Sexual Selection

Chapter Summary

1. In The Descent of Man and Selection in Relation to Sex, Darwin hypothesized that some traits evolved via sexual selection, which "depends on the advantage which certain individuals have over other individuals of the same sex and species in exclusive relation to reproduction."
2. Sexual selection can occur via (1) intersexual selection, wherein individuals of one sex choose individuals of the other sex as mates, and/or (2) intrasexual selection, in which members of one sex compete for mating access to the other sex.
3. The four main evolutionary models of female mate choice are the "direct benefits," "good genes," "runaway selection," and "sensory bias" models.
4. In the direct benefits model, selection favors females who have a genetic predisposition to choose mates that provide them with some tangible resource—for example, protection from predators.
5. In systems in which females do not receive direct benefits, selection may favor females that choose mates that possess so-called "good genes." Evolutionary biologists have hypothesized that traits that are accurate and honest indicators of male genetic quality should be used by females when selecting among males based on good genes.
6. In the runaway sexual selection model, there is a genetic correlation between a male trait and a female preference for that trait. If the frequency of one increases—for example, by selection—the other increases as well due to this correlation. This process can "snowball," resulting in dramatic male traits and strong female preferences.
7. In the sensory bias model of mate choice, females initially prefer a trait in males because the female nervous system already responds to that trait outside the context of mate choice.
8. Males compete with each other for access to females in direct ways, such as one-on-one fights, but also in more indirect, complex ways, such as via "sneaker" strategies and sperm competition.
9. When selection operates differently on males and females, sexual conflict may result, where the traits that evolve in one sex are detrimental to individuals of the other sex.