In his On the Origin of Species, Darwin proclaimed that natural selection was the main, but not exclusive mechanism of change. Alongside natural selection, based on the struggle to survive, was sexual selection, based on the struggle to reproduce. Twelve years later, in his two-volume Descent of Man, and Selection in Relation to Sex, Darwin focused on sexual selection, devoting part of the first volume and the entire second volume to sexual selection, not just in humans but across biodiversity. He used sexual selection to explain traits not easily explained by natural selection. How, for instance, could natural selection form the peacock’s extravagant tail when that tail seemed to be a liability in avoiding predators in the struggle for existence? Perhaps the peacock’s tail was instead a way to charm female peahens in the struggle for a mate. The evolutionists who came after Darwin were less inclined to give female choice such an important role, but in the past fifty years there has been a renaissance, and sexual selection now enjoys the enthusiastic support of many who work in the biological and human sciences. It is currently used to explain even more than Darwin had imagined.

Darwin’s interest in sexual selection long predated his Origin. In his “Sketch of 1842,” he included a passage on sexual selection contrasting it with natural selection (Darwin 1909, 10). This passage reappeared in a slightly modified form in his “Essay of 1844”:

A similar passage was included in the brief and first public statement of his views – the extract of his “big species book” read before the Linnaean Society along with Alfred R. Wallace’s paper in 1858 (Darwin and Wallace 1858, 50).