Sexual selection is the theory proposed by Charles Darwin that states that certain evolutionary traits can be explained by intraspecific competition. Darwin defined sexual selection as the effects of the "struggle between the individuals of one sex, generally the males, for the possession of the other sex". Biologists today distinguish between "male to male combat" (it is usually males who fight each other), "mate choice" (usually female choice of male mates) and sexual conflict. Traits selected for by male combat are called secondary sexual characteristics (including: horns, antlers, etc.) and sometimes referred to as "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 will get rid of a male's sperm without his knowledge. The equivalent in male-to-male combat is sperm competition.
