by
Jun 08. 2016
"이타적 유전자"의 자연선택모델
Inclusive Fitness에 관하여
Inclusive fitness란, 진화론에서 논란의 대상이었던 “이타주의”적 행동이 어떤 식으로 “자연 선택"에 의해 전해지는지에 대한 모델이다. 한 마디로 요약하자면 소위 '이기적인 유전자'가 개체의 그룹 내 이타주의를 어떤 식으로 발생시키는지를 설명해 주는 이론이라고 할 수 있겠다. 이에 대척점이랄 수 있는 이론이 group selection인데, 말하자면 이타적인 구성원이 많은 집단일수록 생존에 유리하여 결국 그것이 진화적으로 선택된다는 이론이다.
우선, 다음 동영상을 시청하기 바란다. (영어 동영상이며 대본은 이 글의 마지막 부분에 있다.)
2014년 2월 18일 (옥스포드대학 노조-_-로 착각하기 쉬운ㅋ) Oxford Union 토론협회에서 주최한 토론회의 QnA 세션인데, 리처드 도킨스가 Inclusive Fitness ("Kin selection" 이라는 말로 직관적 설명이 될듯하다)에 대해 질문자에게 간단한 강의를 해 주고 있다. 여기서 도킨스는 이타주의 자연선택 메커니즘에서 거의 정설로 여겨지고 있는 W. D. Hamilton의 inclusive fitness 이론에 대해 QnA 시간을 이용해 간략하게 (약 14분) 설명하고 있는데, 질문이 구체적으로 나와 있지는 않지만 어째서 E. O. 윌슨이 inclusive fitness 를 거부하고 group selection 으로 돌아섰는지에 대한 질문인 것 같다.
2010년에 E. O. Wilson은 진화학계에서 이타적 행동의 진화를 설명하는 가장 유력한 학설인 inclusive fitness 모델을 거부하고 당시 거의 사장돼 있었던 group selection 이론을 지지하는 논문을 네이쳐지에 게재하여 파문이 일었다. 다음은 문제의 E. O. Wilson 의 논문인데, 링크를 보면 알다시피 많은 양의 (안티)코멘트가 달려 있다.
(1) "The evolution of eusociality"
(2) 이론적 배경(부록)
- 이후 논쟁들:
(1) "Inclusive fitness theory and eusociality"
(2) "Only full-sibling families evolved eusociality"
(3) "Kin selection and eusociality"
(4) "Inclusive fitness in evolution"
(5) "In defence of inclusive fitness theory"
- 관련 한글 논문:
"포괄적합도 이론 논쟁과 의미론적 문제" (＜과학철학＞ 전진권 ＆ 장대익, 한국과학철학회.)
이에 도킨스는 이타주의적 행동에 대한 Hamilton's rule을 설명해 주고, 어째서 윌슨이 이를 거부하는지를 추측해 본다.
Hamilton's rule:
(Relationship)*(Benefit)＞(Cost)일 경우 이타적 행동이 퍼진다 - 즉, 선택압에 대한 유전적 적응/적합도는 직계자손 뿐 아니라 모든 혈연관계도 고려해야 한다 - 는 '이타적 행동'에 관한 진화이론.
"Inclusive fitness 는 1930년대에 네오다위니즘이 대두하면서 수학적 계산에 의해 연역된 법칙이기 때문에 이를 실험적으로 증명하려 한다는 것은 피타고라스의 정리를 삼각형 그려서 실측해서 증명하려는 것과 같다고 본다. 즉 이는 윌슨의 주장과는 달리 실험적 증거를 통해 증명해야만 하는 부산물이라고 볼 수 없다."
"전체 혈통에 대한 이타적 행동이 전혀 없는 생물종의 경우라도, 그냥 R*B≤C 의 관계가 그것을 유발했을 뿐인 것이지, Hamilton's rule 자체에 대한 반증이 되지 못한다."
"윌슨과 동료들이 inclusive fitness를 거부하는 것은, 아마 계산 자체가 어렵기 때문에 필드 생물학자들이 적용하기에 실용적이지 못하기 때문 아닐까."
다만 도킨스가 개인적으로 선호하는 것은 개체가 inclusive fitness를 최대화시키느니 그딴것 복잡하게 생각하지 말고 막바로 유전자 레벨로 들어가서 유전자가 개체를 조종하는 부분에 대해 따져보는 것이라고 한다.
집단유전학에서 나온 inclusive fitness 이론은, 자연 선택으로 인한 생존 경쟁에서 "유전자"가 살아남기 위해 택하는 방법을 설명하기 위한 이론으로, 어째서 "이타주의"가 진화적으로 유리한지를 설명하기 위한 이론인데, 여기서 "fitness," 즉 "적합(適合)"의 진화적 의미를 설명해 주고 있다.
진화에서 fitness (적합)의 의미:
[다윈 시대 - 더 강한 쪽] vs [현대 - 살아남는 쪽]
직계가족 뿐 아니라 일가친척 및 기타 혈연관계에 있는 모든 혈족들의 생존률 및 번식률을 높여주는 방향으로 자연선택이 이루어진다는 것을 1960년대 중반에 W. D. Hamilton 이라는 집단유전학자＆진화학자가 수학적으로 계산했는데, 요약하자면 자연선택에 살아남은 '적합(fitness)'은 개체의 생존보다는 그 개체가 담고 있는 유전자의 생존에 유리함을 말함이다.
(다음은 내가 동영상을 듣고서 직접 받아적은 대본이다. 틀린 부분이 있을 수 있으나 대체적인 의미는 전달되리라고 본다.)
If you look at what actually changes in evolution it is changes in gene frequencies in gene pools as generations go by. I'm talking now about sexually reproducing organisms such as ourselves, such as mammals, birds, such as all vertebrates, and so on. Now, the reason that gene survival is important is that genes in the form of copies of the DNA are immortal. They are potentially immortal, in the sense that what goes on from generation to generation, is the coded information in the DNA. So potentially a bit of DNA code can persist for tens of millions, hundreds of millions of years. Not all of it does, therefore there's an important difference between those bits of DNA code that do persist for over many generations and those that don't. That is natural selection.
The reason why I insist on talking about natural selection at the level of the genes is precisely that it really makes a difference at the level of the gene because some genes are going to go on forever for a very long time and others are not. No other units, no other level in the hierarchy of life has that property; individual organisms don't, groups don't, species don't, nothing else has that property of 'potential immortality' which genes have. Now this is nothing new; this is the way that people have looked at natural selection ever since 1930s when the great founders of the population genetics modified Darwinism, Darwin himself didn't know anything about genetics. It, genetics, was imported properly into evolutionary theory in the 1930s, and that time it was understood that natural selection is the differential survival of genes in gene pools.
Now, as a euphemism, people will sometimes say animals do so and so to perpetuate the species. They realized that survival itself is not enough. Survival, about Darwin realized of course, the survival plus reproduction is what matters. Survival is a means to be end of reproduction. And, a euphemism for reproduction is perpetuating the species. And partly out of the result of that, a sort of myth grew up of what animals are doing, the reason why they do what they do is to perpetuate the species. The real reason is to perpetuate their genes, if you forgive the teleological language, the real reason that is to perpetuate their genes, perpetuating the species is an incidental consequence.
The idea of perpetuating species of perpetuating the group was semi-formalized into theory of group selection, and some biologists tried to explain the evolution of thins like altruism as a result of the differential survival of groups. Those groups, in which individuals care for each other, survive better than groups in which individuals don't care for each other; that was group selection not aimed to survival, but perhaps bad off other daughter groups. That theory of group selection was correctly rubbished in the 1960s by various people, and perhaps the most significant of them was W. D. Hamilton who invented the phrase 'inclusive fitness,' which the questioner referred to, and I say that inclusive fitness was not the most clear way of explaining the point of view but is the one that Hamilton brilliantly put forward.
Fitness, in Darwin's time, fitness meant roughly what we in colloquial language mean by fitness it meant being fast running, it meant being strong, it meant being attractive, all the thing that we in all ordinary language might mean by fitness. When the population genetics revolution happened, the word fitness was given a technical meaning which was pretty much 'that which survives,' which meant that survival of the fittest, the phrase that Darwin took over from Herbert Spencer, survival of the fittest became a tautology because the fittest from whole is defined as that which survives.
The technical meaning of fitness pretty much meant the reproductive success of an individual not only to the next generation but the grandchild generation, the great grandchild generation, and so on. So a 'fit' animal is one who had what it took, who was equipped, to leave a lot of descendants that has obvious connection with the classical meaning of fitness; you can run fast if you got keen eyesight and so on, that helps you, that equips you, to have lots of descendants, but fitness was actually defined mathematically in this precise way.
When Hamilton came along, he realized that, because what really matters is gene survival, it's not enough to count just children and grandchildren and great-grandchildren direct descendants, because collateral relatives - siblings, nephews and nieces and so on - also share genes. Therefore a gene that makes an individual care for a brother or sister and nephew or niece would survive for the same kind of a reason as a gene for caring for children and grandchildren. Hamilton realized that what matters is the statistical probability of a gene being shared and that statistical probability can be calculated whether or not the descent is lineau or whether we're talking about collateral relatives. And he showed that what matters is the quantity called are the 'coefficient of relationship' which is a half for offspring, a half for full siblings, a quarter for half siblings, a quarter for nephews and nieces, an eighth for first cousins, a sixteenth for second cousins, a quarter for grandchildren, and so on. So natural selection then favors animals that care for those other animals that have a statistical likelihood of sharing the same genes.
Hamilton could've left it of that, and that was indeed the way he phrased it in his first paper published in 1963 in the American Naturalist. However, he wanted to take over the concept of fitness and modify it, in the way that it needs to be modified in order to take account of collateral relatives, because his audience, professional biologists, were already accustomed to talking about fitness, and Hamilton, what he said 'look, you can keep your fitness in your equations but you must modify it to take account of collateral relatives.' And the modified former fitness that he invented was 'inclusive fitness' and he did a lot of rather difficult mathematics in order to show how to calculate inclusive fitness. Informally you can define inclusive fitness as that quantity which an individual will appear to be maximizing, when what is really being maximized is gene survival.
The great triumph of Hamilton's theory was the social insects; it works for all animals, it applies to all animals in the form of a simple equation called Hamilton's rule which states that an altruistic act will spread through the population if RB is greater than C, where R is the coefficient of relationship, that fraction I told you about a half a quarter whatever it is, B is the benefit to the recipient, and C is the cost to the donor - the cost to the altruist. But as I say he wanted to express, he wanted to find a modification of fitness that you need in order to the others were rescued the concept of fitness.
And I said that the great triumph of his theory was with social insects but I also said that R be greater than C works for all animals and all plants even if R is so small, or B and CR so large, that it you can more or less, forget about collateral relatives, and that is true for many animals. In the case of the social insects, R is high, social insects are, the workers are sterile, they're closely related to their fertile relatives, so worker ant doesn't reproduce in most cases it doesn't reproduce itself, but is closely related to the young queens and males, which is going to be churned out by the nest. And that's why the ants work, that's why worker ants do work they do, because they share genes with the queens and males which are going to propagate copies of those very same genes into the next generation.
Now the question I was asked why such eminent biologists as E. O. Wilson the great entomologist, the great ant specialist, the world's leading expert on ants indeed, why he has turned against inclusive fitness, it's not entirely clear to me, I mean, it's as though he thinks, and this actually goes back to his great book, 'sociobiology,' which already in 1975 betrays a certain misunderstanding of the idea of inclusive fitness. Wilson clearly thought that caring for relatives is a kind of unparsimonious addition to the ordinary theory of natural selection which was what we were quite happy with for the 1930s onwards. So for Wilson, you don't move on to inclusive fitness unless you absolutely have to, because classical fitness, which applies only to direct offspring and descendants, grandchildren and so on, will do the job.
My reply to that would be, you can't escape from inclusive fitness, you can't escape from looking at collateral relatives whether you like it or not, RB greater than C is the correct formulation of the way of natural selection works, and if you've got at species in which there is no altruism toward collateral relatives that's absolutely fine, it just means that the B's and C's and R's work out that way. But you cannot get away from inclusive fitness; it follows deductively from the neo-Darwinian synthesis from the 1930s; it's not an add-on, it's not an additional thing that has to be justified by empirical evidence. It's there deductively in the mathematics. To go around looking for species of animal and testing Hamilton theory is a bit like testing Pythagoras' theorem by going out with a ruler and measuring right angled triangles - you don't do that, you prove Pythagoras' theorem by mathematical deduction and that's what inclusive fitness is, like.
Now I think, is a way of interpreting why Wilson and his colleagues, I mean nothing else the weakened, watered-down version of their objection which is that inclusive fitness itself is difficult to calculate in practice, and that's true. It is probably a difficult thing to do in practice unless you could measure very, very precisely lot of difficult economic quantities. So, it's a reasonable objection to say that inclusive fitness is not a practical measure for a field biologist. But you cannot say that 'therefore altruistic behavior is not explained by Hamilton's rule, RB greater than C, and my own preference would be actually to forget about inclusive fitness and go straight to the level of the gene, and think in terms of genes manipulating individual organisms, rather than think in terms of the individual organisms trying to maximize their inclusive fitness.
참고하기:
Hamilton, W.D. (1964). The genetical evolution of social behaviour. I. J Theor Biol. 7(1):1-16.
