Fisherian runaway

Fisherian runaway is a model of sexual selection, first proposed by R.A. Fisher in 1915, that suggests an explanation for sexual selection of traits that do not obviously increase fitness of survival.

Fisher's explanation is that selection of such traits is a result of sexual preference; that members of the opposite sex find a trait desirable. This preference makes the trait advantagous, which in a circular fashion makes having a preference for the trait advantagous.

The process is termed "runaway" because over time, it would facilitate the development of greater preference and more pronounced traits, until the costs of producing the trait balance the reproductive benefit of possessing it.

By way of example, the peacock's tail requires a great deal of energy to grow and maintain, it reduces the bird's agility, and it may increase the animals visibility to predators. Yet it has evolved, indicating that birds with longer tails have some advantage. Fisherian runaway explains that if a peahen selects a peacock with a longer and more colorful tail, then her male children are more likely to have long and colorful tails and are more likely to be sexually successful themselves, because other peahens have the same preference for longer tails. Therefore, in a circular fashion, having a preference for longer tails gives an advantage.