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Evolution__3rd_Edition

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338 PART 3 / Adaptation and Natural Selection<br />

Individual selection favors a 50 : 50<br />

sex ratio ...<br />

. . . unaffected by sex differences in<br />

adult mortality<br />

biased sex ratio a one with four females for every one male, for instance. Each male in<br />

the population will fertilize on average four females. This condition could not be stable<br />

for long in evolution, because an average male is producing four times as many offspring<br />

as an average female. There is an advantage to being a male, and an advantage to<br />

a female who produces extra sons a because sons have a higher reproductive success<br />

than daughters.<br />

If a mutant female arose who produced only sons, the total reproductive success of<br />

her offspring would be 20/8 times that of an average female (the mutant produces five<br />

males, each with a relative reproductive success of 4, for every one male and four<br />

females produced by the average female). The mutant would spread. As it spread, the<br />

population sex ratio would become less and less female biased. The same argument<br />

works in reverse for a population with a male-biased sex ratio. The reproductive success<br />

of the average female is then higher than that of a male, and natural selection will<br />

favor mutant females that produce more daughters than sons. Only when the sex ratio<br />

is equal are the relative reproductive successes of the two sexes equal. At that point<br />

there is no advantage in producing more of one sex than the other. The 50 : 50 sex ratio<br />

is the equilibrium that the population moves to, over evolutionary time, and then stays<br />

at. Any population that deviates from the 50 : 50 sex ratio will be shifted back to it by<br />

natural selection.<br />

The fundamental reason why the 50 : 50 sex ratio is stable is that every organism has<br />

one father and one mother. All the females together contribute the same number of<br />

genes to the next generation as all the males together; when members of one sex are in<br />

short supply, their average success must increase.<br />

To be exact, Fisher’s theory predicts a 50 : 50 ratio of investment by the parents in<br />

male and female offspring. This usually translates into a 50 : 50 sex ratio in the zygotes.<br />

The sex ratio among adults may be biased away from 50 : 50 if males have higher or<br />

lower mortality than females. This does not mean that selection favors any compensating<br />

bias at the early stages to produce more of the high-mortality gender.<br />

Suppose, for instance, that males have higher mortality than females on average. The<br />

adult sex ratio will be female biased. Among the individuals that do survive to reproduce,<br />

the average male will have a higher reproductive success than the average female.<br />

But as far as a mother is concerned, the extra reproductive success of her surviving sons<br />

exactly balances the zero reproductive success of those that die before reproducing.<br />

When she produces a son, she cannot “know” in advance whether he will be a survivor<br />

who will have higher than average success, or die and not reproduce at all. At birth, a<br />

male can only be expected to have the success of an average male, and the average male<br />

has the same reproductive success as the average female. The offspring of any one parent<br />

will suffer any sex differences in mortality in the same proportion as the population<br />

as a whole and there is nothing to be gained from producing more of the sex that will<br />

(in the adult stage) be in the minority. Any parent who did so would simply increase the<br />

average mortality rate among her progeny.<br />

However, Fisher’s argument makes various assumptions. When the assumptions are<br />

altered, the predicted sex ratio is altered too. In the past 30 years or so, biologists have<br />

used Fisher’s basic theory to test for biased sex ratios in many peculiar circumstances.<br />

In some cases, the predictions are quantitative and the tests experimental. The sex ratio<br />

has proved to be a remarkably fertile testing ground for theories of adaptation, and the<br />

..

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