Evolution__3rd_Edition

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Darwin provided comparative
evidence for sexual selection
The advantage to a female of
mating with a male that carries a
costly character is not obvious
But Fisher suggested an answer
CHAPTER 12 / Adaptations in Sexual Reproduction 329
stronger. Darwin therefore reasoned that secondary sexual characters would be more
developed in polygynous, than monogamous, species. Polygynous species should have
stronger sexual dimorphism.
Darwin’s book The Descent of Man, and Selection in Relation to Sex (1871) contains a
long review of sexual dimorphism in the animal kingdom. It is still the best (and classic)
demonstration that sexual dimorphism is indeed mainly found in polygynous species.
In polyandrous birds, such as phalaropes, sexual selection is reversed: females compete
for males, and it is the females that are the larger and more brightly colored sex. There
are exceptions, such as monogamous ducks that are sexually dimorphic; Darwin had an
additional theory for them. However, the main point is that Darwin’s principal evidence
for sexual selection came from a comparison of large numbers of species that
showed that species with brightly colored, large, or dangerously armed males are more
often polygynous and species in which males and females are more similar are more
often monogamous.
12.4.3 Females may choose to pair with particular males
For Darwin, female choice among males was an assumption; he was mainly concerned
to show that, if it exists, it can explain extraordinary phenomena like the peacock’s tail.
He did not have much to say about the prior question of why the female preference
should ever evolve to begin with. Selection can work on a female preference just like on
any other character. If females with one type of preference produce more offspring
than females with another, selection will favor the more productive preference. The
difficult case is in an extreme case like the peacock, in which the form of female choice
appears to be disadvantageous to the female. Females are picking males that possess a
costly character that will be passed on to their sons; the female preference therefore
seems to be causing the females to produce inferior sons.
We can spell the problem out more fully in terms of selection on a mutant, nonchoosy
female. Suppose that peahens do prefer peacocks with dazzling tails, and a
mutant female, who does not prefer these males, arises; she might mate at random, or
prefer some other sort of male. What does selection do to this mutation? The mutant
female will produce sons that do not possess the costly character, or at least in a less
extreme form. Her sons will therefore survive better than average. So the mutant
should be favored, the female preference should be lost, and the extreme male forms
should disappear.
Or should it? The mutant female will indeed produce sons that survive better than
the population’s average. But that, as Fisher (1930) first realized, is not enough to guarantee
that the mutation will spread. When the mutant female’s sons grow up, with their
inferior tails, they will be rejected as mates. The mutant female is a rare mutant, in a
population where the majority of females prefer males with long tails, and this majority
preference will work against the mutant’s sons. Despite their superior survival, they
will be condemned to celibacy. The randomly mating mutation, therefore, may not
spread.
Fisher also discussed how the preference for a costly character could evolve to begin
with. After the long male tail has evolved it is costly, but at an earlier evolutionary stage,