Fisherian runaway

R. A. Fisher

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,^[1] and expanded upon in his 1930 book The Genetical Theory of Natural Selection,.^[2] The model did not find strong support, in part because it is difficult to experimentally test. ^{[citation needed]}

Certain male traits are advantageous not because they indicate good quality, but simply because they are attractive to females. Offspring of females choosing males with attractive traits inherit alleles influencing the expression of both the preference (from their mother) and the trait (from their father). The resulting association between preference and trait can lead to a positive feedback cycle of ever stronger preferences and larger display traits. Ornaments can evolve to such extrememes that their severe costs balance the reproductive advantages of having the trait.

Explanation

The peacock feathers illustrate the runaway process.

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.^[3] 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 outweigh 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 offspring 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

• Handicap principle
• Hamilton–Zuk hypothesis (parasite-resistance hypothesis)
• Sexy son hypothesis

References