検索対象: The selfish gene
The Selfish Gene
94 月 gg 尾 立 〃 ん / な ア 4 〃 イ ど 襯 ん substantial proportion of their genes. Each selfish gene therefore has its loyalties divided between different bodies. This is ex- plained in the next ch 叩 ter.
/ 襯 襯 0 ′ ー 4 / CO / な 47 Other genes for its own selfish ends. SO does a gene for crossrng¯ over. There are even genes—called mutators—which manipulate the rates 0f copymg-errors in 0ther genes. BY definition, a COPY- lng error IS tO the disadvantage Of the gene which is mscopied. But if it is t0 the advantage 0f the selfish mutator gene which induces it, the mutator can spread through the gene P001. Similarly, if crossmg—over benefits a gene for crossing-over, that iS a sufficient explanation for the existence Of crossmg—over. And if sexual, as opposed tO non-sexual, reproduction benefits a gene for sexual reproduction, that is a sufficient explanation for the existence Of sexual reproduction. Whether or not it benefits all the rest Of an individual's genes IS comparatively irrelevant. Seen from the selfish gene s point Of view, sex is not SO bizarre after all. ThiS comes perilously close tO being a circular S1nce the existence Of sexuality is a precondition for the whOle chain Of reasoning which leads t0 the gene being regarded as the unit 0f selection. I believe there are ways Of escaping from the circular- ity, but this bOOk is not the place tO pursue the question. Sex exiStS. 、 hat much iS true. lt iS a consequence Of sex and crossmg— over that the small genetic unit or gene can be regarded as the nearest thing we have tO a fundamental, independent agent Of evolution. Sex is not the only apparent paradox which becomes less puzz- ling the moment we learn tO think in selfish gene terms. For lnstance, it appears that the amount ofDNA in organisms IS more than is strictly necessary for building them: a large fraction 0f the DNA is never translated intO protein. From the point Of view Of the individual orgamsm this seems paradoxical. If the purpose of DNA is to supervise the building of bodies, it is surpnsing t0 find a large quantity 0f DNA which does no such thing. Biologists are racking their brains trying t0 think what useful task this apparently surplus DNA is doing. But from the P0int 0f view of the selfish genes themselves, there is no paradox. The true purpose' Of ー ) 、 ・ A iS tO more and no less. sim¯ plest way t0 explain the surplus DNA is t0 suppose that it is a parasite, or at best a harmless but useless passenger, hitching a ride in the survival machines created by the other DNA. Some people object tO what they see as an excessively gene- centred view 0f evolution. After all, they argue, it is whole
外 イ ど . ・ 一 ん ど 〃 ど ル ど 2 / 耘 4 な 215 have tO posit 2 genetic advantage in lmitatiom though that would certainly help. All that is necessary is that the brain should be c 叩 4 み な 0f imitation: memes will then evolve which exploit the capability to the full. I now close the topic of the new replicators, and end the b00k on a note 0f qualified hope. One unique feature 0f man' which may or may not have evolved memically, is his capacity for con— scious foresight. Selfish genes (and, if you allow the speculation 0f this chapter, memes t00 ) have no foresight. They are uncon— scious, blind, replicators. The fact that they replicate, together with certain further conditions means, willy nilly, that they will tend towards the evolution ofqualities which, in the special sense Of this book, can be called selfish. A simple replicator, whether gene or meme, cannot be expected tO forgo short—term selfish advantage even ifit would really pay it, in thelong term, t0 d0 so. We saw this in the chapter on aggression. Even though a 'consprracy 0f doves would be better for ど 認 朝 な 市 朝 〃 4 / than the evolutionarily stable strategy, natural selection is bound t0 favour the ESS. lt is possible that yet another unique quality 0f man a capacity for genuine, disinterested, true altruism. I hope SO' but I am not gomg tO argue the case one way or the Other' nor tO speculate over its possible mem1C evolution. The point I am making now is that, even if we lOOk on the dark side and assume that individual man is fundamentally selfish, our conscious fore— sight—our capaclty tO simulate the future imagination¯could save us from the worst selfish excesses Of the blind replicators. We have at least the mental equlpment tO foster our long—term selfish interests rather than merely our short—term selfish inter— ests. we can see the long-term benefits Of participating in a conspiracy Of doves', and we can sit down together tO discuss ways 0f making the conspiracy work. We have the power t0 defy the selfish genes 0f our birth and, if necessary, the selfish memes Of our indoctrination. we can even discuss ways Of deliberately cultivating and nurturing pure, disinterested altruism¯ something that has no place in nature, something that has never existed before in the whole history 0f the world. We are built as gene machines and cultured as meme machines, but we have the power tO turn against our creators. alone on can against the tyranny 0f the selfish replicators.
6. Genesmanship WHAT is the selfish gene? lt is not Just one single physical bit of DNA. Just as in the primeval soup, it is 4 ″ が な 廰 of a particular bit of DNA, distributed throughout the world. If we allow our- selves the licence 0f talking about genes as if they had consclous always reassurmg ourselves that we could translate our sloppy language back int0 respectable terms if we wanted t0, we can ask the question, what is a single selfish gene trying to do? lt is trying tO get more numerous in the gene POOI. Basically it does this by helping to program the bodies in which it finds itself to survrve and tO reproduce. But now we are emphasizing that 'it' iS a distributed agency, exlsting in many different individuals at once. The key point of this chapter is that a gene might be able to assist r ど 2 〃 Of itself which are sitting in Other bOdies. If SO, this would appear as individual altruism but it would be brought about by gene selfishness. Consider the gene for being an albino in man. ln fact several genes exist which can give rrse tO albinism, but I am talking about just one Of them. lt is recesslve; that is, it has tO be present ln double dose in order for the person to be an albino. This is true Of about I in 20 000 Of us. But it is also present, in single dose, ln about I in 70 Of us, and these individuals are not albinos. Since it is distributed in many individuals, a gene such as the albino gene could, in theory, assist its own survival in the gene POOI by programrmng its bOdies tO behave altruistically towards Other albino bOdies, S1nce these are known tO contain the same gene. The albino gene should be quite happy if some of the bodies which it inhabits die, provided that in doing so they help other bodies containing the same gene tO survive. If the albino gene could make one Of its bOdies save the lives Of ten albino bodies, then even the death of the altruist is amply compensated by the lncreased numbers Of albino genes in the gene POOI. ShouId we then expect albinos to be especially nice to each
Contents 1. Why are people? 2. The replicators 3. lmmortal coils 4. The gene machine 5. Aggression: stability and the selfish machine 6. Genesmanship 7. Family planning 8. Battle of the generations 9. Battle Of the sexes 10. You scratch my back, I'II ride on yours I I. Memes: the new replicators BibIiography lndex and key to bibliography 22 49 95 117 132 151 179 203 217 221
124 4 襯 / ケ が 4 〃 〃 g earlier chapters 0f this book should have prepared you to be sceptical t0 the point of saying that, plausible as it may sound, the evidence for Wynne-Edwards's theory had better be good, or else.... And Unfortunately the evidence is not good. lt consists of a large number of examples which could be interpreted in his way, but which could equally well be interpreted on more orthodox 、 selfish gene' lines. AIthough he would never have used that name, the chief architect of the selfish gene theory of family planning was the great ecologist David Lack. He worked especially on clutch-size ln wild birds, but his theories and conclusions have the merit of being generally applicable. Each bird species tends to have a typical clutch size. For instance, gannets and guillemots incubate one egg at a time, swifts three, great tits half a dozen or more. There is variatlon in this: some swifts lay only two at a time, great tits may lay twelve. lt is reasonable tO suppose that the number of eggs a female lays and incubates is at least partly under genetic control, like any other characteristic. That is say there may be a gene for laying two eggs, a rival allele for laying three, another allele for laying four, and so on, although in practice it is unlikely t0 be quite as simple as this. Now the selfish gene theory requires us tO ask which Of these genes will become more numerous in the gene P001. SuperficiaIIy it might seem that the gene for laying four eggs is bound to have an advantage over the genes for laying three or two. A moment's reflection shows that this simple 'more means better' argument cannot be true, however. lt leads tO the expectation that five eggs should be better than four, ten better still, 100 even better, and infinity best of all. ln other words it leads logically to an absurdity. Obviously there are c な as well as benefits in laying a large number of eggs. lncreased bearing is bound tO be paid for in less efficient caring. Lack's essential point iS that for any glven ln any envn•onmental situatlon, there must be an optimal clutch size. Where he differs 仕 om Wynne-Edwards is in his answer to the question optimal from whose point of view?'. Wynne-Edwards would say the important optimum, t0 which all individuals should aspire, is the optimum for the group as a whole. Lack would say each selfish individual chooses the clutch size which maxrmizes the number Of children she rears. If three is the optimum clutch size for swifts, what this
8 ル 4 尾 / are competing in what Darwin called the struggle for existence, the individual seems best regarded as a pawn in the game, tO be sacrificed when the greater interest Of the species as a whole requires it. TO put it in a slightly more respectable way, a group, such as a species or a population within a species, whose individual members are prepared to sacrifice themselves for the welfare of the group, may be less likely to go extinct than a rival group whose individual members place their own selfish interests first. Therefore the world becomes populated mainly by groups consisting of self-sacrificing individuals. This is the theory of group selection', long assumed tO be true by biologists not familiar with the details of evolutionary theory, brought out into the open in a famous book by V. C. Wynne-Edwards, and popularized by R0bert Ardrey in ど So 4 / Co 厩 ra The orthodox alternative is normally called 'individual selection although I personally prefer to speak of gene selection. The quick answer of the 'individual selectionist' to the argument just put might go something like this. Even ⅲ the group Of altruists, there will almost certainly be a dissenting minority WhO refuse tO make any sacrifice. If there iS Jl-lSt one selfish rebel, prepared to exploit the altruism of the rest, then he, by definition, is more likely than they are to survlve and have children. Each of these children will tend to inherit his selfish traits. After several generatlons Of this natural selection, the 'altruistic group' will be over-run by selfish individuals, and will be indistinguishable from the selfish group. Even if we grant the lmprobable chance existence initially Of pure altruistic groups without any rebels, it is very difficult tO see what is to stop selfish individuals migrating in from neighbouring selfish groups, and, by inter-marriage, contaminating the purity Of the altruistic groups ・ The individual-selectionist would admit that groups do indeed die out, and that whether or not a group goes extinct may be influenced by the behaviour of the individuals in that group. He might even admit that な the individuals in a group had the gift of foresight they could see that in the long run their own best interests lay in restraining their selfish greed, to prevent the de- structlon Of the whole group. HOW many times must this have been said in recent years tO the working people of Britain? But
お の 〃 め ′ が 4 〃 〃 / 〃 g 129 is sparse. We have already agreed that over-crowding is likely t0 foreshadow famine. ()bviously, if a female is presented with reliable evidence that a famine is tO be expected, it is in her own selfish interests to reduce her own birth-rate. Rivals who do not respond tO the warmng signs in this way will end up rearing fewer babies, even if they actually bear more. We therefore end up with almost exactly the same conclusion as Wynne-Edwards, but we get there by an entirely different type of evolutionary reasomng. The selfish gene theory has no trouble even with 'epideictic displays'. You will remember that Wynne-Edwards hypothesized that animals deliberately display together in large crowds in order tO make it easy for all the individuals tO conduct a census, and regulate their birth-rates accordingly. There is no direct evidence that any aggregations are in fact epideictic, but Just suppose some such evidence were found. WouId the selfish gene theory be embarrassed? NOt a bit. Starlings roost together in huge numbers. Suppose it were shown, not only that over—crowding in wrnter reduced fertility in the following spring, but that this was directly due to the birds' listening tO each other's calls. lt might be demonstrated experi— mentally that individuals exposed to a tape-recording of a dense and very loud starling roost laid fewer eggs than individuals ex- posed tO a recording Of a quieter, less dense, roost. By definition, this would indicate that the calls Of starlings constituted an epideictic display. The selfish gene theory would explain it in much the same way as it handled the case Of the mice. Again, we start from the assumption that genes for having a larger family than you can support are automatically penalized, and become less numerous in the gene pool. The task Of an efficient egg-layer is one of predicting what is going to be the optimum clutch size for her, as a selfish individual, in the coming breeding season. You will remember 伝 om Chapter 4 the special sense in WhiCh we are using the word prediction. NOW hOW can a female bird predict her optimum clutch size? What variables should influence her prediction? lt may be that many species make a fixed prediction, which does not change from year to year. Thus on average the optimum clutch size for a gannet is one. lt is possible that in particular bumper years for fish the true optimum
, イ gg 0 〃 ゞ sta み 市 り 4 〃 ノ 励 ど 川 〃 c ん is the ability t0 build efficient survival machines—bodies. We must now amend that statement. The gene pool will become an れ ん 行 0 〃 / ケ st ん 0f genes, defined as a gene pool which cannot be invaded by any new gene. 、 lost new genes which arise, either by mutatlon or reassortment or rmmlgratl()n, are quickly penalized by natural selection: the evolutionarily stable set is restored. ()ccasionally a new gene does succeed in invading the set: it succeeds in spreading through the gene pool. There is a transitional periOd Of instability, termlnating in a new evolution- arily stable set—a little bit 0f evolution has occurred. By analogy with the aggression strategies, a population might have more than one alternative stable point, and it might occasionally flip from one tO another. ProgreSS1ve evolution may be not SO much a steady upward climb as a serles Of discrete steps from stable plateau t0 stable plateau. lt may ok as though the population as a whole is behaving like a single self-regulating unit. But this illusion is produced by selection gomg on at the level of the single gene. Genes are selected on merit'. But merit is judged on the basis of performance against the background of the evolutionarily stable set which is the current gene pool. By focusing on aggressive interactlons between whole individuals, Maynard Smith was able to make things very clear. lt is easy t0 think 0f stable ratios of hawk bodies and dove bodies, because bOdies are large things which we can see. But such inter— actions between genes sitting in ノ ( 〃 と 〃 ー bodies are only the tip Of the iceberg. The vast (a)ority Of significant interactlons between genes in the evolutionarily stable set—the gene pool— go on ル ″ ん individual bOdies. These interactlons are difficult t0 see, for they take place within cells, notably the cells of developing embryos. WeII-integrated bodies exist because they are the product of an evolutionarily stable set of selfish genes. But I must return to the level of interactlons between whole animals which is the maln subject Of this b00k. For understand- lng aggresslon lt was convement tO treat individual animals as independent selfish machines. This model breaks down when the individuals concerned are close relatives—brothers and sisters, cousrns, parents and children. ThiS iS because relatives Share a 93