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When Darwin first proposed the possibility of sexual selection, he
identified two mechanisms, male competition for mates and female choice of
mates. Extending this classification, we distinguish two forms of mate
choice, direct and indirect.
This distinction clarifies the relationship between Darwin's two
mechanisms and, furthermore, indicates that the potential scope for sexual
selection is much wider than so far realized.
Direct mate choice, the focus of most research on sexual selection in
recent decades, requires discrimination between attributes of individuals
of the opposite sex.
Indirect mate choice includes all other behavior or morphology that
restricts an individual's set of potential mates.
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Possibilities for indirect mate choice include advertisement of fertility
or copulation, evasive behavior, aggregation or synchronization with other
individuals of the same sex, and preferences for mating in particular
locations. In each of these cases, indirect mate choice sets the
conditions for competition among individuals of the opposite sex and
increases the chances of mating with a successful competitor.
Like direct mate choice, indirect mate choice produces assortative mating.
As a consequence, the genetic correlation between alleles affecting
indirect choice and those affecting success in competition for mates can
produce self-accelerating evolution of these complementary features of the
sexes.
The broad possibilities for indirect mate choice indicate that sexual
selection has more pervasive influences on the coevolution of male and
female characteristics than previously realized.
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This expanded perspective on sexual selection developed from a
classification of behavioral mechanisms of mate choice. By emphasizing a
distinction between direct and indirect mate choice, we reached two
conclusions.
First, Darwin's two mechanisms of sexual selection, mate choice and
competition for mates, are inseparable. It seems likely that competition
for mates by one sex always depends on conditions set by indirect mate
choice by the other sex.
Even when male-male competition takes exaggerated forms, females are not
"passive", as Darwin originally suggested. Instead, indirect mate choice
by females sets conditions for male competition. Studies of sexual
selection and mating systems so far have largely overlooked this aspect of
mate choice.
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Second, indirect mate choice evolves in the same way as direct choice.
In special cases, indirect mate choice might lead to runaway evolution of
both male and female traits, just as direct mate choice might.
Furthermore, in situations in which females have limited opportunities to
interact with males before mating, indirect mate choice might allow more
reliable and less costly identification of competitive mates than would
direct choice.
Future research on sexual selection and mating systems should strive to
redress the imbalance so far in our attention to direct and indirect forms
of mate choice.
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