Evolution__3rd_Edition

338 PART 3 / Adaptation and Natural Selection
Individual selection favors a 50 : 50
sex ratio ...
. . . unaffected by sex differences in
adult mortality
biased sex ratio a one with four females for every one male, for instance. Each male in
the population will fertilize on average four females. This condition could not be stable
for long in evolution, because an average male is producing four times as many offspring
as an average female. There is an advantage to being a male, and an advantage to
a female who produces extra sons a because sons have a higher reproductive success
than daughters.
If a mutant female arose who produced only sons, the total reproductive success of
her offspring would be 20/8 times that of an average female (the mutant produces five
males, each with a relative reproductive success of 4, for every one male and four
females produced by the average female). The mutant would spread. As it spread, the
population sex ratio would become less and less female biased. The same argument
works in reverse for a population with a male-biased sex ratio. The reproductive success
of the average female is then higher than that of a male, and natural selection will
favor mutant females that produce more daughters than sons. Only when the sex ratio
is equal are the relative reproductive successes of the two sexes equal. At that point
there is no advantage in producing more of one sex than the other. The 50 : 50 sex ratio
is the equilibrium that the population moves to, over evolutionary time, and then stays
at. Any population that deviates from the 50 : 50 sex ratio will be shifted back to it by
natural selection.
The fundamental reason why the 50 : 50 sex ratio is stable is that every organism has
one father and one mother. All the females together contribute the same number of
genes to the next generation as all the males together; when members of one sex are in
short supply, their average success must increase.
To be exact, Fisher’s theory predicts a 50 : 50 ratio of investment by the parents in
male and female offspring. This usually translates into a 50 : 50 sex ratio in the zygotes.
The sex ratio among adults may be biased away from 50 : 50 if males have higher or
lower mortality than females. This does not mean that selection favors any compensating
bias at the early stages to produce more of the high-mortality gender.
Suppose, for instance, that males have higher mortality than females on average. The
adult sex ratio will be female biased. Among the individuals that do survive to reproduce,
the average male will have a higher reproductive success than the average female.
But as far as a mother is concerned, the extra reproductive success of her surviving sons
exactly balances the zero reproductive success of those that die before reproducing.
When she produces a son, she cannot “know” in advance whether he will be a survivor
who will have higher than average success, or die and not reproduce at all. At birth, a
male can only be expected to have the success of an average male, and the average male
has the same reproductive success as the average female. The offspring of any one parent
will suffer any sex differences in mortality in the same proportion as the population
as a whole and there is nothing to be gained from producing more of the sex that will
(in the adult stage) be in the minority. Any parent who did so would simply increase the
average mortality rate among her progeny.
However, Fisher’s argument makes various assumptions. When the assumptions are
altered, the predicted sex ratio is altered too. In the past 30 years or so, biologists have
used Fisher’s basic theory to test for biased sex ratios in many peculiar circumstances.
In some cases, the predictions are quantitative and the tests experimental. The sex ratio
has proved to be a remarkably fertile testing ground for theories of adaptation, and the
..