From The Art and Popular Culture Encyclopedia
Sexual selection is the theory proposed by Charles Darwin that states that the frequency of traits can increase or decrease depending on the attractiveness of the bearer. Biologists today distinguish between "male to male combat" (it is usually males who fight), "mate choice" (usually female choice of male mates) and "mate coercion" (forced mating). Traits selected for by male combat are called "weapons", and traits selected by mate choice are called "ornaments". Much attention has recently been given to cryptic female choice, a phenomenon in internally fertilising animals such as mammals and birds, where a female may simply dispose of a male's sperm without his knowledge. The equivalent in male-to-male combat is sperm competition.
Male to male combat is also classified as intrasexual competition, while mate choice and mate coercion are also known as intersexual competition.
Females often prefer to mate with males with external ornaments - exaggerated features of morphology. These can plausibly arise because an arbitrary female preference for some aspect of male morphology initially increased by genetic drift, creating, in due course, selection for males with the appropriate ornament. This is known as the sexy son hypothesis. Alternatively, genes that enable males to develop great ornaments may simply show off greater disease resistance or a more efficient metabolism - features that also benefit females. This idea is known as the good genes hypothesis.