Charles Darwin proposed that all living species were derived from common ancestors.
The primary mechanism he proposed to explain this fact was natural selection: that is, that organisms better adapted to their environment would benefit from higher rates of survival than those less well equipped to do so.
However he noted that there were many examples of elaborate, and apparently non-adaptive, sexual traits that would clearly not aid in the survival of their bearers. He suggested that such traits might evolve if they are sexually selected, that is if they increase the individual’s reproductive success, even at the expense of their survival (Darwin 1871).
Darwin noted that sexual selection depends on the struggle between males to access females. He recognized two mechanisms of sexual selection: intrasexual selection, or competition between members of the same sex (usually males) for access to mates, and intersexual selection, where members of one sex (usually females) choose members of the opposite sex.