Evolution and sexual selection
Charles Darwin first theorized in On the Origin of Species that the peafowl's plumage had evolved through sexual selection. This idea was expanded upon in his second book,The Descent of Man and Selection in Relation to Sex.
The sexual struggle is of two kinds; in the one it is between individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners.
Sexual selection is the ability of male and female organisms to exert selective forces on each other with regard to mating activity. The strongest driver of sexual selection is gamete size. In general, eggs are bigger than sperm and females produce fewer gametes than males. This leads to eggs being a bigger investment, and therefore to females being choosy about the traits that will be passed on to her offspring by males. The peahen's reproductive success and the likelihood of survival of her chicks is partly dependent on the genotype of the mate.
Females generally have more to lose when mating with an inferior male due to her gametes being more costly than the male's.