A peacock's tail made Darwin feel physically ill, until he found an explanation
Charles Darwin once confessed to a colleague that looking at a peacock feather made him sick. A giant, glittering tail seemed to help its owner survive not at all. His answer was a second force besides natural selection: competition for mates, where losers do not die but leave few or no offspring.
Sexual selection works in two ways. Members of one sex choose partners from the other, and members of the same sex compete for access. Either way, some individuals out-reproduce others because they are more attractive or pick more attractive partners. Darwin raised the idea in On the Origin of Species in 1859 and developed it in The Descent of Man in 1871, arguing that where males and females live alike yet differ in colour or ornament, mate competition is mostly responsible.
Many contemporaries resisted. Victorians found female choice uncomfortable for what it implied about human sexuality, and few believed insects could have any aesthetic sense. Alfred Russel Wallace accepted the process but thought it weak; he counted male rivalry as ordinary natural selection, explained the peahen's drab colouring as camouflage, and doubted creatures like beetles could judge beauty. Edward Bagnall Poulton's 1890 book The Colours of Animals pushed back, citing 1889 observations by George and Elizabeth Peckham of female spiders closely watching dancing males and frequently turning them down anyway. Wallace attacked the book in a review in Nature.
The statistician Ronald Fisher supplied the mathematics in 1930, in a book titled The Genetical Theory of Natural Selection. His runaway model starts with a small preference, say for longer tails. Daughters inherit the taste and sons the trait, so the two become genetically linked and reinforce each other, accelerating exponentially until physical limits call a halt. Fisher expected the most extraordinary plumage to reflect sudden spurts rather than slow steady change. Richard Dawkins later explained the same logic without equations in The Blind Watchmaker.
The long-tailed widowbird is a textbook case: females not only prefer long-tailed males but are drawn to tails even longer than any that occur naturally. Similar pressures produced the plumage of birds-of-paradise and the antlers of deer, and in some species the usual roles are reversed. Sexual selection turns up widely among animals and has been found in plants and fungi too.
Source: Sexual selection
