Sexual selection is the theory that competition for mates between individuals of the same sex drives the evolution of certain traits.
These traits, which give an individual an advantage over its rivals in courtship, without being directly involved in reproduction, are called secondary sex characteristics. Sex differences directly related to reproduction and serving no purpose in courtship are called primary sex characteristics.
The theory of sexual selection was first proposed by Charles Darwin in his book The Origin of Species. A later book, The Descent of Man, and Selection in Relation to Sex dealt with the subject exhaustively.