In this section you will be introduced to a variant of Darwinian concept of natural selection which he termed sexual selection. You may have observed very distinct and striking differences in appearance between opposite sexes of various animals groups, more prominently in birds and mammals. When differences in appearance occur between sexes, the animals are said to exhibit sexual dimorphism, the phrase simply meaning that themales and females are not uniform in appearance, but present two different forms. In general the males of the species are strikingly  different from females although there may be exceptions to this rule. The male of the species may be more aggressive, brilliantly coloured or possess elaborate combs or bright plumage or extensive horns or ornamental hooks, tusks, spurs etc. Why are the male members different from the females? Is it not that the possession of such extra structures easily identify them to the enemies and make them more susceptible to their attack? Can these characters therefore be regarded as disadvantageous or ill-adaptations? If that be the case, is not the theory of natural selection contradicted?

As an answer to these questions, Darwin proposed the theory of sexual selection. According to the theory, the females of the species select their males and therefore a male which is more attractive or stronger than the other members of its sex has better chances of mating and leaving behind its offspring. In males there'will be a constant evolution of such characters which are attractive to their female counterparts. Sexual selection can be regarded as competition among the members belonging to a sex to win the members of. the opposite sex.