What is sexual selection, and why did the peacock's tail trouble Darwin?

Sexual selection is a mechanism Darwin proposed in which traits evolve because they help an individual win mates, even if they do not help it survive. The peacock's tail troubled him because it looked like a burden that natural selection, which favours survival, could not account for. Darwin's answer was that females preferred showy males, so the tail spread because it brought mating success. See [nature.com](https://www.nature.com/scitable/knowledge/library/sexual-selection-13255240/?error=cookies_not_supported&code=00898932-6991-4c9d-b514-186cf872a95c) and [darwin200.christs.cam.ac.uk](https://darwin200.christs.cam.ac.uk/sexual-selection). [1] [2]

According to the [darwinproject.ac.uk](https://www.darwinproject.ac.uk/commentary/evolution/sexual-selection), Darwin realised that natural selection could not explain the marked differences between males and females of the same species, such as lion manes, antlers, or bright male bird plumage. He proposed a secondary mechanism operating alongside natural selection. He described it as depending 'not on a struggle for existence, but on a struggle between the males for possession of the females'. [3]

The [nature.com](https://www.nature.com/scitable/knowledge/library/sexual-selection-13255240/?error=cookies_not_supported&code=00898932-6991-4c9d-b514-186cf872a95c) overview says Darwin recognised two forms. Intrasexual selection is competition within one sex, usually males, for access to mates. Intersexual selection is choice by one sex, usually females, among members of the other. The [birds.fieldmuseum.org](https://birds.fieldmuseum.org/stories/selection/sexual-selection) account quotes Darwin that the result for the losing competitor is 'not death', but 'few or no offspring'. [1] [4]

In an April 1860 letter to Asa Gray, Darwin wrote that the sight of a peacock's feather 'makes me sick!', as reported by [theguardian.com](https://www.theguardian.com/science/2017/may/19/a-peacocks-tail-how-darwin-arrived-at-his-theory-of-sexual-selection). The tail seemed to be a hole in his theory. It appeared to serve no survival purpose and may be a handicap, since it makes escaping predators harder, as [darwin200.christs.cam.ac.uk](https://darwin200.christs.cam.ac.uk/sexual-selection) notes. [5] [2]

There was a wider stake as well. The [darwinproject.ac.uk](https://www.darwinproject.ac.uk/commentary/evolution/sexual-selection) says the tail was bound up with questions of beauty in nature, and opponents such as the duke of Argyll used it to argue for divine creation. The [theguardian.com](https://www.theguardian.com/science/2017/may/19/a-peacocks-tail-how-darwin-arrived-at-his-theory-of-sexual-selection) article adds that Victorian thinking treated beauty as divine design. [3] [5]

Darwin argued that in the past, peahens preferred males with slightly longer or more flamboyant tails. Those traits passed to the next generation, and over many generations tails grew longer and brighter. The mating advantage outweighed the survival cost, as [darwin200.christs.cam.ac.uk](https://darwin200.christs.cam.ac.uk/sexual-selection) explains. The [darwinproject.ac.uk](https://www.darwinproject.ac.uk/commentary/evolution/sexual-selection) notes that he argued such plumage could be acquired in small successive steps, and that females choosing the males most attractive 'according to their standard of beauty' shaped later generations. [2] [3]

Later explanations of why females prefer such traits vary. [darwin200.christs.cam.ac.uk](https://darwin200.christs.cam.ac.uk/sexual-selection) says there are various suggestions, including that traits signal fitness directly because they depend on good health or diet, or indirectly because they show a male can survive despite the cost. The [birds.fieldmuseum.org](https://birds.fieldmuseum.org/stories/selection/sexual-selection) account reports evidence that females prefer males with more eyespots in their trains. [2] [4]