It was Charles Darwin who originally proposed that the so-called secondary sexual characteristics of male animals -- such as the elaborate tails of peacocks, bright plumage or expandable throat sacs in many birds, large racks in mooses, deep voices in men -- evolved because females preferred to mate with individuals that had those features. Sexual selection can be thought of as two special kinds of natural selection, as described below. Natural selection occurs when some individuals out-reproduce others, and those that have more offspring differ genetically from those that have fewer.