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Why group selection is not true

I occasionally buy the magazine Science Reporter, run by CSIR, a government funded research body. The quality of the articles is not usually great, but in the March 2012 issue, there is an article on evolution of sex which exposes a fundamental problem in thinking about Natural Selection. So let us have a look at what that is so that we understand Natural Selection better.

I have earlier mentioned that evolution by natural selection is best understood when looked at from the perspective of the gene. Talking about the genes as if they are conscious entities striving to maximise the number of copies of itself in the population is a good way to understand Natural Selection. Of course, genes are no more goal seeking entities than a table or a chair, but a gene that happens to contribute towards the individual’s well being, will leave more copies of itself (by leaving more children), and so by default such genes tend to survive well.

If the purpose of a gene is to make maximum copies of itself, then sex seems to be a disadvantage because from the gene’s perspective sexual reproduction means that it has only a 50% chance of having a copy of itself in the children. For example, a gene in your mother will only have a 50% chance of being in you. So is the case with every gene in your father. But in case of asexual reproduction, every gene in the parent will definitely be in the child (except for the occasional random mutation), since the child is just a clone of the parent. So genes in asexually reproducing organisms will leave twice the number of copies when compared to sexually reproducing organisms. This is referred to as the dual cost of sex. To compensate for this, sex must some how bestow benefits to a sexually reproducing gene that is at least twice that of an asexually reproducing organisms.

And it is this advantage that scientists are facing a tough time to understand. It is not for lack of attempts though. I don’t intend to lay out what scientists think on this. If you want to understand that you can refer to this absolutely fantastic book, by Matt Ridley, called The Red Queen. I cannot recommend it enough. It cost me somewhere between Rs. 250 and Rs.300. For such a price, the book is a treasure.

Now coming to the common understanding of natural selection. At one time, there was this idea of “Group Selection” that was popular among evolutionary biologists. The idea is that a gene that gives an advantage to the group of individuals to which it belongs, will tend to survive more. For example, consider a gene that makes the individual ready to sacrifice one’s life to save the clan he/she belongs to. It is definitely an advantage to the clan, because such dedicated soldiers will do much better in protecting the clan than those who are worried about their own lives. Thus, it was supposed, a gene that works for the betterment of the group will leave more copies (after all the clan is protected).

This argument, that something that is good for the group will spread, in spite of the disadvantage to the individuals, was widely touted and believed. The problem with such an argument is simple. Imagine a group that has individuals ready to put down their lives for the group’s sake. And suppose, that in this group a child is born who is more bothered about his own life than about the clan’s. Such individuals will stay away from any clan fights and thus have very little chances of dying without reproducing. So the genes of such individuals begin to spread. Thus the group will slowly start to have more and more individuals, who are selfish. Thus the gene for sacrifice will stop spreading.

One might say that, this is the case even within a single individual. After all, an individual too is a collection of genes, and since the gene’s eye view is the best perspective on natural selection, the genes too can be selfish and need not be bothered about the individual. But the difference between the group and the individual is the idea of a reproductive bottleneck. For a gene in an individual, the only way for it to make copies is to make more copies of the individual, since that is the only way they can leave offspring. Thus, all the genes within an individual will cooperate for the benefit of the individual. But in case of a group, all the individuals within the group do not reproduce in one go, or through one such common pathway. Each one can reproduce on its own. This lack of a reproductive bottleneck leaves Group Selection without a mechanism to work. That is the reason why it is not considered to be true.

There are exceptions to this which beautifully illustrate the point being made. Imagine a group of bees. As is common knowledge, bees usually have one queen that does all the reproducing. The rest of the worker bees are sterile in that they cannot reproduce. Thus if the genes in the sterile workers have to leave copies, the only way to do that would be to help the queen to reproduce (since the workers share genes with the queen, who is after all their mother). In this case, the queen acts as the reproductive bottleneck and thus in bees, you find individuals who are ready to put down their lives for the group.

Thus the existence of a trait cannot be explained by listing out the advantages it gives to the population. The only way to explain a trait is to explain the benefit to the individual and thereby to the gene. Now I come to the article that I referred to in the beginning. The author talks about how the sex is an advantage because it protects us from parasites and thus prevents the species from going extinct. If I have done my job well, so far, you will spot the problem right away. The genes are not worried about the species going extinct. It is only bothered about itself, and about the individual (to the extent that the individual benefits the gene, which in most cases is quite a bit). And that is what is wrong with the article.

So I wrote a letter to the editor pointing this out. I am reproducing it here.

Dear Editor,

This is regarding the article titled “Evolution of Sex” in the March 2012 issue of Science Reporter. In this the article talks about how sex evolved and why it is advantageous, but there is a fundamental problem in the author’s approach to this question of sex.

It has long been recognised by most evolutionary biologists, except for a few on the fringe, that group selection does not work and there is no evidence for it happening. But the idea was popular at one time. And that has left a hangover. This hangover is visible in this article too. The last paragraph of the article says:

“Why did sex evolve? From the evolutionary point of view sex is definitely an inefficient way to reproduce, but it acts as a safeguard against extinction”

That traits are selected to prevent species from going extinct is a group selectionist argument. This assumes that natural selection works to help species or groups survive. This is not true. Natural selection works only for the benefit of the gene. All the genes in an individual have to reproduce via the individual, who acts as a genetic bottleneck, and thus in most cases what is good for the individual is also good for the gene. There are exceptions for that too. But no gene is selected for the survival of the groups (except in cases like eusocial insects, where there is a genetic bottleneck like the queen). Thus the idea that something is good because it prevents extinction of the species, is flawed.

Earlier in the article too, one can see this sentence.

“If all organisms continue to reproduce asexually, the genetic variations of its species as a whole will slowly grind to a halt and it becomes likely that a parasite that can kill one member of the species can wreak havoc on the entire population, which will not be able to get rid of the harmful effects of mutation”.

Here too, the author talks about parasites being the bane of the species. That parasites are considered an important cause for the evolution of sex is true, but that is not because it will “wreak havoc on the entire population” but because without sex, it puts the individual at a disadvantage when facing parasites. The difference, though seemingly subtle, is key to a correct understanding of evolution by natural selection.

Madhav.

The author of the science reporter article is a “Retired reader and Head, Department of Zoology, Madurai College, Madurai. He has a teaching experience of 35 years”. What surprises me is how somebody who has so much experience misses this very important point.

PS: For a recent kerfuffle on this topic, you can refer to this post by Jerry Coyne. The name of this post is inspired from the name of his website (he does not like people calling it a blog).

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