by FRANK H. T. R HODES The University of Michigan ILLUSTRATED BY
REBECCA MERRILEES
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RUDY ZALLINGER
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GOLDEN PRE...
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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
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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
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KK 81
1
XI
II
8
7
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c h rom os o m e s i n s p e r m a tog o n i u m
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spermatogon i u m
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THE DEVELOPMENT OF
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. JI" •• • • , . .,.,_ lfC
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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
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fi rst ' � ; polar body
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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
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
•
,.,"),