Although this post is about individual vs group selection theories, I would like to focus on the example put forward. – Why the sex ratio is bound to remain approximately 1:1 in many organisms? I discuss this often in my own general bio class when we come to stabilizing pressure on populations, but I think I will start using this extreme example to make the point even clearer.
I’m teaching introductory evolution this quarter, and am using as a textbook Doug Futuyma’s Evolution (second edition, Sinauer). Today’s lecture will be on the maintenance of genetic variation via natural selection (heterosis, etc.), and in the textbook under “frequency dependent selection,” I see this on page 319:
Why is the sex ratio about even (1:1) in many species of animals? This is quite a puzzle, because from a group-selectionist perspective, we might expect that a female-biased sex ratio (i.e., production of more females than males) would be advantagesous because such a population could grow more rapidly. [JAC: such a sex-ratio-biased group would then outcompete other groups and predominate]. If sex ratio evolves by individual selection, however, and if all females have the same number of progeny, why should a genotype producing an even sex ratio have an advantage over any other?
The answer, first realized by Ronald Fisher…
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