Biology Reference
In-Depth Information
Chapter 7
Sexual Selection,
Sperm
Competition and
Sexual Conflict
Photo © Joah Madden
In the second part of
'The Descent of Man and Selection in Relation to Sex'
, Charles Darwin
(1871) proposed his theory of 'Sexual Selection'. He had been puzzling over the
extravagant traits which often occurred in one sex only, usually the males. Why, for
example, is it only male kudu (an African antelope) that have enormous horns, and
only male birds of paradise that have such remarkable, ornamented plumage (Fig. 7.1)?
Darwin argued that these structures could not be essential for survival, otherwise surely
the females would have them too. Instead, he proposed that these traits had evolved
simply because they were of advantage in competition for mates, a process he called
'Sexual Selection'.
Darwin suggested that: 'the sexual struggle is of two kinds; in the one it is between the
individuals of the same sex, generally the males, in order to drive away or kill their rivals,
the females remaining passive; whilst in the other, the struggle is likewise between the
individuals of the same sex, in order to excite or charm those of the opposite sex,
generally the females, which no longer remain passive, but select the more agreeable
partners'. The first process, where victory goes to the more powerful competitor, might
explain the evolution of male weapons (such as kudu horns) and other male attributes
(e.g. large size and strength) which give a male an advantage in direct combat with rival
males. The second process, where victory goes to the most charming competitor, might
explain the evolution of male ornaments (such as those of a bird of paradise). Somewhat
confusingly, the first process is now often referred to as 'intrasexual selection' or 'male-
male competition', and the second as 'intersexual selection' or 'female choice'. This is
confusing because, as Darwin recognized, both processes involve intrasexual competition,
in the first case to win mates by force and in the second to win them by charm.
Before examining the evidence for sexual selection, and its consequences, we need
to consider why it is usually the males who compete for females, rather than vice
versa. This takes us right back to the beginning, to the fundamental differences
between male and female.
Darwin's theory
of sexual
selection:
selection for traits
that increase
mating success
Competing for
mates by force or
by charm
