Sexual selection

Sexual selection is a special kind of natural selection. It is a theory of Charles Darwin that certain traits can be explained by competition "within" a species.
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". It is usually males who fight each other. Traits selected by male combat are called secondary sexual characteristics. They include horns, antlers, which are sometimes referred to as 'weapons'. 
Traits selected by mate choice are called 'ornaments'. Females often prefer to mate with males with external ornaments exaggerated features of morphology. Genes that enable males to develop impressive ornaments or fighting ability may simply show off greater disease resistance or a more efficient metabolism. This idea is known as the 'good genes' hypothesis. Sexual selection is still being researched and discussed today.
One author says studies on sexual selection have something important to tell us, namely "how males and females came to be what they are".
Modern views.
Ernst Mayr said:
Competition between members of the same species.
Today, biologists would say that certain evolutionary traits can be explained by competition between members of the "same" species. Competition can be "before" or "after" sexual intercourse.
Sexual conflict.
Finally, sexual conflict is said to occur between breeding partners, sometimes leading to an evolutionary arms race between males and females. This is based on the simple fact that the interests of males and females in reproduction are fundamentally different. 
Males: their interest is to mate with a large number of completely faithful females, thus spreading their genes widely in the population.
Females: Their interest is to mate with a large number of fit males, thus producing a large number of fit and "varied" offspring.92
Female mating preferences are widely recognized as being responsible for the rapid and divergent evolution of male secondary sexual traits. Females of many animal species prefer to mate with males with external ornaments – exaggerated features such as elaborate sex organs. Alternatively, genes that enable males to develop impressive ornaments or fighting ability may simply show off greater disease resistance or a more efficient metabolism. "These" features would be likely to be inherited by offspring of both sexes.