austra l i a beca use of t h a t con t i n e n t ' s c o n ti n ui n g i s o l a t i o n . ( After S i m p s o n ) born a re m ore m a t ure t h a n a re m a rsupials, pres u m a b l y a n i m ­ po rta nt evol utionary adva ntage. The o l d est ( Cretaceous ) were s h re w l i k e i n sectivore s .

131

EARLY CENOZO I C MAMMALS s h owe d a period of ex­ plos ive rad iation, re p l a c i n g the Mesozoic ru l i n g reptiles i n a l most eve ry e nviro n m e nt. Reco n structio n a bove of scene some 50 m il l io n years ago i n c l u d e d a ncestra l le­ m u r Notharctos ( 1 ) , carn ivo res Oxyaen a ( 2 ) , Mesonyx ( 3 ) , hoofed m a m m a l s Palaeosyops (4), a tita no there (5 ) , and the a m blyods Eobasileus (6), Uinta­ therium (7), a n d Coryphodon ( 8 ) . Phe n a codus

ARCHAl C HERBIVOROUS, H O O F E D MAMMALS i n cl u ded

Phen acodus, an adva nced con­ d y l a r t h , with a l o n g ta i l , fi ve toes, a n d a carn ivore l i k e s k u l l . Co n te m pora ries were a m b l y ­ pods a n d u i n ta t h e re s ( a bove ) , w i t h teeth m o d i fi ed for chew­ ing v e g e t a tio n . T h e clawed toes of a n ces tra l fo r m s later beca m e m o d i fi e d to h ooves.

EVOLUTION OF C A R N I VO R E S (After Colbert)

UJ J z � UJ u u

J ) ' J )

o ,.... � � CL

�

u '

u w

� 5u u

�L. 0 UJ

ARC H A I C C A R N I V O R O U S MAM­ M A L S - th e creodo n ts - were

mostly s m a l l , s l e n d e r, long­ ta i l ed creatures. T h ey deve l ­ o p e d c laws, s h a rp teet h , a n d s u p p l e l i m bs. So m e rea c h e d the s i ze of l io n s . Most c reodonts beca m e e xt i nct in the Eoce n e . F r o m wease l - l i ke m e m bers of t h i s g ro u p t h e re s u bs e q u e ntly developed t h e a n cestors of l iv­ ing cats, dogs, a n d bears.

i n c l uded LATER C A R N IVO R E S a n ces tra l for m s of fl s s i pe d (s p l it­ foote d) cats, dog s , hye n a s , a n d weasels, a l l o f w h i c h a pp e a red a t d iffere n t t i m e s . We b - footed carn ivores (sea l s , wa lru ses) i n ­ vaded t h e ocea n s i n Miocene t i mes. T h e c l osely re lated ceta­ cea n s , i n c l u d i n g d o l p h i n s a n d whales, a p peared i n t h e Eoce n e a n d a re s u perbly a d a pted to m a r i n e l ife.

1 33

M O D E R N M A M M A L S a rose i n Eocene a n d O l i g ocene

ti m es. P l eistocene recon struction s h ows g i a nt g round sloth Megatherium ( 1 ) , b ison (2}, s a b e r-tooth cat ( 3 ) , horses (4), wool l y m a m m oth ( 5 ) , c a m e l - l i ke C a m elops (6), g lyptodo n t (7}, a h u g e beaver Castoroides ( 8 ), a n d g ro u n d sloth Mylodon ( 9 ) . MODERN

H O O FE D

MAMMALS

( U n g u la tes ) i n c l u d e od d - toed horses, tapirs , a n d rh i n os , a n d eve n - toed, clove n - h oofed cat-

tie, pigs, ca m e l s , and d eer, which d i s p l aced the earlier o d d - toed u n g u lates. C h a n ges i n veg etation h a d a n ef fect . Cottle

Camels

.>.'�'

P r i m i tive Ruminant

(t .

1 34

·�f:f(v

•

.��

;i.»..y·; \; C h evroto i n s FAM I LY T R E E O F EVEN-TOED U N G U LATES

P r i m a tes

I nsectivo re s

D e r m o pterans

,t!jf>��.'

�� .>. � . t' ·

C h i ropterans

Pyrotheres

Adaptive rad i a ti o n of place n t a l m a m m a l s .

( After Col bert. )

Ra d ia t i o n of m a m m a l s i nto every e nvi ro n m e n t is typified by t h ei r m a stery o f t h e a i r a n d t h e ocea n s as wel l a s t h e l a n d . Bats a n d a n cestral w h ales both a p ­ pea red i n the ea rly Tertiary. On t h e l a n d , specia l i z e d m a m m a l i a n g ro u p s · d eveloped . Rodents a n d rabbits a d a pted to a variety of food s a n d ways of l ife, i n cl u d ­ i n g bu rrowi n g . Prim ates, m a ny a d a pted t o l ife i n t h e trees, a rose ea rly i n t h e Tertia ry. Elepha nts a n d e d e n ­ totes (sloths a n d a r m a d i l los) represent fu rther special­ ization i n a d a ptati o n s .

135

EVI DE N CE OF EVO LUT I O N of o n e species i n to a n ­

o t h e r over geo l o g i c t i m e i s p rovi d e d by m a n y m a m ­ m a l i a n g rou p s . Two typica l exa m p l es a re g iven h e re . TITA N OTHERES were a g rou p o f large Te rtiary m a m m a l s . Their evo l u t i o n a ry deve l o p m e n t is shown i n h i storical seq u e n ce

below. Other for m s a l so e x­ i sted, a n d t h e i r evo l u t i o n i s a l so we l l documented. (After Os­ born.)

B rontotherium

Ill c Ill

platyceras

v

0

-�

6

4; � _g

_,.,

*"

Brontotherium leidyi

t

y

cr.;.;• � � .� � --, .

Ill c Ill v

Dolichorh inus

0

w 4;

hyognathus

a. a. ::>

'\

Ma nteoceras ma nteoceros

Ill -o ·-o -

Ill c Ill v

0 ::E w

�{'/-�.; .

.·

\ . . ..· .

·

·1. ; t:::;t �>

Ill c Ill

v

0

w

Lambdotherium popagilum

•

,.,"),<

Mesatirhi � us petergon1

;;f

....

��·� �

Eotitanops

Eotitanops

prin ceps

gregoryi

EVO LUTION O F PROBOSCI D EANS, G R EATLY S I M PL I FI ED. ( A FTER OS BO RN . )

E le p ha s

Pleist.- Rec .

Stegodon

P l i o .- Pieist.

P l io .

Mammut

M i o .- P i i o .

Gomphotherium Moe ritherium

Eo c . - O i ig .

Palaeomastodon

Olig.

Mio.-Piio.

1 37

G E O G RAPH I C D I ST R I B U T I O N of livi n g m a m m a l s re­

fl ects the pattern of i n terco n nection betwee n conti­ n e nts during the geologic past. E U ROPE, ASIA, AMERICA were

AND

N ORTH

co n n e cted for m u c h of Cen ozoic t i m e , a l low­ ing m ig ra tion a n d e x p l a i n i n g m a n y si m i l a ri ties o f t h e i r pre s ­ e n t fa u n a s . T h e d i fferen ces t h a t do e x i s t reflect differi n g c l i ­ m a ti c e n v i ro n m e n ts a n d rece n t develo p m e n t o f desert a n d

m o u n ta i n ba rriers to m i g ra t io n . The fa u n a s o f South A merica, Austra l ia, a n d Africa south of the Sahara a re q u ite disti n ct. These conti n e n ts have been separated fro m o n e a nother throu g hout t h e Cenozoi c. North Africa n m a m m a l s a re m ore s i m ­ i l a r t o those o f Europe.

Reindeer

Bison

W i l d Horse PlAEARCTIC

Hedgehog

.. .

...

O R I ENTAl

I n d ia n E l e p h a n t

Flying Phalanger

1, -t�-

••

..

..

Marco Polo Sheep

Koa l a AU STRA l i A N

K a n g a roo

I S O LATION of South America a n d Australia p rod uced very d ifferent ma m ma l i a n fau nas, i n wh ich marsupials were a t fi rst the dom i n a nt for ms. They re­ main abundant in Austra l ia be­ cause of its conti n u i n g isola­ tion. I n tercon nection of North and South America i n the late Tertiary led to com petition and exti nction of m a n y South America n p l a ce n ta ls. Convergent evolution i n ex­ ternal form between North

American placental m a m mals and fossil South Am erica n mar­ supials (p. 1 3 1 ) de monstrates the i nfluence of natural selec­ tion in ada ptation to s i m ilar modes of l ife. Geog ra phic distribution of other a n i m a l g roups does not n ecessa rily s h ow s a m e bou n d ­ aries a s m a m m a ls . Pla n ts a n d m a ri n e i n vertebrates, f o r e x ­ a m ple, have q uite differe n t dis­ persal m e a n s , a n d therefore different d istribution pa ttern s .

Mou n ta i n Goat M u s k Ox

N EA RCTIC

Caribon

Porc u p i n e

Pro n g h o r n A ntelope

K i n k a j ou Howler Monkey

Capybara

1 39

Arboreol i n sectivores

Relation s h ips between the m a i n g ro u ps of pri m a tes. bert. )

PRIMATES a re t h e m a m m a l ia n

( After Co l ­

o r d e r to w h i c h l e ­ m u rs, ta rsiers, m o n keys, a pes, a n d m a n be lon g . They ten d to be rather ra re a s fossils, l a rgely because of their c h a r a cteristi c a l l y a rborea l h a bits . Most prim ates show two fu n d a m ental a d a ptations to t h e i r tree-dwe l l ­ i n g existen ce : stereoscopic v i s i o n a n d h a n d s c a p a b l e of g ra s p i n g . These two features, present i n a l l but the m ost p r i m itive m e m bers, a l l ow the p ri m ates to j u d g e d i stan ces accu rately a n d t o swi n g fro m b ra n ch to bra n c h . They were a lso i m porta nt, together with his l a rge brai n , i n t h e d evelo p m e n t of g ro u n d - dwel l i n g m a n , a l lowi n g h i m t o develop i n crea s i n g s ki l l s i n m a k­ i n g a n d u s i n g tools.

1 40

PR O S I M I A N S (pre-monkeys) i n ­

clude living l e m u rs, aye-ayes, bushbabies, a n d the more mon­ key- l i ke tarsioids. They a rose in the Pa leoce n e , probably from arborea l i n sectivores, a n d be­ came d ivers ified d uring the early Tert i a ry. T h ey d e c l i n e d i n n u m be rs d u r i n g l a t e Tertiary t i m e s, proba b l y beca u s e of com petition from their des c e n ­ da nts, t h e a n t h ropoids. Prosi m ­ i a n s st i l l s u rvive i n s u c h places a s Madag asca r and Southeast A s i a . Prosi m ia n s have less w e l l ­ developed b i noc u l a r v i sion a n d g rasp i n g l i m b s t h a n ot her pri ­ mates.

Notharctos, an Eocene prosi m­ ian, a bout 1 8 i n ches h i g h .

i n c l u de mon­ keys, a pes, a n d men. They de­ veloped i n t h e O l i g oce n e a n d Miocene from pri m i tive p ros i m ­ i a n a n cestors . O l d World m o n ­ keys s h ow funda m e ntal diffe r· e n ces fro m those of t h e New World. Flat-nosed, pre h e n s i le-

ta i l e d South A m erica n form s, such a s m a r mosets, ca p u c h i n s , a n d spider m o n keys , see m t o be more pri m i tive. O l d World and New World m o n keys a rose i n depen d e n t l y fro m prosi m i a n s . Their s i m i la rities a re t h e res u l t of converg e n t evo l ution .

ANTHROPO I D S

P L E I ST O C E N E

-

Ne w World Monkeys

;\

�-

,_____

Australopithecus-Homo

o n key

. .

;,�

PI i o p i t h e c u s

l

O reop

"'!!!_ _ _ _ __j----

___ ________

- -

-

_

t

�

ec

�

- - - - -

l_o ;;;pith; us

APES

�

"

It

R a m a pithecus MEN

EVOLUTION OF P R I MATES (After McAlester)

1

f.l�� ,.,....

•� · · #,

.: •

•

�\u'!

!.

.

•

• • •

•

Gibbon

•

1)., . .

� •

.

• *

C h i m p a n zee

... Gori l l a

, Pon g i d a e

Hylobati d a e

(Apes)

RECENT

PLEI STOCE N E

Oreopithecidae

PLIOCENE

' ' ' ... , - - - - - - -- - - - ---

M I OC E N E

' ' '

' I

' '. - - - - - - - - - - ''- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ·'

(�' l \

\.,. .,. --

•'''�� : .;

.. ..

Pliopithecus

Oreopithecus

Dryopithec us

HOMINOIDS-a pes a nd men-a re cl a ssified together

i n a s i n g l e superfa m i ly, Hominoidea. They show fewer d ifferences fro m o n e another t ha n d o the O l d World fro m the New World mon keys, which a re i n sepa rate superfa mi l i e s . The h istory of the h o m i n o i d s a bove shows their possi b l e evol utio n a ry relatio n s h i p s as re­ vea led by s k u l l s a n d by d ental patterns. 1 42

.

.,. -,, ,,, , .. . ., I> ., ., .

Ora n g u t a n

Homo

Austra lopithecus Hominidae

(Men)

R a m a p ithecus . I I I . I I I

· - - - - - - - - - - - -'

I

·- - - - - - - - - - - - - - - - - - - - -

. I J

?

R a m a p ithecus

LIVI N G APES i n c l u de the ch i m­

panzee a n d gori l l a , w hi ch a re c h iefly g rou n d - l i v i n g forms, a n d t h e g i b bon a n d t h e ora n g uta n, which a re bea uti f u l ly ada pted to a rboreal l ife. All lack the typical ta i l of mon keys. Al l , e x ­ cept perha ps the g i bbons, seem to h ave arisen from g e nera l -

Austra lopithecus

iz:ed a pes that were wides pread in t h e Old World in M iocene and P l ioce ne times. Dryopithe­ cus (Proconsu/J, w h i c h i n c l u ded severa l forms most probab l y of apelike proporti o n s , may a l s o h ave given rise to t h e a n cestors of m a n . ( I l l u s tra tio n s a d a p ted from m a n y a u t h o rs . )

1 43

THE FAMILY O F MAN w h ich s p a n s t h e l a st 2 m i l l i o n

yea rs, i n c l u d e s t h ree g e n e ra . Two of t h e m a re n o w ex­ t i n ct. Because of the ra rity of pri m ate fossils a n d per­ haps a l so because of the i nten s e i n terest in the o ri g i n o f m a n , there i s so m e d i s a g re e m e n t con ce r n i n g t h e deta i l ed relatio n s h i ps o f particu l a r species wit h i n t h e broad patte r n o f evo l uti o n a ry develo p m e n t . N ew d is­ coveries a re sti l l bei n g m a d e, a n d the p rovisio n a l ac­ cou n t g iven here m ay well req u i re l ater m o d ification . RAMAPITHECUS, a

sti l l poorly known h o m i n id, has bee n fou n d i n late Miocene a n d Pl io­ cene rocks of I n dia a n d Africa. The pa.ttern of its teeth s hows a rather s mooth s e m i c i rcular outli ne, which is far more si m i­ lar to that of livi ng m a n t h a n to the quadrate pattern o f the

apes (p. 1 4 2). Little is k nown . about other pa rts of t h e s kele­ ton ; a n d because of this, the reco n struction shown below is very te n ta tive. But the tooth pattern i s so " h u m a nistic" t h a t i t seems p robable t h a t Ramapith­ ecus wa s close ly re lated to modern m a n .

(Southern­ ape) i s a lso regarded a s closely re lated to modern m a n , prob­ a b ly d i rectly a n ces tra l to t h e genus Homo t o which w e as­ sig n o u r. own species, Homo AUSTRALO P J TH E C U S

sapiens.

A u s t ra lo p i t h ec i n e s , o n c e w i d e­ sp read i n Afri ca, a re now t h o u g h t t o i n c l u d e two s p e c i e s ( p . 1 4 3 ) . T h e y were g ro u n d d w e l l e r s a b o u t 4 f e e t ta l l . A l re a d y , how­ ever, t h ey had a n u p r i g h t pos­ t u re . Bones f o u n d w i t h t h e i r re­ m a i n s s u g g est t h a t t h ey were ca r n i vores, but t h i s i s not cer­ ta i n . In d e n ta l pattern a n d i n g e n e r a l s k u l l fo r m , t h e y were

very m a n l i ke, desp ite their rat h ­ e r p rotru d i n g jaws a n d brow ridges. T h e i r bra i n capac ity (about 600 cc) was only half that of modern man. There is sti l l s o m e dou bt w het her crudel y c h i pped stone tool s associated with fossi l de­ posits were made and u sed by a us tra lopi t h ec i n e s or by t h e i r desce n da nts a n d u l t i m ate con­ te m poraries, Homo erectus. Austra lopithecines beca m e e x­ tinct a bout h a l f a m i l l io n years ago. Rece nt d is cove ries in Ke nya suggest t h a t early forms m ay date back a s far a s 2.6 m i l l io n yea rs.

1 45

W H AT I S M A N ? T h i s question is s u rp ri s i n g l y d iff icu l t t o a n sw e r w h e n a p p l ied t o fos s i l s . I t see m s b ette r to restrict t h e term " m a n " to o u r own s p e c i e s , Homo s a ­ piens, a n d to re g a rd o t h er c l o s el y r e l a ted fo r m s a s p re­ h u m a n , t h o u g h so m e of t h e s e m a n l i k e c reatu res d i d s h a re t h e h u m a n c h a racte r i stic o f too l m a n ufa c t u r e . Mod e r n m a n a p pea red a bo u t 5 0 0 , 0 0 0 yea rs a g o . HOMO ERECTUS i s known fro m fos s i l s fou n d i n t h e Ple istoce n e sed i m e nts ra n g i ng fro m about 750,000 to 200,000 years in age. Though sometimes de­ scribed by other names ( m ost co m m o n ly Pithecanthropus}, i n ­ d ividuals of the species a re k n own from Java , C h i n a , Af­ rica, and Asia. H . erectus was an erect, g rou n d - dwe l l i n g i n d i ­ vidual w h o fash ioned various

tool s a n d was a pparently a h u nter. Ma n l i ke i n stru cture a n d i n a p peara nce, H . eredus h a d a b ra i n capac ity of 900- 1 1 00 cc, i n termed iate between that of A ustrafopithecus and modern man. H. erectus was a contem­ pora ry a n d perhaps a com peti­ tor of l a te r a u s tralopithec i n es from w hose e a r l i e r m e m bers "he" evolved. H. e re cf us used fire and led a com m u nal life.

MODERN MAN, Homo sapiens, seems to have a risen from H . erectus. For a l m ost 200,000 years, t h e two s pecies were con te m poraries. Modern man is cha racterized by less cons piCu ­ o u s brows a n d j a w s t h a n t h e earlier h o m i n i d s a n d h a d a m uc h larger b ra i n ( av. capa­ city about 1 3 5 0 cc ) . Severa l races were i n vo lved in the fos s i l h istory of m a n . Ne·

g a rded as a race of o u r own species, l ive d t h roughout Eu­ rope , the Med iterra nean area, and parts of Asia Minor from about 1 1 0,000 to 35,000 years ago, a period that i n c l u ded t h ree e pisodes of g l aciation. Nea nderthal m e n l ived in caves a n d were s k i l lf u l too l m a kers a n d h u nters. T h ey were n ot t h e s t u p i d brutes t h ey a r e often pictu red as being. MAN replaced Nea n dert h a l Ma n i n Europe about 35,000 yea rs ago, prob­ a bly m i g rati n g fro m the Mid­ d l e East. Physically s i m i la r to modern m a n , Cro- M a g n o n m a n m a n u fa c t u red s u perior tools and prod u ced m a sterpieces of art and scu l p t u re ( p . 1 4 9 ) .

CRO-MA G N O N

a n dertha l m a n , l o n g regarded a s a d isti nct species, was a race of h eavy- browed , m uscular in­ d ividuals. The later C ro-Magnon race had facial features that more c l osely rese m bled those of modern m a n . Alth o u g h these d ifferences are rea l , t h ey seem a n a logous o n ly to those of l iv­ ing human races, betwee n wh ich i n terbreeding freq u e ntly ta kes place.

T H E EVOLU T I O N O F TOOLS. We a po n s, societies, a n d

c u l t u res a ri s e fro m a n d reflect m a n ' s p hysic a l a n d m en t a l evo l utio n . B i n o cu l a r vision , m a n u a l d exterity, a n d i n crea si n g m e ntal capa city were p a ra l leled by t h e i n crea si n g perfectio n o f m a n ' s work a s a craftsm a n . T H E O LD E S T TOOLS were prob­ ably used b u t not made, con­ sisting of stones a n d bou lders conve n i e n tly shaped by natu re. La ter too ls were crudely c h i pped and s h a ped axes and scra pers . These were gradually s u pple­ m e nted and replaced by d e l i ­ cately c h i p ped b l a d e s a n d ar­ rowheads of a va riety of mate­ r i a l s , i n c l u d i n g bone.

About 1 0,000 years a g o, t h e early c u l t u re of c h ipped i m ­ p l e m e n ts, t h e Pa l eo l i t h ic, g a ve way in E u rope to the Neol i t h ic, w h i c h was m a r ked by f i n ely g rou n d and pol i s h e d too l s a n d wea pons. About 5 ,000 yea rs ago, man f i rst learned to fa s h io n i m p l e m e n ts of m e ta l . Th e "sto n e a ge" sti l l pers i sts a m o n g s o m e l iv i n g peo p l e s . SCRAPER Mousterian

HAND AXES Acheulian

SPOKESHAVE A u ri g n a c i a n

BONE A N D A N T L E R W E A PONS Magdelinian

W E A PON H EADS So l u trean

M A M M O T H C A RV I N G Magdelinian

C U LTURAL EVOLUTI O N of m a n

i s g l i m psed i n c ave p a i n t i n g s a n d carv i n g s t h a t d a t e t o about 2 8 ,0 0 0 yea rs ago. Both a re c h iefly depicti o n s of h u n t i n g a n d ferti l i ty. T h e y m ay h a ve had " m a g i ca l " s i g n i fi ca n ce .

Ma n 's a n c i e n t belief in s u r­ vival a fter death is s h own by N ea n d e rt h a l a n d C ro-Ma g n o n s ke l e to n s b u ri e d i n feta l o r s l e e p i n g pos i t i o n s , w i t h i m p l e ­ m e n ts a n d to k e n s to be u s e d i n the new life.

149

EVO L U T I O N O F H U MAN SOCI E T I E S a rose fro m m a n ' s g rowing a d a ptation to h is e nviro n m e nt. Such l a n d ­ m a r ks a s t h e d iscovery a n d u s e o f fi r e b y Homo erec­ tus a n d the d eve l o p m e n t of crop cultivation , a n i m a l h us b a n d ry, a n d pottery b y Neolithic m e n prod uced ra d i cal c h a n g es in the patter n s of h u m a n life. Man , ori g i n a l ly a n o m a d i c h u nter a n d herbivore, cou l d then constru ct dwe l l i n g s a n d gather into g roups who estab­ lished settl e m e nts. The need for com m u n i cation fostered the d evelop­ m en t of in creasi n g ly sop h i sticated l a n g u a g e . The g row­ i n g size of co m m u n ities n ecessitated the d ivision of the various ta sks involved i n s u rviva l and the creation of some fo rm of govern ment. We know l ittl e of t h e de­ tailed d evelo p m e n t of a n y of th ese, for writi n g was not i nvented until a bout 5,000 yea rs a g o . Early re­ cord ed history i s very patc hy, bei n g i n fi n itely m o re co m pl ete for some pa rts of the world, such a s Egypt, t h a n for others . Th e earli est societies a n d thei r u nwrit­ ten l a n g u a ges, u n l i ke the i m p lements of ea rly m a n , l eft no records i n t h e sto n e .

1 50

TH E M EAN I N G OF EVOL UTI O N Even before D a rwin publ ished The Origin o f Spe­ cies, s o m e rel i g ious l e a d e rs atta c ked t h e concept of evo l ution beca use t h ey t h o u g h t it t h re a t e n e d t h e i r view p o i n t; o t h e rs h a ve e m b ra ced it a s a n ew i n si g h t i n to t h e wor k o f G o d i n t h e created worl d . Evo l utio n ­ a ry t h eorists h ave c l a i m e d evo l u t i o n a s a j u stifica tion for m i l ita nt politi c a l tactics; others h ave endorsed it as i l l ustrati n g t h e i n evita b i l ity of h a rmonious political de­ velo p m ent. So m e eco n o m ists h ave clai m ed it as an ar­ g u m en t for la issez-faire eco n o m i c policies, while a few scientists h ave u sed it a s a basis for a n ew code of ethics. So m e po p u l a r writers, atta c k i n g trad ition a l re­ ligious belief, h ave a d o pted evol utio n i s m as a n ew re­ l i g i o n . Se l d o m h a s a scientific theory so q u i ckly beco m e a l l t h i n g s t o a l l m e n . Seldom h a s a n atu ral pro cess been so ca relessly used as a n expository basis for the whole pattern of h u m a n life . Contem porary ca rtoon and verse i n d icates Victorian i n ter­ est i n the m ea n i n g of evo l u tion •

.

A m I Satyr o r M a n ? P r a y te l l m e w h o c o n , A n d sett l e m y p l ace i n t h e s c a l e , A m o n i n a pe' s s h a pe , An a n t h ropoid a p e , Or m o n ke y d e p r i ved o f h i s ta i l ? T h e Ves t i g es ta u g h t , That all came from naught By "deve l o p m en t , " so c a l l e d , ' ' prog ress i v e ; " T h a t i nsects a n d w o r m s Ass u m e h i g h e r f o r m s By m od i f i c a t i o n e x cess i v e , T h e n D A RW I N set f o rt h , I n a book of m u c h w o r t h T h e i m p o rta nce of " N a t u re ' s s e l e c t i o

M O N K E Y A N A.

V i ctor i a n ca rtoon d e p icts a pu zzled Darw i n a n d h i s a n cestors.

The I m p l i cations of Evo lution The proces s of evo l u tion i s a fact. N u m e ro u s l i n e s of evi ­ dence i n d i cate t h e desce n t of new s pecies by m odification of a n cestral form s over extended perio d s . A l t h o u g h t h e m ech a ­ n i s m i s s ti l l t h eore t i c a l , t h e re is very s tro n g evidence t h a t n a t u ra l s e l ecti o n , g e n e t i c v a r i ­ a t i o n , a n d i s o l a tion a re t h e c h ief co m po n e n ts . ( p. 1 0 2 ) Evol u t io n , l i k e a n y other nat­ u ra l process o r scientific t h e­ ory, i s t heolog ica l ly n e u tra l . I t d e scri bes m e c h a n i s m s, b u t n ot m ea n i n g . I t i s based u po n t h e recog n i ti o n of order b u t i n ­ corporates n o concl u s io n co n ­ cern i n g t h e o ri g i n o f t h a t or­ der a s either p u rposef u l or p u rpos e l e s s . A l t h o u g h evo l u ti o n i n vo lves t h e i n terpret a t i o n of n a t u ra l eve n ts b y n a t u ra l processes, i t

1 52

n e i t h e r a s s u m e s n o r provides parti c u l a r co n c l u s i o n s co ncern ­ i n g t h e u l t i m a te s o u rces or t h e s i g n i fi c a n ce of m a teria l s , events o r processes. Evo l u t i o n p rov ides no ob­ concern i n g co n c l u s i o n s vious p o l i t i c a l o r eco n o m i c syst e m s . Evo l u t i o n n o m o r e s u pports evo­ l u t io n a ry pol i t i cs (whatever t h e y m i g h t b e ) t h a n d o e s t h e Seco n d L a w o f T h e rmodyna m i cs s u p­ port political d i sorder or eco­ n o m i c c haos . Evo l u ti o n offers no b a s i s for ethics. It i s n o t s e l f -ev i d e n t t h a t s u rviva l i s t h e h i g h e s t good a n d that any mea n s of i t s a t ­ ta i n m e n t i s v i rt u o u s . T.H . H u x ­ l e y w rote " T h e e t h ica l pro g ­ res s of society depen d s , n o t o n i m i ta ti n g t h e co m i c proces s , s ti l l less i n r u n n i n g away from i t, b u t i n co m ba t i n g i t . "

fo r man, h e n ce is a s i g n ifi c a n t contri bution to h u m a n u n der­ sta n d i n g . Reco g n iti o n of the i m m ensity of the spa n of geolog i c ti m e , t h e aweso m e scale of cos m i c d i m e n ­ s i o n s a n d p ro cesses i n volved i n t h e l o n g period o f p re­ o rg a n i c evo l ution, a n d the p l a ce of m a n h i m self wit h i n the e n d l ess d iversity o f t h e tee m i n g life on t h e fra i l su rfa ce o f o u r p l a n et-a l l these hel p t o e n l i g hten a n d susta i n m a n a s he faces the c h a l l e n g e, the d i l e m m a , a n d t h e mystery o f h i s h u m a n con d ition . EVOLUTION

PROV I D ES

A

PERS PECTIVE

Ma n k i n d , t h e p ro d u c t of o r g a n i c e vo l u t i o n , i s n o w te c h n i c a l l y e q u i p p e d w i t h p o w e r , i f n o t t li e wi l l , to c o n ­ t r o l t h e f u t u r e d e ve l o p m e n t of l i fe o n e a rt h . P s y c h o so c i a l evo l u t i o n

h a s n o w d i s p l a c e d t h e o l d e r p ro c e s s e s of

o rg a n i c evo l u ti o n i n h u m a n co m m u n i t i e s . K n o w l e d g e , t r a d i ti o n s , va l u e s , a n d s k i l l s a r e n o w t ra n s m i tt e d fro m o n e g e n e r a t i o n to a n o t h e r t h ro u g h b o o k s a n d te a c h i n g i n st i t u t i o n s

rath er

than

being

learned

a n ew

"fro m

s c r a t c h " by e a c h n ew i n d i v i d u a l .

Psyco-Social

E a c h seg m e n t

Evolution

represents a p p ro x i m a t e l y 5 0 m i l l io n years

of Earth Appearance of oldest known foss i l s

1 53

THE FUTUR E EVOLUTION OF MAN, of other species,

a n d perhaps of the whole i ntricate ecosystem of which we are a p a rt now sta n d s i n jeopa rdy. The pol l ution of the atmosph ere on which o u r existe n c e depends h a s now reached a c r i s i s of m a j o r p roportions in most i n ­ d ustri a lized a reas o f t h e worl d . Rapidly dwi n d l i n g re­ serves of s uch essential co m m od ities as petro l e u m a n d ma ny m etals th reaten t h e future not o n ly of i n d ustria l pro d u ction but a l so of technologically b a s e d society. A conti n u i n g explosion of h u m a n population, espe­ cia l ly i n the less i n d ustri a l i zed areas of the worl d , raises the awesome possi bil ity of wid es p read fa m i n e a n d sta rvatio n within the n ext 3 0 yea rs. Man a l ready possesses the tech n i cal power to solve these th ree m a j o r prob l e m s : poll ution, dwi n d l i n g m i n ­ era l resou rces, a n d overpopu lation . Whether h e has the wisdo m , t h e wil l , and the e n e rgy to solve them re­ m a i n s to be seen . It is i ro n i c that the future of the age­ lon g p rocess of o rg a n i c evo l ution m ay n ow depend on the conscious c h o ice of m a n, a p roduct of that process. The d a n g e r, the c h a l l e nge, a n d t h e choice i nvolve m a n­ kind i n a com m o n peril a n d a com m o n hope. Evo l ution p rovi des n o easy a n swers to m a n ' s long search for m e a n i n g a n d no i n sta nt sol utions to m a n 's most p ress i n g p ro b l e m s . It i s rather for m a n h i m self now to p rovi d e the i n put-i n the recog n ition of an eth i c beyo n d th a t of s u rviva l , of a p urpose b eyo n d that o f g a i n , a n d o f a v i s i o n o f l ife beyo n d that of m e c h a n i s m a n d p rocess. On such col lective com m it­ ments of m e n a n d of n ations depend the s u rvival of m a n k i n d a n d the future cou rse of evo l ution .

1 54

Po l l ution of a t m o s p h ere in i n d ustri a l areas is a worldwid e prob­ l e m . World fa m i n e poses an i n creasi n g ly s evere t h reat a s bur­ ge o n i n g population co m petes for l i m i ted res o u rces. World popu­ lation projections suggest g loba l population of 2 5 bi l l i o n by 2 0 70 . 2 5 b i l l i o n p e o p l e b y 2 0 70

World population q u a d rupled by 2044

1 6 1: ·a. 0 ..c:

12 � 10 5 ::: 14 U Q;

V)

Asia : q u a d rupled by 2040 Wor l d : d o u b l e d by 2 0 0 7 ; po s s i b l e fa m i n e Asia : doubled by 2 0 0 5

MORE

I N FORMAT I O N

T h e fo l l o w i n g l i s t of books i s o n l y a n i n trod u c t i o n to t h e vo l u m i n o u s l i te ra t u re on evo l u t i o n . Many m u se u m s a l so prov ide d i sp l a y s , ta l k s , a n d l i terat u re . H i stori c a l D a rw i n , C h a r l e s , T h e O r i g i n o f Spe cies, O x f o r d U n ivers i ty Press, N . Y . , 1 9 5 6 . ( 6 t h ed . , 1 8 7 2 re p r i n ted : T h e W o r l d C l a s s i c s . )

Gre e n e ,

J . C . , D a r w i n a n d t h e M odern World V i e w , Mentor Books, N . Y . , 1 9 6 3 . A . , D a r w i n a n d t h e S e a g l e , H a rper a n d Row , N . Y . 1 9 6 9 .

Moore h e a d ,

T h e Proce s s o f E v o l u t i o n D e B e e r , G . , A t las of E v o l u t i o n , N e l son a n d S o n s , l o n d o n , 1 9 64 . Mayr, E . , A n im a l Spec ies and Evolution , Oxford U n ivers i ty Press, N . Y . 1 9 6 3 . Moore , R . , Evolution , T i m e - L i fe, I n c . , N . Y . , 1 9 6 2 . S a v a ge ,

J . M . , E v o l u t i o n , H o l t , R i n e h a r t o n d W i n s to n , N . Y . , 1 9 6 3 .

Sheppard, P . M . , N a t u r a / S e le c t i o n a n d Heredity , H u tch i n so n , l o n d o n , 1 9 5 8 . S i m p s o n , G . G . , T h e Major Features o f E v o l u t i o n , Co l u m b i a U n ivers i ty Press,

N . Y . , 1 95 3 . Smith,

J . M . , T h e Th eory o f E v o l u t i o n , Pe ng u i n B o o k s , H a r m o n d s w o r t h , 1 9 5 8 .

Tax, S . , E v o l u t ion after D a r w i n , U n ivers i ty o f Ch i c a g o Press, C h icag o , 1 9 6 0 . T h e Course o f E vo l ut i o n C o l bert, E . H . , E v o l u t ion o f t h e Vertebrates, J o h n W i l e y , N . Y . , 1 95 5 . Rhode s ,

F . H . T. , T h e E v o l u t ion o f l ife , Peng u i n Books, B a l t i m ore, 1 9 7 4 . F . R . S h affe r, Foss ils, a G u ide to Pre h istor ic

Rhode s , F . H . T . , H . S . Z i m , a n d L if e , G o l d e n Press, N . Y . , 1 9 6 3 .

The E vo l ut i o n of M a n

T . , M a n k i n d E v o l v i n g , Y a l e U n iversity P r e s s , N e w Have n , 1 9 6 2 . F. C l ark, Ear ly M a n , T i m e - l i fe B o o k s , N . Y . , 1 9 6 5 . LeGros C l ark, W. E . , H istory of t h e Primates, B r i t i s h M u se u m of N a t u r a l H i s­

Dobzh a n s k y , Howe l l ,

tory, l o n d o n , 1 954; T h e Foss i l Ev idence of H u m a n E v o l u t ion , U n i v e r s i ty of Ch i c a g o Pre s s , C h i c a g o , 1 96 4 . O a k l e y , K . P., M a n t h e Tool-Maker, B r i t i s h M u se u m af N a t u ra l H i story, l o n ­ d o n , 1 96 3 . T h e Me a n i n g o f E v o l ut i o n Barbour, Lack, D . ,

I . G . , Issues in Science and R e l i g i o n , S C M l td . , l o n do n , 1 9 6 6 .

E v o l u t ionary Theory a n d Christian B e fief; th e Un reso lved Con f l i c t ,

Meth u e n a n d Co . , l td . , l o n d o n , 1 9 5 7 . S i m p s o n , G . G . , T h e M e a n i n g of E v o l u t ion , M e n t o r B o o k s , N . Y . , 1 9 5 1 . Te i l h ard de Chard i n , 1 95 9 .

P. , T h e P h e n o m enon of M a n , H a rper a n d B r o t h e r s , N . Y . ,

PHOTO C R E D I TS : p . 6 - 7 , To u r g o P h o t o Serv i ce; p . 1 0 , Bett m o n n A r c h ives; p . 1 2 , D ra w i n g by Sa m u e l l a w re n ce , p h o t o g r a p h repri n ted by perm i s s i o n of George Rai n b i rd , l td . , Bett m a n n A rch ives; p . 1 4 , Bettm a n n A rc h i ve s ; p . 1 5 , C u l v e r Pic­ t u res; p. 1 6 , George R a i n b i rd , l td . , p . 1 8 , Reyno l d H. C h ose; p . 1 9 , Rey n o l d H . C h o se; p . 2 0 , Bett m a n n Arc h ives; p . 2 2 , C u l ver P i c t u res; p . 2 3 , Bet t m o n n A r­ c h ives; p . 2 4 , U n i ted N o t i o n s ; p . 2 6 , Bettm a n n A rc h i ves; p . 2 7 , Bell m a n A rc h i ves; p . 2 8 , Victur A . M c K u s i c k ; p . 4 6 , T h e American M u se u m of N a t u ra l H i story; p . 4 7 , T h e A m e r i ca n M u s e u m of N a t u r a l H i s to ry; p . 7 2 , Photo Resea r c h e rs ; p . 1 1 0 , E . S . Borg h o o r n , ( H a rv a rd U n i v e rs i t y ) ; p . 1 5 1 , P u n c h M a g a z i n e 1 8 6 1 ; p. 1 5 2 , from Ev o l u t i o n a ry T h eory of C h r i s t i a n Be l i ef, p . 1 54 , U n i ted N o t i o n s ; p . 1 5 5 , U n i ted N o t i o n s .

1 56

I N DEX A c a n t h od i a n , 1 1 5 Acorn worms, 1 1 4 Ada m , 6 Ada p ta t i o n s , 2 5 , 3 3 , 4 2 , 54, 79, 8 1 ' 88 Agnatha, 1 1 4, 1 1 6 A l g a e , 5, 1 1 0 , 1 2 0 A l lele, 6 1 , 66 A m b l y pod , 1 3 2 A m i no acids, 70, 7 1 A m m o n i a , 1 05 , 1 06 A m m o n i te , 1 09 A m p h i b i a n , 1 1 7 , 1 1 9 , 1 24 A n a l o g o u s s t r u c t u re s , 3 9 An c o n s h ee p , 7 4 A n e m i a , s i c k l e - ce l l , 9 2 A n g i os pe r m s , 1 2 1 , 1 2 2 , 1 23 A n i m a l s , n u m be r o f , 5 o l dest, 1 1 0 A n ky l osa u rs , 1 2 7 A n t e a t e r , s p i n y , 53 , 1 3 0 A n t h ro p o i d s , 1 4 1 A n t i a rc h , 1 1 4 Ape, 1 40 , 1 4 1 , 1 4 2 , 1 4 3 , 1 44 A ra u c a r i a , 5 3 A r c h a eoptery x , 5 2 , 9 4 , 1 29 A r i s t o t l e , 7, 1 0 A r m ad i l l o , 1 3 5 A rs i n a t h e r e s , 1 3 5 Art h r od i re , 1 1 5 A rt h ropod s , 5, 1 1 2 , 1 1 8 , 1 19 A rt i f a c t s , 1 09 A r t i o d a c ty l s , 1 3 5 Astra p o t h e re s , 1 3 5 A t m o s p h ere, 1 0 7 ATP, 40 A u s t ra l i a , 1 1 0 , 1 3 8 , 1 3 9 A us t r a l op i t h e c u s , 1 4 3 , 1 45, 1 46 A y e - a ye , 1 4 1 Bacte r i a , 1 1 0 , 1 2 0 Bat, 1 35 B e a g l e , voya g e o f , 1 6 , 1 7, 1 9 Bea r , 8 6 , 87 Beaver, 1 3 4 Beri n g S t r a i t s , 77

B e r k e n d e r , L C . , 1 07 " B i g b a n g " t h eory, 1 04 B i rd s , 1 1 9 , 1 27 , 1 2 8 , 1 2 9 B irke n i a , 1 1 4 Bison, 1 34 B i ston b e t u la r i a , 47 Blending, 61 B l ood p i g m e n t s , 40 Bove r i , T . , 28 B ra c h i opod s , 1 1 1 , 1 1 2 , 1 1 3 B reed i n g , s e l e c t i v e , 4 4 B rya n , W i l l i a m J e n n i n g s , 30 B r y o p h ytes, 1 2 0 B ryozoa n , 1 1 2 , 1 1 3 Bush boby, 1 4 1 Coec i l i o n , 1 1 7 , 1 2 4 Co l o m o i t e s , 1 20 Co l o m oph y t o n , 1 2 1 Ca m b r i a n , 1 07, 1 1 1 , 1 1 2 , 1 13 Ca m e l , 1 2 6 , 1 3 1 , 1 3 4 Ca m e l ops, 1 3 4 Cope V e r d e I s l a n d , 4 3 Capuch i n , 1 4 1 C a r b o n , 3 5 , 97, 1 0 5 Ca r b o n d i o x i d e , 1 05 , 1 06 Ca r bo n i fe r o u s , 1 1 8 Carcho rodon , 1 1 5 C a r n i v o re s , 1 3 3 , 1 3 5 Carti l a g i n o u s f i s h , 1 1 5 , 116 Castoroides, 1 34 Cot, 1 33 , 1 34 C o to s t r o p h i s i s , 1 4 Cott l e , 1 3 4 Cove bea r , 8 6 Ce l l d i v i s i o n , 5 8 s t r u c t u re , 3 2 Ce n oz o i c , 1 1 6 , 1 2 9 , 1 3 1 , 1 3 2, 1 34, 1 3 8 Cep h a l op o d s , 1 1 2 , 1 1 3 Cerotops i o n s , 1 27 Cetacea n s , 1 3 3 , 1 3 5 C h o l i co t h e r e s , 1 3 1 C h a m e l eo n , 4 2 C h i asmata, 73 C h i m pa n zee , 1 4 2 , 1 4 3 Ch i ro pte ro n s , 1 3 5 C h oo n i c h t h ye s , 1 1 7 C h o n d r i c h t hyes, 1 1 5

C h o n d r i te s , c a r b o n a ce o u s , 1 05 C h ordata , 1 1 4 C h o rd a t e s , 5 C h r o m a t i n , 67 Ch r o m o s o m e s , 2 8 , 2 9 , 5 6 , 57, 58, 59, 63, 65, 66, 67, 73, 74 C lodose loch e , 1 1 5 C l o m s , 4 9 , 1 09 , 1 1 9 Classificat i o n , 36 Climati us, 1 1 5 C l u b m o s s , 1 20 , 1 2 1 C o c k r oa c h , 1 1 8 Cocos I s l a n d , 83 C o l o r b l i n d n e s s , 66 C o m m u n i t i e s , 35 C o n dy l o rt h , 1 3 2 , 1 3 5 C o n i fe r s , 5 , 1 2 1 , 1 2 3 C o n v e rg e n ce , 1 2 8 C o ra l s , 1 1 2 , 1 1 3 C o rd a i te s , 1 2 1 , 1 2 3 Coryph odon , 1 3 2 C o t y losaurs, 1 2 6 C reod e n t , 1 3 3 Cretaceo u s , 5 0 , 1 2 3 , 1 2 9 , 1 3 1 ' 1 32 Crocod i l e s , 1 2 7 C r o - M o g n o n , 1 47 , 1 49 C r o s s i n g - ov e r , 7 3 Crosso pte ryg i a n , 9 4 , 9 5 , 1 1 6 , 1 1 7, 1 2 4 C r u stacea n , 1 1 9 C u l t u re , e v o l u t i o n o f , 1 49 Cuvier, Georges, 1 4 C y c a d s , 1 20 , 1 2 3 C y n o g n a t h us, 5 3 D a r w i n , C h a r l es , 1 5 , 1 6 , 1 7, 1 8 , 1 9, 20, 2 1 , 22, 23, 24, 25, 28, 42, 43, 44, 80, 82, 91 ' 1 51 D a r w i n , E ra s m u s , 1 2 , 1 3 Deer, 1 34 Demes, 4 5 , 76 Dem ocrit o n , 7 D e r m optero n s , 1 3 5 Devon i a n , 5 5 , 1 1 2 , 1 1 4 , 1 1 5, 1 1 6, 1 1 7, 1 1 8, 1 1 9, 1 2 1 , 1 22 , 1 24

1 57

de V ri e s , 27 D io t r y m o , 1 2 9

D i c y n od o n t s , 1 2 6 D i n os a u rs , 37, 50, 1 09 , 1 26 D i p l oid, 57 D ip loverte bron , 95 D i s t r i b u t i o n , g e o g ra p h i c , 43 D N A , 5 6 , 6 7 , 6 8 , 6 9 , 70, 74 Dobz h o n sky, 8 1 Dog , 1 2 3 Do l p h i n , 1 2 8 , 1 3 3 Dreponospis, 1 1 4 Drepo n i d i d s , 8 8 , 8 9 D r os op h i l a , 2 8 , 2 9 , 8 1 Dryop i t h e c u s , 1 4 2 , 1 43 D u n k loste us, 1 1 5 E a r t h , 9 6 , 9 7 , 1 04 , 1 0 5 , 1 07 E c h i d n a , 5 3 , 1 30 E c h i n ode r m , 1 1 2 , 1 1 4 Ecosyste m , 3 5 Edentotes, 1 3 5 E g g s , d ev e l o p m e n t of, 57 Elephants, 1 35 E m bryo n i c deve l o p m e n t , 38 Eobos i l e u s , 1 3 2 Eocene, 1 3 3 , 1 4 1 Eospe m o top t e r i s , 1 2 1 Eryops, 1 1 9 Esch e r i c h ia c o l i , 4 6 Essa y on Pop u la t i o n , 2 0 E u rypte r i d s , 1 1 2 , 1 1 3 E u s t h e n op t e ro n , 95 Evo l u ti o n , 2 3 a n d f u t u re of m a n , 1 5 4 c o u rse of, 1 05 i nd i c a t i o n s of, 3 0 patte r n s o f , 1 0 2 perspec t i v e prov ided by, 1 53 p r e - o rg a n i c , 1 04 processes of, 5 6 - 1 OJ role, 1 00 s y n t h e s i s t h eory of, 2 8 Fe r n , 5 , 1 20, 1 2 2 t ree, 1 2 1 Fiji, 43 F i n c h e s , Ga l a p a g o s , 1 9 , 82, 83, 88

1 58

F i s h , 1 1 4, 1 1 5 , 1 1 6, 1 1 7 F i s s i pe d s , 1 3 3 F l ower i n g p l a n t s , 5, 1 2 2 , 1 23, 1 32 F o ra m i n i fe r a , 4 8 Fossi I s , 48- 5 3 , 86 living, 53 o l de s t , 1 1 0 rec o r d , 5 4 types of, 1 08 , 1 09 F rog , 1 24 F u c us, 1 20 F u n g i , 5, 1 20 G a l a pa g o s I s l a n d s , 1 8 , 1 9, 43, 82 G a m etes, 5 7 , 63 , 7 5 G e n e s , 5 6 , 60, 6 3 , 6 6 , 67, 72, 74 Genes i s , Book of, 6 G e n e t i c d r i f t , 75 G e n o type, 6 1 , 65, 7 3 Geo l o g i c t i m e s c a l e , 98-99 Gi bbon, 1 42 , 1 43 G i n kg o , 5 3 , 1 2 1 , 1 2 3 G l ossople r i d s , 1 20 G lyptoda n , 1 7, 1 3 4 G o r i l l a , 1 4 2 , 1 43 G rad u a l i s t , 1 4 G y m n osperm s , 1 2 3 H a p l o i d , 57 H a rd y - We i n berg p r i n c i p l e , 65 H a wa i i , 8 8 He l i u m , 97, 1 0 5 Helix, 1 1 9 Hemop h i l ia , 66 Henderson I s land, 43 H e r b i vores, 1 3 3 H e reford c a tt l e , 74 H e spe rorn i s , 1 2 9 Heterozyg o u s , 6 1 H o m o l d o t h ere, 1 3 1 H o m i n oi d s , 1 4 2 H o m o e r e c t u s , 1 4 5 , 1 46 , 1 47 , 1 50 sap iens, 1 45 , 1 4 6 , 1 47 H o m o l o g o u s s t r u c t u res, 39 H o m ozyg o u s , 6 1 H o r s e s , 5 1 , 1 00 , 1 0 1 , 1 3 1 , 1 34 H u tt o n , J a m e s , 1 5

H u x ley, T . H . , 2 3 , 1 5 2 H y d rog e n , 3 5 , 1 04 , 1 0 5 , 1 06 H y d rosp here, 1 0 7 Hyena, 1 33 Hyro c o i d s , 1 3 3 H yrocot h e r i u m , 5 1 l ch t h yorn i s , 1 2 9

I c h thyosaur, 1 26, 1 28 l c h t h yostego, 9 5

l c h t h yosteg i d s , 9 5 , 1 24 I cterus, 3 6

I g uana, 1 8 I n g ro m , Ver n o n , 9 3 I n heritonce, 56 lows of, 2 6 , 6 2 mechan i s m , 27, 66 patte r n s o f , 6 0 p r o b a b i l i t i es , 6 3 , 6 4 varia bi l ity, 72 I n sectiv ores, 1 3 5 , 1 4 1 I n s ects, 1 1 8 I s l a n d s p ec i e s , 4 3 I solation, 1 39 genetic, 76 geog rap h i c , 1 9 , 7 6 J e l l yf i s h , 1 1 0 , 1 1 2 , 1 1 3 J u p i te r , 1 05 J u ra s s i c , 5 2 K a n g a ro o , 1 3 1 Kett l e we l l , H . B . D . , 8 5 K u r te n , B . , 8 6 L a byri n t h od o n t , 9 5 , 1 1 7 , 1 24 L o c k , D a v i d , 83 Logom orphs, 1 3 5 Lam arck, 1 2 , 1 3 L oncelets, 1 1 4 L ea d , 97 l eo d -.t h or i u m , 9 7 L e m u r, 1 40 , 1 4 1 L e p i d od e n d r o n s , 1 20 lepos p o n d y l e s , 1 1 7 L i fe , a b u n d a nce of, 4 , 5 adapta t i o n s , 3 3 b i oc h e m i c a l s i m i l o r i ti e s , 40 ce l l s t r u c t u re , 3 2 c l assification o f , 1 0 c o n t i n u i ty of, 3 1 , 44 dev e l o p m en t o f , 6

d i ve rs i ty of, 5, 1 2 e v o l u t i on o f , 1 07 g rowt h , 3 2 h i story, 1 4 i n terdepe n d e n c e , 3 5 m e t a bo l i s m , 3 2 n a t u re of, 3 4 on land, 1 1 8 o r i g i n of, 8 , 1 8 , 1 06 protop l a s m , 3 2 reprod u c t i o n , 3 2 rese m b l o n ce , 3 6 , 3 8 sero l og i c a l s i m i l a r i t i e s , 41 s i m i l a r i t i es o f , 3 6 , 3 8 u n ity o f , 3 2 variation of, 2 4 , 44, 4 5 , 7 2 , 7 3 , 75 l i g h t , s pectros c o p i c o n o l y s i s of, 1 05 L i n g u la , 1 00 l i n n o e u s , 1 0, 1 1 l i n n o e o n S o c i ety, 2 2 l i ptopte r n , 1 3 1 l i verwort, 1 20 l izards, 1 27 l o b ef i n , 9 5 , 1 1 7 , 1 24 l o be l i a , 8 8 , 8 9 l u ngfish, 1 1 6, 1 1 7 lycopod , 1 2 1 , 1 2 2 Lye l l , C h a r l es , 1 5 , 1 6 lyre b i rd , s u p e r b , 9 1 Ma l a r i a , 9 2 , 9 3 Mo l t h u s , R o b e r t , 2 0 , 2 1 Mam m a l s , 1 1 9 , 1 2 8 , 1 30 , 1 3 1 ' 1 3 2 , 1 3 3 , 1 34 , 1 3 5 , 1 3 6 , 1 37 , 1 3 8 , 1 3 9 , 1 40 , 1 4 1 , 1 4 2 , 1 43 Ma m m ot h , woo l l y , 1 0 8 , 1 34 Ma n , 1 40 , 1 4 1 , 1 4 2 , 1 44 , 1 4 5 , 1 46 , 1 47 , 1 4 8 , 1 49 , 1 50 , 1 54 Marm oset, 1 4 1 Marsh a l l , l . C . , 1 07 Mars u p i a l s , 1 3 1 , 1 3 9 Medus ion , 1 1 0 Mego t h e r i u m , 1 3 4 Meios i s , 5 9 , 7 3 Me l a n i s m , 4 7 , 8 5 Mend e l , G re g o r , 2 6 , 2 7 , 2 8 , 2 9 ' 5 6 , 60, 6 6 Meson y x , 1 3 2

Mesoz o i c , 1 1 6 , 1 1 8 , 1 2 2 , 1 2 3 , 1 24 , 1 2 5 , 1 2 6 , 1 27 , 1 30 , 1 3 2 Meta b o l i s m , 3 3 Meteo r i t e s , 1 0 5 Met h a n e , 1 0 5 , 1 06 Michelangelo, 6 M i g ra t i o n , 77 M i l l e r , Sta n l ey, 1 06 M i m i c ry , 90 M i ocene, 1 3 3 , 1 4 1 , 1 43 , 1 44 " M i s s i n g l i n k s , " 5 2 , 94 Mitosis, 58 Mo l l u s k s , 5 M o n key, 1 40 , 1 4 1 , 1 4 2 M o n o t r e m e s , 5 3 , 1 30 M o rg a n , T. H . , 2 8 M o rp h o l og y , 3 8 Mososo u rs , 1 2 7 Mos s , 5 , 1 20 Mot h , peppered , 4 7 , 8 4 , 85 M u ta t i o n , 2 7 , 2 8 , 4 6 , 7 2 , 7 4 , 7 9 , 1 03 Myolino, 4 9 M y lodo n , 1 3 4

Oposs u m , 1 3 1 O ra n g u ta n , 1 43 O rc h i d , s l i pper, 9 0 O r d ov i c i a n , 1 1 2 , 1 1 3 Oreop i t h e c u s , 1 4 2 Orioles, 36 O r n i t h o pods , 1 27 Oste i c h tyes, 1 1 6 Osteolepis, 1 1 7 Ostrocod e r m , 1 1 4 Overpop u l a t i o n , 2 4 , 2 5 Oxyoe n o , 1 3 2 Oxyg e n , 3 5 , 1 05 O z o n e , 1 07 Poch ycep h o lo pec tora lis ,

1 02 Pa l eocene, 1 4 1 Pa l eo l i t h i c c u l tu re , 1 48 Pa l eozo i c , 1 1 3 , 1 1 4 , 1 1 6 , 1 2 2 , 1 24 Pontodonts, 1 3 5 Porus, 4 5 Paste u r , 9 Peng u i n , 1 1 9 , 1 2 8 Pe n n s y l va n i a n , 1 2 2 Perissoda cty l a , 1 3 5 Perm i a n , 5 3 , 1 1 4 , 1 2 5 N a t u r a l s e l ec t i o n , 2 0 , 2 1 , Pet r i f i ed Forest N o t i o n a l Pork, 1 23 2 5 , 78, 79, 80, 84, P h e n o codus, 1 3 2 9 2 , 9 4 , 1 00 P h e n otype, 6 1 , 6 5 N e a n d e rt h a l m a n , 1 47, P h o l i d oto , 1 3 5 1 49 P h os p h o ro u s , 1 0 5 N eo l i t h i c c u l tu re, 1 4 8 , P h otosyn t h es i s , 1 0 6 1 50 Phyla, 5 N e pt u n e , 1 0 5 P i g , 1 34 N e w G u i ne a , 4 3 P i n n i pe d s , 1 3 3 N e w t , 1 24 P i t h ecanth rop u s , 1 46 Newto n , 2 3 P l o ce n t o l s , 1 3 1 , 1 3 5 N i trog e n , 1 05 P l acode r m , 1 1 5 , 1 1 6 N oa h ' s F l oo d , 1 5 P l o c od o n t , 1 26 N o m e n c l a t u re, b i n o m i a l , P l a n t s , 1 06 , 1 20 11 n u m be r of s p e c i e s , 5 Noth orctos , 1 3 2 , 1 4 1 o l dest, 1 1 0 N o t h o s o u rs , 1 26 vasc u l ar, 1 2 1 N ot oc h o rd , 1 1 -4 P lasmod i u m , 9 3 N ot o u n g u l otes, 1 3 5 P l ato, 7 P l a ty p u s , 5 3 , 1 30 Obelia, 1 1 1 P l e s i osa u rs , 1 2 6 Olenellus, 1 1 1 P l i oc e n e , 1 43 , 1 44 O l i g oc e n e , 1 4 1 P l i op i t h e c u s , 1 -4 2 On th e Ori g i n of Species, P n e u m ococcus, 6 7 1 6 , 2 2 , 2 3 , 52, 80, 1 51 Po l y p l o i d y , 74 Ophrys, 90 Porpoise, 1 1 9

1 59

Potassi u m - a rg o n , 9 7 Preca m br i a n , 98, 1 1 0 , 1 1 1 ' 1 20

P r i m ates, 1 3 5 , 1.40 , 1 4 1 ,

1 4 2 , 1 4 3 , 1 44 , 1 4 5 ,

1 46 , 1 47 ' 1 4 8 , 1 4 9 P r i n c ip les of G e o l o g y , 1 5, 1 6

Proba b i l i t i e s , 6 3 , 64 Probosc i d e a n s , 1 3 5 , 1 3 7 Procon s u l , 1 4 3

Prosi m ia n s , 1 4 1 Prote i n s , 6 7 , 7 0 , 1 0 7 Proto lep idodendro n , 1 2 1

Protop l a s m , 3 2 Protoz o a n s , 5 Ps i l a p h y t e s , 1 2 0 Psi lopsid, 1 2 1 , 1 2 2 Ptera sp i s , 1 1 4

Pterosa u r s , 1 2 7 Pyrotheres, 1 3 5

Ra b b i t , 1 3 5 Races, 45, 76 Rad i a t i o n , a d a p t i v e , 1 2 8 , 1 35

Rad i oa c t i v e e l e m e n t s , 9 6 , 97

Rad i u m , 9 7 R a m ap i t h e c u s , 1 4 3 , 1 4 4

Ray , 1 1 5 Ray, Joh n , 1 0 Rece n t , 1 3 0 , 1 3 1 Reco m b i n a t i o n , g e n e t i c , 72, 73

Red i , Fra ncesco, 9 Rep rod u c t i o n , sex u a l , 3 3 , 5 9 , 7 3 , 1 07

Rept i l e , 1 1 7 , 1 1 9 , 1 2 5 , 1 26

R h i n oceros , 1 3 1 , 1 3 4 R h y n coce p h a l i a n s , 1 2 6 R N A , 67, 70, 7 1 Roden ts, 1 3 5 R u b id i u m -stront i u m , 97 R u s h , sco u r i n g , 1 2 1

Sa l a m a n d e r , 1 2 4 Sa t u r n , 1 0 5 S a u ropods , 1 2 7 Sca l e i n sects , 4 6

1 60

S c o rp i o n , 1 1 8 Sea l , 1 3 3 Sea sq u i rt s , 1 1 4 S eed f e r n s , 1 2 0 , 1 2 3 Selecti on , sexua l , 9 1 S e x , determ i n a t i o n o f , 6 6 Shark, 1 1 5 , 1 28 Sickleb i l ls, 88, 89 S i c k l e - ce l l g e n e , 9 2 , 9 3 Sil urian, 1 1 2, 1 1 4, 1 1 8, 1 19

T h a l l o p h yte s , 1 2 0 T h ecodo n t, 1 2 7 Therapsid, 1 30 T h e r iodont, 53, 1 2 6 T h e ropod s , 1 2 7 T i m e sca l e , g eo l og i c , 98-99

T i t , g reat, 4 5 T i ta n o t h eres, 1 3 2 , 1 3 6 To ad , 1 2 4 Too l s , 1 4 8 To r to i se s , 1 8 , 1 9

S i re n i a n s , 1 3 5 S ka t e s , 1 1 5 Toxodo n , 1 7 S l oth, 1 34 , 1 3 5 T r a c h e o p h yte s , 1 2 1 Snai l , 1 1 2, 1 1 9 Trans m u t a t i o n of S p e c i e s , S n a ke s , 1 2 7 20 Societies, h u m a n , 1 50 Tree h o p pe r , 9 0 Solomon I s lands, 4 3 , 1 02 Triassic, 53 , 1 30 S o u t h e r n - a pe , 1 4 5 Tr i c e rat ops, 5 0 Spassky, 8 1 T r i l o b i te , 1 0 8 , 1 0 9 , 1 1 1 , S pec i e s , c h a n g e s i n , 4 6 1 1 2 foss i l , 4 8 T u b u l identes, 1 35 island, 43 T u rt l e , 1 1 9 , 1 2 6 new, 1 03 n u m be r of, 5 5 , 1 0 3 U n g u l a te s , 1 3 3 , 1 3 4 S p e r m , deve l o p m e n t of, 57 U n i f o r m i t a r i a n i s m , 1 5 S p h e n ops i d , 1 2 2 U n i ntatheri u m , 1 3 2, 1 3 6 Spider, 1 1 8 U ra n i u m , 9 7 S p i d e r m o n key, 1 4 1 U ra n u s , 1 0 5 S p i n y a n teater, 5 3 , 1 3 0 U rey, H a ro l d , 1 0 6 Sponge, 1 1 2 U rod e l es , 1 1 7 S p o n t a n e o u s g e n e ra t i o n , 9 Ursus arctos, 8 6 Spr i g g i n a , 1 1 0 spe l a e u s , 8 6 S t a rf i s h , 1 1 3 Stegosa u r s , 1 27 V a sc u l a r p l a n t s , 1 2 1 S te reospo n d y l a , 1 1 7 Ve rte b ra tes , 5 4 , 1 1 9 Stone age, 1 48 " c l asses o f , 5 5 S t ro m a to l i te s , 1 1 0 oldest, 1 1 4 S u b s pec i e s , 4 5 V e s ti g i a l s t r u c t u re s , 3 9 S u l f u r, 1 0 5 S u rv i v a l of t h e f i ttes t , 7 8 W a l l a ce, A l f red R u s se l , S u tt o n , Wa l te r , 2 8 1 5 , 2 0 , 2 2 , 43

Ta p i n ocep h a l i d s , 1 2 6 Ta p i r , 4 3 , 1 3 4 Tars i e r s , 1 40 , 1 4 1 Tarsioids, 1 4 1 T a x od o n t , 1 3 1 Taxonomy, 36 T e r t i a r y , 1 00 , 1 3 4 , 1 3 5 ,

Wa l r u s , 1 3 3 Wease l , 1 3 3 Wh a l e , 1 1 9 , 1 33 , 1 35 W h istler, g o lden , 1 0 2 Wolf, 1 3 1 Wood pec k e r s , 4 2 Worm , 1 1 0 , 1 1 2 W r i g h t , Sewa l l , 7 5

1 36, 1 39, 1 4 1 Tex t u laria , 4 8

Zygotes, 57, 65

B

C

D

E