Fisherian runaway
Fisherian runaway is a model of sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...

, first proposed by R.A. Fisher
Ronald Fisher
Sir Ronald Aylmer Fisher FRS was an English statistician, evolutionary biologist, eugenicist and geneticist. Among other things, Fisher is well known for his contributions to statistics by creating Fisher's exact test and Fisher's equation...

 in 1915, and expanded upon in his 1930 book The Genetical Theory of Natural Selection
The Genetical Theory of Natural Selection
The Genetical Theory of Natural Selection is a book by R.A. Fisher first published in 1930 by Clarendon. It is one of the most important books of the modern evolutionary synthesis and is commonly cited in biology books.-Editions:...

, that suggests an explanation for sexual selection of traits that do not obviously increase fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...

 of survival, based upon a positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...

 "runaway" mechanism.


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 a circular fashion 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.

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 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
Nash equilibrium
In game theory, Nash equilibrium is a solution concept of a game involving two or more players, in which each player is assumed to know the equilibrium strategies of the other players, and no player has anything to gain by changing only his own strategy unilaterally...

 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 respectively reducing mobility and increasing visibility to predators would no longer be incentivized.

Human sexual selection

Peter Frost
Peter Frost (anthropologist)
Peter Frost is a Canadian anthropologist.He was awarded his Ph.D. from Université Laval in 1995.He is best known for hypotheses on the evolution of hair, eye and skin colors...

 has proposed a sexual selection thesis in respect to human skin color
Human skin color
Human skin color is primarily due to the presence of melanin in the skin. Skin color ranges from almost black to white with a pinkish tinge due to blood vessels underneath. Variation in natural skin color is mainly due to genetics, although the evolutionary causes are not completely certain...

. He brings together evidence of preference for a paler female partner and a darker male partner, by age and fecundity, and concludes the preference for paler skin tones may be rooted in a hardwiring of the human brain for such a preference. However, his thesis remains to be proven.

Frost found that, on average, women of a given ancestry have a lighter skin tone than men of the same ancestry, and that there is a sexual preference for paleness in women and darkness in men in many cultures throughout the world. In his foreword to Frost's book, University of Washington
University of Washington
University of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...

 sociologist Pierre L. van den Berghe summarizes:
A consequence of this is that, since higher-ranking men get to marry the perceived more attractive women, the upper classes of a society generally tend to develop a lighter complexion than the lower classes by sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...


Studies have shown that lighter skin has generally been preferred in most cultures and races. Exceptions to this have appeared in modern times in Western culture, where tanned skin is often considered more attractive. Tanned skin has been shown in the United States to be viewed both as more attractive and more healthy than pale skin.

See also

  • Assortative mating
    Assortative mating
    Assortative mating , and the related concept Disassortative mating, is the phenomenon where a sexually reproducing organism chooses to mate with individuals that are similar or dissimilar to itself in some specific manner...

  • Handicap principle
    Handicap principle
    The handicap principle is a hypothesis originally proposed in 1975 by biologist Amotz Zahavi to explain how evolution may lead to "honest" or reliable signaling between animals who have an obvious motivation to bluff or deceive each other...

  • Sexual conflict
    Sexual conflict
    Sexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly the mode and frequency of mating, leading to an evolutionary arms race between males and females. The conflict encompasses the actions and behaviors of both sexes to influence...

  • Sexy son hypothesis
    Sexy son hypothesis
    The sexy son hypothesis of evolutionary biology was first proposed by Patrick J. Weatherhead and Raleigh J. Robertson of Queen's University in 1979. It proposes that a female animal's optimal choice among potential mates is a male whose genes will produce male offspring with the best chance of...

Further reading

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