Fisherian runaway

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Fisherian runaway refers to a model of the evolution of sexual selection first proposed by R.A. Fisher in 1915.

The model provides a mechanism that would result in sexual selection for a trait that does not increase fitness (evolutionary success). The following is a simple example involving a hypothetical trait to illustrate the process. If a female acquires a heritable preference for a male trait (through a novel mutation, for example), the female will tend to mate with a male possessing that trait. If the male trait is also heritable, the male offspring will be more likely to possess that trait, and the female offspring will be more likely to prefer that trait in their mates than the offspring in the general population. If the female preference for the male trait becomes more prevalent in this population through this process, males possessing the trait will be more likely to mate with females with the preference. This assumes that females without the preference mate at random with regard to the trait.

In this scenario, we assume that the female preference is only expressed in the female and the male trait is only expressed in the male. However, after a female with the preference and a male with the trait mate, all the offspring (or at least a higher percentage of them, depending on the mechanism of heredity) will possess the genes for both the trait and the preference. Therefore, the sons will not express the preference, but will still carry the genes, and the daughters of the sons will express it, and vice versa.

In the end, the trait and the preference become linked. This is not due to any evolutionary advantage of either the male having the trait or of the female having a mate with the trait; rather, it is the result of assortative mating. Further mutations could feasibly reinforce this selection. It is even possible that the trait could invoke some slight cost on the female. Even with a cost, the trait and preference for it could still increase in the population depending on how great the cost is relative to the preference.

The process is termed "runaway" because it would facilitate the development of greater preference and more pronounced traits. For example, if the trait is long tail feathers in a bird species, females that prefer long tails would be even more attracted to males with extra-long tails. This would increase the fitness of males with extra-long tails, and the tails of the population would become longer on average due to selection. This would continue until the costs of producing the longer tails (more visible to predators, require more resources to grow the extra feathers) are equal to the reproductive benefits of possessing them.

It should also be noted that although females usually choose male mates based on Fisherian traits, the opposite does occur in nature.