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 in 1930, is that there is individual “frequency-dependent” selection that enforces an even sex ratio. Consider, for instance, a population in which females predominated, and males were rare. In that population, a female who produced more males would have more grandchildren than other females, for the average reproductive success of her offspring will be higher. (Imagine if there were only one male and elebenty gazillion females in a population. A mutant female producing mostly or all males would have huge numbers of grandchildren, for her male offspring would inseminate most of the females. Evolutionarily, whatever genes gave her that male-biased sex ratio would increase in the population.)
The reverse would be the case if males predominated in the population: any mutant individual producing more females would leave more grandchildren.
In this case, then, the rarer sex always has a reproductive advantage, and any variant individual producing the rarer sex would have an evolutionary advantage. The upshot is that the sex ratio will reach equilibrium only when there IS no rarer sex, i.e., when there are equal numbers of males and females. In such a case no new mutant individual will have a reproductive advantage. This has been tested experimentally by varying sex ratios in species which have three sex chromosomes, and populations always settle down at the 50/50 sex ratio.
Although there are some exceptions to a 50/50 sex ratio in animals, most conform to the 50/50 value. This is precisely what is expected if sex ratio is a result of individual and not of group selection. Ergo, when the two are in evolutionary conflict, as they are here, individual selection wins. And evolution is the answer to a question you’ve probably never asked yourself: why are there as many females as males?
I still know of no adaptation in nature that is explained more plausibly by group selection than by individual or kin selection; but there are plenty of adaptations, like sex ratio, easily explained by individual or kin selection.
It’s time for biologists to stop banging on about group selection until we find evidence that it has actually operated in nature. We don’t have time to waste on theoretically plausible but infrequent mechanisms for which there’s no evidence.