Sexual selection, as distinguished by Charles Darwin, is known as a variance in the number of mates.
Differences in mating success results in adaptations for obtaining mates, rather than adaptations for survival.
Sexual selection acts to refine secondary sexual characters of the phenotype such as morphological differences between males and females, or differences between male types.
Sexual selection has the capacity to evolve maladaptive traits, by producing individuals with such elaborate ornaments that they must be either energetically costly to develop, maintain, or disrupt the direct survival of the individual.
This idea of sexual selection gave an aid in answering the question of why some species had acquired such bizarre traits.