Transcription
TheExtendedPhenotypeThe Gene as the Unit of SelectionRichard D a w k i n sUniversity of OxfordOxford University Press
aHongDar es ord uttaLumpurNairobiWaltonNewCityof OxfordNicosiaUniversityPublished in the United Statesby Oxford University P r e s s , c.byCopyright hardcoverFreemanissued f HarperbyRudyardof DoubledayRichardeditionandanLibraryDawkins,1971of Mrs.Hymn'1982W.First by permissionpermissionOriginalH.Song' panyOxfordKipling.&byLimitedUniversityinPress olutionI.Titlephenotype.2.575-12Gene expressionQH.WISBNAllrightsreserved.No partof this publicationmaybereproduced,stored in a retrieval system, or transmitted, in any form or by any photocopying,of Press)orPressLtd,Suffolkotherwise,without
PrefaceT h e first c h a p t e r d o e s s o m e o f t h e w o r k o f a P r e f a c e , i n e x p l a i n i n gw h a t the b o o k d o e s a n d d o e s n o t set o u t t o a c c o m p l i s h , s o I c a n b e b r i e f h e r e .It is n o t a t e x t b o o k , n o r an i n t r o d u c t i o n to an e s t a b l i s h e d field.It is ap e r s o n a l l o o k a t t h e e v o l u t i o n o f life, a n d i n p a r t i c u l a r a t t h e l o g i c o f n a t u r a ls e l e c t i o n a n d t h e l e v e l i n t h e h i e r a r c h y o f life a t w h i c h n a t u r a l s e l e c t i o n c a nb e said t o a c t . I h a p p e n t o b e a n e t h o l o g i s t , b u t I h o p e p r e o c c u p a t i o n s w i t ha n i m a l b e h a v i o u r will not b e too n o t i c e a b l e . T h e i n t e n d e d scope o f the b o o kis w i d e r .T h e r e a d e r s for w h o m I a m m a i n l y w r i t i n g a r e m y p r o f e s s i o n a l c o l l e a g u e s sts,ecologists,andphilosophers and humanists interested in e v o l u t i o n a r y science, including, edisciplines.T h e r e f o r e , a l t h o u g h this b o o k i s i n s o m e w a y s t h e s e q u e l t o m y p r e v i o u sb o o k , The Selfish Gene, it a s s u m e s t h a t t h e r e a d e r h a s p r o f e s s i o n a l k n o w l e d g eo f e v o l u t i o n a r y b i o l o g y a n d its t e c h n i c a l t e r m s . O n t h e o t h e r h a n d i t i sp o s s i b l e to e n j o y a p r o f e s s i o n a l b o o k as a s p e c t a t o r , e v e n if n o t a p a r t i c i p a n ti n the p r o f e s s i o n . S o m e l a y p e o p l e w h o r e a d this b o o k i n d r a f t h a v e b e e n k i n de n o u g h , o r p o l i t e e n o u g h , t o c l a i m t o h a v e l i k e d it. I t w o u l d g i v e m e g r e a tsatisfaction t o b e l i e v e t h e m , a n d I h a v e a d d e d a g l o s s a r y o f t e c h n i c a l t e r m sw h i c h I h o p e m a y h e l p . I h a v e also t r i e d t o m a k e t h e b o o k a s n e a r a s p o s s i b l eto being enjoyable to read. T h e resulting tone m a y possibly irritate someserious p r o f e s s i o n a l s . I v e r y m u c h h o p e n o t , b e c a u s e s e r i o u s p r o f e s s i o n a l s a r ethe p r i m a r y a u d i e n c e t o w h o m I w i s h t o s p e a k . I t i s a s i m p o s s i b l e t o p l e a s ee v e r y b o d y i n l i t e r a r y s t y l e a s i t i s i n a n y o t h e r m a t t e r o f taste, a n d s t y l e s t h a tg i v e the most positive pleasure to s o m e p e o p l e are often the most a n n o y i n g toothers.C e r t a i n l y the tone of the b o o k is not c o n c i l i a t o r y or a p o l o g e t i c — s u c h isnot the w a y o f a n a d v o c a t e t h a t s i n c e r e l y b e l i e v e s i n his c a s e — a n d I m u s tp a c k all a p o l o g y i n t o t h e P r e f a c e . S o m e o f t h e e a r l i e r c h a p t e r s r e p l y t ocriticisms of my previous book, w h i c h m i g h t recur in response to the present
viPrefaceo n e . I a m s o r r y t h a t this i s n e c e s s a r y , a n d I a m s o r r y i f a n o t e o f e x a s p e r a t i o nc r e e p s i n f r o m t i m e t o t i m e . I trust, a t l e a s t , t h a t m y e x a s p e r a t i o n r e m a i n sg o o d h u m o u r e d . I t i s n e c e s s a r y t o p o i n t t o p a s t m i s u n d e r s t a n d i n g s a n d try t aggrievedimpression that m i s u n d e r s t a n d i n g has b e e n w i d e s p r e a d . It has been confinedto n u m e r i c a l l y v e r y limited quarters, b u t in s o m e cases rather v o c a l ones. Ia m g r a t e f u l t o m y c r i t i c s for f o r c i n g m e t o t h i n k a g a i n a b o u t h o w t o e x p r e s sdifficult m a t t e r s m o r e c l e a r l y .I a p o l o g i z e to readers w h o m a y find a favourite and relevant w o r k missingfrom the b i b l i o g r a p h y . T h e r e are those c a p a b l e o f c o m p r e h e n s i v e l y andexhaustively s u r v e y i n g the literature of a large field, but I h a v e never beena b l e t o u n d e r s t a n d h o w t h e y m a n a g e it. I k n o w t h a t t h e e x a m p l e s I h a v e c i t e dare a small subset of those that could h a v e been cited, and are sometimesthe w r i t i n g s or r e c o m m e n d a t i o n s of my friends. If the result a p p e a r s biased,w e l l , of course it is biased, a n d I am sorry. I think nearly e v e r y b o d y must bes o m e w h a t b i a s e d i n this k i n d o f w a y .A b o o k i n e v i t a b l y reflects the c u r r e n t p r e o c c u p a t i o n s of the author, andt h e s e p r e o c c u p a t i o n s a r e l i k e l y t o h a v e b e e n a m o n g the t o p i c s o f his m o s tr e c e n t p a p e r s . W h e n t h o s e p a p e r s a r e s o r e c e n t t h a t i t w o u l d b e a n artificialcontrivancetochangethe words,Ih a v e not hesitatedto reproduce ap a r a g r a p h almost v e r b a t i m here and there. T h e s e paragraphs, w h i c h will bef o u n d i n C h a p t e r s 4 , 5 , 6 a n d 1 4 , a r e a n i n t e g r a l p a r t o f t h e m e s s a g e o f thisbook, a n d to o m i t t h e m w o u l d be just as artificial as to m a k e gratuitouschanges in their w o r d i n g .T h e o p e n i n g sentence of C h a p t e r1d e s c r i b e s the b o o k a s a w o r k o fu n a b a s h e d a d v o c a c y b u t , w e l l , p e r h a p s I a m j u s t a little bit a b a s h e d ! W i l s o n( 1 9 7 5 , p p . 28—29) h a s r i g h t l y c a s t i g a t e d t h e ' a d v o c a c y m e t h o d ' i n a n y s e a r c hfor s c i e n t i f i c t r u t h , a n d I h a v e t h e r e f o r e d e v o t e d s o m e o f m y f i r s t c h a p t e r t o ap l e a o f m i t i g a t i o n . I c e r t a i n l y w o u l d n o t w a n t s c i e n c e t o a d o p t the l e g a ls y s t e m i n w h i c h p r o f e s s i o n a l a d v o c a t e s m a k e t h e best c a s e t h e y c a n for ap o s i t i o n , e v e n i f t h e y b e l i e v e i t t o b e false. I b e l i e v e d e e p l y i n t h e v i e w o f lifet h a t this b o o k a d v o c a t e s , a n d h a v e d o n e so, a t least i n p a r t , for a l o n g t i m e ,c e r t a i n l y since the time of my first published paper, in w h i c h I c h a r a c t e r i z e da d a p t a t i o n s a s f a v o u r i n g ' t h e s u r v i v a l o f the a n i m a l ' s g e n e s . . . '(Dawkins1 9 6 8 ) . T h i s b e l i e f — t h a t i f a d a p t a t i o n s a r e t o b e t r e a t e d a s 'for t h e g o o d o fs o m e t h i n g , that s o m e t h i n g is the g e n e — w a s the f u n d a m e n t a l assumption ofm y p r e v i o u s b o o k . T h e present b o o k goes further. T o d r a m a t i z e i t a bit, i ta t t e m p t s t o free t h e selfish g e n e f r o m t h e i n d i v i d u a l o r g a n i s m w h i c h h a s b e e nits c o n c e p t u a l p r i s o n . T h e p h e n o t y p i c effects o f a g e n e a r e t h e tools b y w h i c hi t l e v e r s i t s e l f i n t o t h e n e x t g e n e r a t i o n , a n d these tools m a y ' e x t e n d ' faro u t s i d e t h e b o d y i n w h i c h t h e g e n e sits, e v e n r e a c h i n g d e e p i n t o t h e n e r v o u ssystems of other organisms. S i n c e it is not a factual position I am a d v o c a t i n g ,b u t a w a y of seeing facts,Iwantedto w a r nthe reader not to expect
Prefacevii' e v i d e n c e ' i n the n o r m a l sense o f t h e w o r d . I a n n o u n c e d t h a t t h e b o o k w a s awork of a d v o c a c y , because I was anxious not to disappoint the reader, not tol e a d h e r o n u n d e r false p r e t e n c e s a n d w a s t e h e r t i m e .T h e l i n g u i s t i c e x p e r i m e n t o f t h e last s e n t e n c e r e m i n d s m e t h a t I w i s h Ih a d h a d the c o u r a g e t o i n s t r u c t the c o m p u t e r t o f e m i n i z e p e r s o n a l p r o n o u n sat r a n d o m t h r o u g h o u t the text. T h i s is not o n l y b e c a u s e I a d m i r e the currentawareness of the m a s c u l i n e bias in o u r l a n g u a g e . W h e n e v e r I w r i t e I h a v e aparticular imaginary reader in mind(different i m a g i n a r y r e a d e r s o v e r s e ea n d 'filter' t h e s a m e p a s s a g e i n n u m e r o u s s u c c e s s i v e r e v i s i o n s ) a n d a t leasthalf myimaginaryreadersare,likeatleasthalf myfriends,female.U n f o r t u n a t e l y i t i s still t r u e i n E n g l i s h t h a t t h e u n e x p e c t e d n e s s o f a f e m i n i n epronoun,whereaneutralmeaningisintended,attention of most readers, of either sex.Ip r e v i o u s p a r a g r a p h w i l l s u b s t a n t i a t e this.seriouslydistractstheb e l i e v e the e x p e r i m e n t of theW i t h regret,therefore,Ihavef o l l o w e d the s t a n d a r d c o n v e n t i o n i n this b o o k .F o r m e , w r i t i n g is almost a social activity, a n d I am grateful to the m a n yfriends w h o h a v e , s o m e t i m e s u n w i t t i n g l y , p a r t i c i p a t e d t h r o u g h d i s c u s s i o n ,argument and moral support.I cannot thank them all by n a m e .MarianS t a m p D a w k i n s has not o n l y p r o v i d e d sensitive a n d k n o w l e d g e a b l e criticismo f the w h o l e b o o k i n several drafts. S h e has also k e p t m e g o i n g b y b e l i e v i n gi n the p r o j e c t e v e n t h r o u g h t h e t i m e s w h e n I lost m y o w n c o n f i d e n c e . A l a nGrafen a n d M a r k R i d l e y , officially m y g r a d u a t e students, r e a l l y , i n theirdifferent w a y s , m y m e n t o r s a n d g u i d e s t h r o u g h d i f f i c u l t t h e o r e t i c a l t e r r i t o r y theirnamesseemed to creep in on almost every p a g e , a n d it w a s only the p a r d o n a b l eg r u m b l i n g s of a referee that c o m p e l l e d me to banish to the P r e f a c e mya c k n o w l e d g m e n t o f debt t o t h e m . C a t h y K e n n e d y m a n a g e s t o c o m b i n e closef r i e n d s h i p for m e w i t h d e e p s y m p a t h y for m y b i t t e r e s t c r i t i c s . T h i s h a s p u ther in a u n i q u e position to advise m e , especially o v e r the earlier c h a p t e r sw h i c h a t t e m p t t o r e p l y t o c r i t i c i s m . I f e a r t h a t s h e w i l l still n o t l i k e t h e t o n eo f these c h a p t e r s , b u t s u c h i m p r o v e m e n t a s t h e r e m a y b e i s l a r g e l y d u e t o h e rinfluence and I am v e r y grateful to her.I w a s p r i v i l e g e d t o h a v e t h e first d r a f t c r i t i c i z e d i n its e n t i r e t y b y J o h nMaynardSmith,DavidC.Smith, JohnKrebs,PaulHarvey,andRicC h a r n o v , a n d the f i n a l d r a f t o w e s m u c h t o a l l o f t h e m . I n a l l c a s e s I a c t e d o ntheir a d v i c e , e v e n if Id i d n o t a l w a y s t a k e it.Others kindly ichaelthechapterona r t e f a c t s , P a u l i n e L a w r e n c e t h a t o n p a r a s i t e s , E g b e r t L e i g h t h a t o n fitness,A n t h o n y H a l l a m the section o n p u n c t u a t e d e q u i l i b r i a , W . F o r d D o o l i t t l ethat o n selfish D N A , a n d D i a n e D e S t e v e n b o t a n i c a l s e c t i o n s . T h e b o o k w a sf i n i s h e d a t O x f o r d , b u t b e g u n d u r i n g a visit t o t h e U n i v e r s i t y o f F l o r i d a a tGainesville on a sabbatical leave kindly granted by the U n i v e r s i t y of O x f o r da n d the W a r d e n a n d F e l l o w s o f N e w C o l l e g e . I a m g r a t e f u l t o m y m a n y
viiiPrefaceF l o r i d a n friends for g i v i n g m e s u c h a p l e a s a n t a t m o s p h e r e i n w h i c h t o w o r k ticismofp r e l i m i n a r y d r a f t s , a n d D o n n a G i l l i s , w h o also d i d m u c h o f t h e t y p i n g . Ib e n e f i t e d , t o o , f r o m a m o n t h ' s e x p o s u r e t o t r o p i c a l b i o l o g y a s the gratefulguest of the S m i t h s o n i a nI n s t i t u t i o n i n P a n a m a d u r i n g t h e w r i t i n g o f theb o o k . F i n a l l y , it is a pleasure o n c e a g a i n to thank M i c h a e l R o d g e r s , formerlyo f O x f o r d U n i v e r s i t y Press a n d n o w o f W . H . F r e e m a n a n d C o m p a n y , a 'Ks e l e c t e d ' e d i t o r w h o r e a l l y b e l i e v e s i n his b o o k s a n d i s t h e i r tireless a d v o c a t e .OxfordJuneRichard Dawkins1981
Contents1 NeckerCubes2 GeneticDeterminism3 ConstraintsandonBuffaloes 1andGenePerfectionSelectionism 9304 A r m s Races and Manipulation555 The Active Germ-Line Replicator816 Organisms, Groups and M e m e s : Replicators or Vehicles? 977 Selfish Wasp or Selfish Strategy?8 OutlawsandModifiers1181339 Selfish DNA, Jumping Genes, and a Lamarckian Scare10 An Agony in Five Fits 17911 The Genetical Evolution of Animal Artefacts 19512 Host Phenotypes of Parasite Genes13 Action at a Distance22814 Rediscovering the O r g a n i s mGlossary283250209156
T h e Extended Phenotype
1 Necker C u b e s and BuffaloesT h i s is a w o r k of u n a b a s h e d a d v o c a c y . I w a n t to a r g u e in favour of aparticular w a y of looking at animals and plants, and a particular w a y ofw o n d e r i n g w h y they do the things that they d o . W h a t I am a d v o c a t i n g is nota n e w t h e o r y , n o t a h y p o t h e s i s w h i c h c a n b e v e r i f i e d o r falsified, n o t a m o d e lw h i c h c a n b e j u d g e d b y its p r e d i c t i o n s . I f i t w e r e a n y o f t h o s e t h i n g s , I a g r e ewith Wilson(1975, p.28)that the ' a d v o c a c y m e t h o d ' w o u l dbe inap-propriate a n d reprehensible. But it is not a n y of those things. W h a t I ama d v o c a t i n g i s a p o i n t o f v i e w , a w a y o f l o o k i n g a t f a m i l i a r facts a n d i d e a s ,and a w a y of asking new questions about them. A n y reader w h o expects ac o n v i n c i n g n e w t h e o r y i n t h e c o n v e n t i o n a l sense o f t h e w o r d i s b o u n d t o b eleft, t h e r e f o r e , w i t h a d i s a p p o i n t e d 'so w h a t ? ' f e e l i n g . B u t I a m n o t t r y i n g t oc o n v i n c e a n y o n e of the truth of any factual proposition. R a t h e r , I am t r y i n gto show the r e a d e r a w a y of seeing b i o l o g i c a l facts.T h e r e i s a w e l l - k n o w n v i s u a l i l l u s i o n c a l l e d the N e c k e r C u b e . I t c o n s i s t s o fa line d r a w i n g w h i c h t h e b r a i n i n t e r p r e t s a s a t h r e e - d i m e n s i o n a l c u b e . B u tthere are t w o possible orientations of the p e r c e i v e onandtheboth arepaper.Weu s u a l l y b e g i n b y s e e i n g o n e o f the t w o o r i e n t a t i o n s , b u t i f w e l o o k for s e v e r a ls e c o n d s t h e c u b e 'flips o v e r ' i n t h e m i n d , a n d w e see t h e o t h e r a p p a r e n to r i e n t a t i o n . A f t e r a few m o r e s e c o n d s t h e m e n t a l i m a g e f l i p s b a c k a n d i tcontinues to alternate as l o n g as we look at the picture. T h e point is thatneither of the t w o perceptions of the c u b e is the correct or 'true' o n e . T h e ya r e e q u a l l y c o r r e c t . S i m i l a r l y t h e v i s i o n o f life t h a t I a d v o c a t e , a n d l a b e lw i t h the n a m e o f t h e e x t e n d e d p h e n o t y p e , i s n o t p r o v a b l y m o r e c o r r e c t t h a nthe o r t h o d o x v i e w . I t i s a different v i e w a n d I s u s p e c t t h a t , a t l e a s t i n s o m er e s p e c t s , it p r o v i d e s a d e e p e r u n d e r s t a n d i n g . B u t I d o u b t t h a t t h e r e is a n yexperiment that could be done to prove my claim.T h e p h e n o m e n a that I shall c o n s i d e r — c o e v o l u t i o n , arms races, m a n i p u l a t i o n o f hosts b y p a r a s i t e s , m a n i p u l a t i o n o f t h e i n a n i m a t e w o r l d b y l i v i n gt h i n g s , e c o n o m i c ' s t r a t e g i e s ' for m i n i m i z i n g costs a n d m a x i m i z i n g b e n e f i t s —1
2N e c k e r C u b e s and Buffaloesare all familiar e n o u g h , a n d are a l r e a d y the subject of intensive study. W h y ,then, should the busy reader bother to go o n ? It is tempting to borrowS t e p h e n G o u l d ' s w i n n i n g l y i n g e n u o u s a p p e a l at the b e g i n n i n g of a moresubstantial v o l u m e ( 1 9 7 7 a ) a n d simply say, 'Please read the b o o k ' a n d y o uw i l l f i n d o u t w h y i t w a s w o r t h b o t h e r i n g t o d o s o . U n f o r t u n a t e l y I d o noth a v e t h e s a m e g r o u n d s for c o n f i d e n c e . I c a n o n l y s a y t h a t , a s o n e o r d i n a r eviewpointr e p r e s e n t e d b y t h e l a b e l ' e x t e n d e d p h e n o t y p e ' h a s m a d e m e see a n i m a l s a n dt h e i r b e h a v i o u r d i f f e r e n t l y , a n d I t h i n k I u n d e r s t a n d t h e m b e t t e r for it. T h ee x t e n d e d p h e n o t y p e m a y n o t c o n s t i t u t e a t e s t a b l e h y p o t h e s i s i n itself, b u t i ts o far c h a n g e s t h e w a y w e see a n i m a l s a n d p l a n t s t h a t i t m a y c a u s e u s t ot h i n k o f t e s t a b l e h y p o t h e s e s t h a t w e w o u l d o t h e r w i s e n e v e r h a v e d r e a m e d of.L o r e n z ' s ( 1 9 3 7 ) d i s c o v e r y t h a t a b e h a v i o u r p a t t e r n c a n b e t r e a t e d like a na n a t o m i c a l o r g a n w a s n o t a d i s c o v e r y i n t h e o r d i n a r y sense. N o e x p e r i m e n t a lresults w e r e a d d u c e d i n its s u p p o r t . I t w a s s i m p l y a n e w w a y o f s e e i n g rnethology( T i n b e r g e n 1 9 6 3 ) , a n d it seems to us t o d a y so o b v i o u s that it is hard tounderstand that it ever needed 'discovering'. Similarly, D ' A r c y Thompson's( 1 9 1 7 ) celebrated c h a p t e r ' O n the theory of t r a n s f o r m a t i o n s . . . ' is widelyr e g a r d e d a s a w o r k o f i m p o r t a n c e a l t h o u g h i t d o e s n o t a d v a n c e o r test ah y p o t h e s i s . I n a sense i t i s o b v i o u s l y n e c e s s a r i l y t r u e t h a t a n y a n i m a l f o r rmation,a l t h o u g h it is not o b v i o u s that the transformation will be a simple one. Ina c t u a l l y d o i n g i t for a n u m b e r o f s p e c i f i c e x a m p l e s , D ' A r c y T h o m p s o ni n v i t e d a 'so w h a t ? ' r e a c t i o n f r o m a n y o n e f a s t i d i o u s e n o u g h t o insist t h a tscience p r o c e e d s o n l y b y the falsifying o f specific hypotheses. I f w e readD ' A r c y T h o m p s o n ' s c h a p t e r a n d t h e n ask o u r s e l v e s w h a t w e n o w k n o w t h a twe didnot k n o wbefore,the a n s w e r m a y w e l l b e not m u c h .But ouri m a g i n a t i o n is fired. We go b a c k a n d look at animals in a n e w w a y ; and wethink abouttheoretical problems,inthis c a s e t h o s e o f e m b r y o l o g y a n dp h y l o g e n y and their interrelations, in a new w a y . I a m , of course, not sop r e s u m p t u o u s as to c o m p a r e the present modest w o r k w i t h the masterpieceof a g r e a t b i o l o g i s t . I u s e t h e e x a m p l e s i m p l y to d e m o n s t r a t e t h a t it is possiblefor a t h e o r e t i c a l b o o k t o b e w o r t h r e a d i n g e v e n i f i t d o e s n o t a d v a n c e t e s t a b l eh y p o t h e s e s b u t s e e k s , i n s t e a d , t o c h a n g e t h e w a y w e see.A n o t h e r g r e a t biologist o n c e r e c o m m e n d e d that to understand the actualwe must c o n t e m p l a t e the possible: ' N o practical biologist interested in nsequences ex-p e r i e n c e d b y o r g a n i s m s h a v i n g t h r e e o r m o r e s e x e s ; y e t w h a t else s h o u l d h ed o i f h e w i s h e s t o u n d e r s t a n d w h y t h e sexes a r e , i n f a c t , a l w a y s t w o ? ' ( F i s h e r1930a, p . i x ) . W i l l i a m s ( 1 9 7 5 ) , M a y n a r d S m i t h (1978a) and others h a v et a u g h t u s t h a t o n e o f t h e c o m m o n e s t , m o s t u n i v e r s a l f e a t u r e s o f life o n E a r t h n.Indeed,its
N e c k e r C u b e s and Buffaloes3e x i s t e n c e turns o u t t o b e p o s i t i v e l y s u r p r i s i n g w h e n set a g a i n s t t h e i m a g i n e dpossibility of asexual r e p r o d u c t i o n . To i m a g i n e a s e x u a l r e p r o d u c t i o n as ah y p o t h e t i c a l p o s s i b i l i t y i s n o t difficult, s i n c e w e k n o w i t i s a r e a l i t y i n s o m eanimals and plants. B u t are there other cases w h e r e o u r i m a g i n a t i o n r e c e i v e sn o s u c h p r o m p t i n g ? A r e t h e r e i m p o r t a n t facts a b o u t life t h a t w e h a r d l ynotice simplybecause welacktheimaginationtovisualisealternativesw h i c h , like Fisher's three sexes, m i g h t h a v e existed in some possible w o r l d ? Ishall try t o s h o w t h a t t h e a n s w e r i s y e s .Playing with an imaginary world, in order to increase our understandingof the actual w o r l d , is the t e c h n i q u e of ' t h o u g h t e x p e r i m e n t ' . It is m u c h usedby p h i l o s o p h e r s . F o r i n s t a n c e in a c o l l e c t i o n of essays onMind (ed.Glover1976),The Philosophy ofvarious authors i m a g i n e surgical operations inwhich one person's brain is transplanted into another person's b o d y , andt h e y use t h e t h o u g h t e x p e r i m e n t t o c l a r i f y t h e m e a n i n g o f ' p e r s o n a l i d e n t i t y ' .A t times philosophers' t h o u g h t e x p e r i m e n t s are p u r e l y i m a g i n a r y a n d w i l d l yi m p r o b a b l e , b u t this d o e s n ' t m a t t e r g i v e n t h e p u r p o s e for w h i c h t h e y a r em a d e . A t o t h e r t i m e s t h e y a r e i n f o r m e d , t o a g r e a t e r o r lesser e x t e n t , b y factsfrom t h e r e a l w o r l d , for i n s t a n c e t h e facts o f s p l i t - b r a i n e x p e r i m e n t s utionaryb i o l o g y . W h e n I w a s a n u n d e r g r a d u a t e o b l i g e d t o w r i t e s p e c u l a t i v e essays o n'the origin o f the C h o r d a t e s ' a n d other topics o f r e m o t e p h y l o g e n y , o n e o f m ytutors r i g h t l y tried t o s h a k e m y faith i n t h e v a l u e o f s u c h s p e c u l a t i o n s b ysuggesting that a n y t h i n g c o u l d , in principle, e v o l v e into a n y t h i n g else. E v e ninsects c o u l d e v o l v e i n t o m a m m a l s , i f o n l y t h e r i g h t s e q u e n c e o f s e l e c t i o npressures w e r e p r o v i d e d i n the r i g h t o r d e r . A t t h e t i m e , a s m o s t z o o l o g i s t sw o u l d , I d i s m i s s e d t h e i d e a a s o b v i o u s n o n s e n s e , a n d I still, o f c o u r s e , d o n ' tb e l i e v e t h a t the r i g h t s e q u e n c e o f s e l e c t i o n p r e s s u r e s e v e r w o u l d b e p r o v i d e d .N o r d i d m y t u t o r . B u t a s far a s t h e p r i n c i p l e i s c o n c e r n e d , a s i m p l e t h o u g h texperiment shows it to be nearly incontrovertible. We need only prove thatt h e r e exists a c o n t i n u o u s series of s m a l l steps l e a d i n g f r o m an i n s e c t , s a y as t a g b e e t l e , t o a m a m m a l , s a y a s t a g . B y this I m e a n t h a t , s t a r t i n g w i t h t h ebeetle, we could lay out a sequence of hypothetical animals, e a c h one ass i m i l a r t o t h e p r e v i o u s m e m b e r o f t h e series a s a p a i r o f b r o t h e r s m i g h t b e ,a n d the s e q u e n c e w o u l d c u l m i n a t e i n a r e d d e e r s t a g .T h e proof is easy, p r o v i d e d only that we a c c e p t , as e v e r y o n e does, thatb e e t l e a n d d e e r h a v e a c o m m o n a n c e s t o r , h o w e v e r far b a c k . E v e n i f t h e r e i sn o o t h e r s e q u e n c e o f steps f r o m b e e t l e t o d e e r , w e k n o w t h a t a t least o n esequence must be obtained by simply tracing the beetle's ancestors b a c k tothe c o m m o n a n c e s t o r , t h e n w o r k i n g f o r w a r d s d o w n t h e o t h e r l i n e t o t h edeer.Wehaveprovedthatthereexistsatrajectoryof stepwisechangeconnecting beetle to deer and, by implication, a similar trajectory from a n ymodern a n i m a l to any other m o d e r n animal. In principle, therefore, we m a y
4N e c k e r C u b e s and Buffaloesp r e s u m e t h a t a series o f s e l e c t i o n p r e s s u r e s c o u l d b e a r t i f i c i a l l y c o n t r i v e d t op r o p e l a l i n e a g e a l o n g o n e of these trajectories.It was a q u i c k thoughte x p e r i m e n t a l o n g t h e s e lines t h a t e n a b l e d m e t o s a y , w h e n d i s c u s s i n g D ' A r c yT h o m p s o n ' s t r a n s f o r m a t i o n s , t h a t ' I n a sense i t i s o b v i o u s l y n e c e s s a r i l y truethat a n y a n i m a l form c a n be turned into a related form by a m a t h e m a t i c a lt r a n s f o r m a t i o n , a l t h o u g h it is n o t o b v i o u s t h a t t h e t r a n s f o r m a t i o n w i l l be asimpleone.'InthisbookIshallmakefrequentuseof thethought-e x p e r i m e n t t e c h n i q u e . I w a r n t h e r e a d e r o f this i n a d v a n c e , s i n c e scientistsare s o m e t i m e s a n n o y e d by the lack of realism in such forms of reasoning.T h o u g h t e x p e r i m e n t s are not supposed to be realistic. T h e y are supposed toclarify our thinking a b o u t reality.O n e f e a t u r e o f life i n this w o r l d w h i c h , l i k e s e x , w e h a v e t a k e n for g r a n t e dand m a y b e should not, is that living matter comes in discrete p a c k a g e s nfunctionalexplanationu s u a l l y a s s u m e t h a t t h e a p p r o p r i a t e u n i t for d i s c u s s i o n i s t h e i n d i v i d u a lo r g a n i s m . T o us, ' c o n f l i c t ' u s u a l l y m e a n s c o n f l i c t b e t w e e n o r g a n i s m s , e a c ho n e s t r i v i n g t o m a x i m i z e its o w n i n d i v i d u a l 'fitness'. W e r e c o g n i z e s m a l l e ru n i t s s u c h a s c e l l s a n d g e n e s , a n d l a r g e r u n i t s s u c h a s p o p u l a t i o n s , societiesa n d e c o s y s t e m s , b u t t h e r e i s n o d o u b t t h a t the i n d i v i d u a l b o d y , a s a discreteu n i t o f a c t i o n , e x e r t s a p o w e r f u l h o l d o v e r the m i n d s o f z o o l o g i s t s , e s p e c i a l l ythose interested in the a d a p t i v e significance of a n i m a l b e h a v i o u r . O n e of mya i m s i n this b o o k i s t o b r e a k t h a t h o l d . I w a n t t o s w i t c h e m p h a s i s from thei n d i v i d u a l b o d y a s f o c a l u n i t o f f u n c t i o n a l d i s c u s s i o n . A t t h e v e r y least Iw a n t t o m a k e u s a w a r e o f h o w m u c h w e t a k e for g r a n t e d w h e n w e l o o k a t lifeas a collection of discrete i n d i v i d u a l organisms.ThethesisthatIshalls u p p o r t isthis.Itislegitimatetos p e a k ofa d a p t a t i o n s a s b e i n g 'for t h e b e n e f i t o f s o m e t h i n g , b u t t h a t s o m e t h i n g i s bestn o t s e e n as t h e i n d i v i d u a l o r g a n i s m . It is a s m a l l e r u n i t w h i c h I c a l l thea c t i v e , g e r m - l i n e r e p l i c a t o r . T h e m o s t i m p o r t a n t k i n d o f r e p l i c a t o r i s the' g e n e ' or small g e n e t i c f r a g m e n t . R e p l i c a t o r s are not, of course, selectedd i r e c t l y , b u t b y p r o x y ; t h e y a r e j u d g e d b y t h e i r p h e n o t y p i c effects. A l t h o u g hfor s o m e p u r p o s e s i t i s c o n v e n i e n t t o t h i n k o f t h e s e p h e n o t y p i c effects a sbeing p a c k a g e d together in discrete 'vehicles' such as individual organisms,this i s n o t f u n d a m e n t a l l y n e c e s s a
selected' editor who really believes in his books and is their tireless advocate. Oxford Richard Dawkins June 1981 . Contents 1 Necker Cubes and Buffaloes 1 2 Genetic Determinism and Gene Selectionism 9 3 Cons
