Evolution__3rd_Edition

..
. . . which may explain extreme
characters, such as the peacock’s
tail
The “handicap” theory is a second
evolutionary theory of female
choice
CHAPTER 12 / Adaptations in Sexual Reproduction 331
mating advantage. As female choice grows commoner and the tail length grows past the
optimum for survival, the relative importance of the two advantages shifts over, until
we reach a third stage at which further elongation is driven purely by female choice.
As the population evolves past the point of optimum tail length, the selective forces
at work have become almost absurd. The runaway process will only come to a stop
when the death rate of males, due to their feathery excess, is so high that their success in
mating no longer makes up for it. The tail length will then reach an equilibrium. That
equilibrium, according to Fisher, is what we are now observing in birds like peacocks
and birds of paradise.
The original problem was to explain the evolution of a set of apparently deleterious
characters. Darwin’s solution was that they could be maintained by female choice. He
did not, however, explain why females should come to choose males with deleterious
characters, nor why the choice would not be lost by natural selection. In Fisher’s theory,
when the choice first evolved, the male character was much smaller and choice then
favored males with higher survival. Genes for choice could thus increase in frequency
from being rare mutants to being the majority form in the population. Once nearly all
the females in a population choose mates in a certain way, mutant females that pick
some other sort of male are selected against (because of the effect on the kind of sons
they produce). The cost of the male character at the final equilibrium serves no function
for the female. The male character is maintained by female choice but is the useless
end product of an initially useful process.
12.4.4 Females may prefer to pair with handicapped males, because
the male’s survival indicates his high quality
We now turn to a second theory, in which the costliness of the male character is
positively useful to the female, called the handicap theory (Zahavi 1975). (“Handicap”
is Zahavi’s term for what we have been calling a costly or deleterious character that
reduces survival.) The argument runs like this. Suppose that the males in the population
vary in their quality. We shall concentrate on species, like peacocks, in which males
contribute only sperm; in them, quality must mean genetic quality, because nothing
else is transferred. We are thus assuming that some males have genes that confer higher
fitness (“good genes”) than do other males (who have “bad genes”). In practice there
could be all degrees between good and bad, but the point can be explained more easily
in the simple dichotomous case.
If a female mates at random, her mates will have good and bad genes in the same proportions
as the good and bad genes have in the whole population; if half the males in the
population have good genes and half have bad, then 50% of her mates will have good
genes and 50% bad. Now suppose that some of the males in the population possess a
handicap or character that reduces their survival. If only males with good genes can
survive possessing this handicap, then a female who mates preferentially with such
handicapped males will only mate with males with good genes (Table 12.3). The choice
will be favored by selection if the advantage through the superior genes outweighs the
cost of the handicap: then the net quality of the choosy female’s offspring will be higher
than those of the randomly mating female.