# Samir Okasha - Philosophy of Function, Fitness, Adaptation

## Метаданные

- **Канал:** Closer To Truth
- **YouTube:** https://www.youtube.com/watch?v=y1k80UQsDUE

## Содержание

### [0:00](https://www.youtube.com/watch?v=y1k80UQsDUE) Segment 1 (00:00 - 05:00)

Samir, is it fair to say that the um modern synthesis that was developed in the early mid 20th century so-called neody Darwinism had its core on the concept of uh and the development of population genetics? And if so, what is population genetics? How can we understand it? — Yeah, I would say it's true to say that population genetics was at the core of the modern synthesis, the modern evolutionary synthesis. So population genetics refers to a discipline that arose in the 1920s and 30s primarily thanks to the work of RA Fischer and JBS Haldane in the UK and Su Wright in the US and their achievement was to integrate Darwin's theory of evolution by natural selection with the recently rediscovered principles of Mandelian genetics. um that that's the idea that um genetic inheritance follow works according to the Mandelian rules. Now for a long time those ideas were considered incompatible and what they managed to do was to demonstrate that they were not just compatible but excellent bedfellows. So what in essence population genetics does is start with a highly abstract description of a population in terms of its genetic composition. That's to say we describe a population by specifying the frequency of every genetic alil, every gene at every locus in on each chromosome in a in a population in a hypothetical population. uh when we're making a model and the aim then of population genetics is to describe how the population's genetic composition will change with time um dependent on the evolutionary forces operating on that population such as natural selection mutation what's sometimes called random drift which essentially just means chance changes in gene frequency that arise in a finite population and migration into the population. So that's in essence what population genetics is all about. Describe the change in the genetic composition of a population over time in response to the fundamental evolution. — And how does the genotype relate to the phenotype? — Well, I mean how the genotype relates to the phenotype is of course a highly important question. Uh but what population genetics does is abstract away from that by in its in the simplest formulations of population genetics, one simply describes all of the possible genotypes in a population. Mhm. And then assigns them a fitness value. So the idea is simply that an organism with one genotype may survive better or leave more offspring or both than an organism of a different genotype. And when that's the case, when there are differences in fitness between the individuals of the different genotypes, that will be because they have a different phenotype as a result. But in population genetics, one ignores that middle bit and doesn't attempt to specify the phenotype at all. — It's just the results of the phenotype precisely, which is the rep the reproduce the frequency of reproducibility and how one displaces another. — Right. That's right. So it's an indirect description of how evolution works because I mean the way evolution really works is that how successful organisms are depend on their phenotypes not only and only indirectly on their genotypes. However, the pattern of inheritance of phenotypes is extremely complicated and dealt with in a somewhat different body of theory called quantitative genetics that does explicitly deal with the genotype phenotype relationship. But what's population genetics in the strict sense simply ignores that relationship and it's there implicitly and directly considers um the the difference in fitness of the genotypes themselves. — What what's an example of um of an evolutionary scenario in which population genetics has made a definitive contribution? — Um one can think of a number. So one is in explaining how it is that genetic variation can be maintained in a population. That was a long-standing interest of population geneticists was to understand why it is that populations are not always just entirely homogeneous genetically. Where does all this variation come from? One might wonder why natural selection doesn't render populations genetically homogeneous. I mean if all if the good genotypes prosper and the poorer ones filtered out. Surely everyone will end up with the good ones. So where does all the genetic variation come from? [snorts] Um there are a number of answers to that question. In part it's simply that mutation continually reintroduces it. In part it may be because there's spatial or temporal variation in the population. So some genotypes do better in certain places or

### [5:00](https://www.youtube.com/watch?v=y1k80UQsDUE&t=300s) Segment 2 (05:00 - 10:00)

in certain times than others. And yet another example known as heterrozygote superiority which arises when the heterroygote who has [clears throat] different copies of who has different alles of a single gene is superior in viability to the homozygot — which often code for diseases or — it's possible. So the mo I mean the best known example is um the existence of um the cickle cell — um alil which is maintained in a population because in certain populations in Africa because if you have although if you have two copies of the cickle cell alil then you you're you'll suffer um a high fitness cost. If you just have one copy then that gives you protection against malaria. So if you're heterroygote then you're superior in fitness to the homozygote for the cickle cell al and to the homoygote for the wild type for the other normal. — So it's maintained in society because of that. And so what population genetics does is to formally show how it's possible for um a population to be in genetic equilibrium while in this case that unfortunate directions but that it causes a certain kind of balance. — Exactly is precisely is maintained — and you might not have thought that was possible until it had been demonstrated with very simple population genetic formalism. That's a a terrific example. What about inclusive fitness? Uh h how that that's been an area of great controversy. You've dealt with it. — Inclusive fitness is a notion that was invented by the English biologist WD Hamilton in his papers in the early 1960s uh where he was concerned with the problem of how altruistic behavior can spread in in evolution by natural selection. a famous puzzle that had troubled Darwin as well. Um, and Hamilton showed in in essence that it is possible for altruistic behaviors to evolve by natural selection despite the being individually costly. So an individual who behaves altruistically, that's to say who helps others pays a fitness cost. But nonetheless, the genes encoding that altruistic behavior can spread by natural selection, Hamilton showed, so long as the beneficiaries of the altruistic behavior are also more likely than average to be altruists. And it here it's here that the notion of inclusive fitness arises. So if you think of say um an extreme of of altruism in the living world. So take a worker honeybee for example that doesn't reproduce itself at all but devotes its whole life to aiding the queen's reproduction in the colony indirectly by foraging you know tending the larae um protecting the nest against invaders and so on behaving altruistically in the biological sense. Now clearly the reproductive fitness of that individual honeybee is zero. So we couldn't possibly think that it's its behavior is been selected to maximize or improve its [clears throat] fitness. But what Hamilton did is to show that in certain circumstances there's a different quantity called the inclusive fitness of an organism which takes into account its reproductives the reproduction of its relatives and that roughly speaking in the circumstances in which an altruistic behavior will evolve by natural selection the net effect will often be that organisms appear as if their phenotypes including their behaviors are designed to maximize their inclusive fitness rather than their own directness. — Why is that such a controversial field and and what kind of criticisms have been leveled against it? — It has led to enormous controversy somewhat surprisingly in part because um the whole area the whole topic of altruistic behavior and the associated levels of selection discussion has always polarized evolutionists. So some of Hamilton's contemporary supporters have argued that the inclusive fitness concept is the a fundamental Darwin a fundamental contribution to evolutionary theory and fundamentally generalizes the notion of Darwinian fitness to cover um social behavior — and so they think of it as at the heart of the important generalization of evolutionary principles to social behavior. Detractors of the inclusive fitness idea, however, have pointed to certain technical limitations with the concept, such as, for example, that in

### [10:00](https://www.youtube.com/watch?v=y1k80UQsDUE&t=600s) Segment 3 (10:00 - 11:00)

Hamilton's earliest um models describing how inclusive fitness works, he makes a number of restrictive modeling assumptions, in particular, an assumption known as weak selection, um which is rather critical to the derivations. And people have pointed out that if one relaxes those assumptions, then inclusive fitness, it's no longer possible to demonstrate that inclusive fitness will be maximized um by the evolutionary process and more fundamentally that in some contexts inclusive fitness may not be a mathematically well-defined quantity at all. Those objections are well taken I think but the fact remains that it has been a heristically extremely valuable concept for evolutionists particularly for biologists in the field who will study it of sometimes study a social behavior under the assumption that it must somehow contribute to the organism's inclusive fitness because otherwise we wouldn't see it. Um and so it gives them a way of applying adaptive reasoning or adaptationist reasoning if you like to social behavior despite the fact that such behavior it does not benefit an individual's personal fitness. So that's a flavor of why this has been so controversial. On the one hand heristically enormously valuable well- definfined in the context of certain specific models. Its limitation being that precisely it's only in those contexts that it's well defined and plays the evolutionary role that has sometimes been claimed for

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*Источник: https://ekstraktznaniy.ru/video/40342*