Fisherian or Fisher's runaway is a model of inter-sexual selection that helps to explain traits that do not obviously increase survival. This model is based upon a positive feedback "runaway" mechanism. It was first proposed by British evolutionary biologist R. A. Fisher in 1915, and expanded upon in his 1930 book The Genetical Theory of Natural Selection,. The model did not find strong support, in part because it is difficult to experimentally test.
Fisherian runaway is an explanation for sexually dimorphic secondary traits that do not play a role in intra-sexual selection. 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 advantageous, which in turn makes having a preference for the trait advantageous. 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. A frequently discussed problem with Fisherian runaway is how the process initially began. Why would there be a preference for a trait in the first place if there is no increase in fitness? Discussions about this often include one sex exploiting the opposite sex's sensory or preference bias.
For 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 animal's 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. Given this preexisting pattern, having a preference for longer and more colorful tails gives an advantage just as having a longer and more colorful tail does. However, all members of the species are less well off than they would be if none of the peahens (or only a small number) had a preference for a longer or more colorful tail, because in the absence of such a preference, the possession of these maladaptive traits reducing mobility and increasing visibility to predators would no longer be incentivized.
Alternative models of sexual selection 
- Fisher, R.A. 1915 The evolution of sexual preference, Eugenics Review 7: 184–192
- Fisher, R.A. 1930 The Genetical Theory of Natural Selection, Clarendon Press, Oxford
- Andrew Pomiankowski and Yoh Iwasa. Runaway ornament diversity caused by Fisherian sexual selection. National Academy of Sciences, Apr 28, 1998.