Evolution__3rd_Edition

328 PART 3 / Adaptation and Natural Selection
The peacock’s tail is an example of
a costly sexual character
Sexual selection works mainly
by...
. . . male competition ...
. . . and female choice
these characters exist. If a population contains some types with higher survival than
other types, natural selection will fix the former and eliminate the latter.
Characters that reduce survival can be called “deleterious” or “costly.” One large
class of apparently costly characters are those found usually only in males and which
Darwin called secondary sexual characters. The primary sexual characters are things
like genitalia that are needed for breeding. The secondary sexual characters are not
actually needed for breeding, but they function during reproduction. The peacock’s
“tail” (or, more exactly, train) is an example. In many other bird species too, the males
have tails or other extravagantly developed and brightly colored structures. A peacock
could inseminate a female just as well without his remarkable tail, and in that sense it is
a secondary, not a primary, sexual organ. The peacock’s tail almost certainly reduces
the male’s survival (though this disadvantage has never actually been demonstrated) as
the tail reduces maneuverability, powers of flight, and makes the bird more conspicuous;
its growth must also impose an energetic cost. Why are these costly characters not
eliminated by selection?
12.4.2 Sexual selection acts by male competition and female choice
Darwin’s solution was his theory of sexual selection. He defined the process by saying
that it “depends on the advantage which certain individuals have over other individuals
of the same sex and species, in exclusive relation to reproduction.” A structure produced
by sexual selection in males exists not because of the struggle for existence, but
because it gives the males that possess it an advantage over other males in the competition
for mates. Darwin’s idea is that the reduced survival of peacocks with long, colorful
tails is more than compensated by their increased “advantage in reproduction.”
Darwin discussed two kinds of sexual selection. One is for males to compete among
each other for access to females. Male competition can take the form of direct fighting,
or it can be more subtle. Some male insects, for instance, can remove sperm from
females they are copulating with a sperm that was stored from matings with previous
males. However, we shall not discuss adaptations of sperm competition here, or other
adaptations of male competition, because they do not pose deep theoretical questions.
The situation is different for Darwin’s other mechanism: female choice.
A structure like the peacock’s tail cannot plausibly be explained by male competition.
It would be no use in fighting a indeed it would reduce the male’s fighting power
a and no one has ever thought up a more subtle competitive function for the tail.
Darwin suggested that the tail exists instead because females preferentially mate with
males that have longer, brighter, or more beautiful tails. If they do, the mating advantage
of males with longer tails will compensate a corresponding amount of reduced male
survival.
Darwin’s main argument for the importance of sexual selection was comparative.
Sexual selection should operate more powerfully in polygamous than in monogamous
species. In a polygynous species, in which several females mate with one male (and
other males do not breed at all), a single male can potentially breed with more females
than under monogamy; selection in favor of adaptations that enable males to gain
access to females (whether by male competition or female choice) is proportionally
..