Fisherian Runaway

genetical theory of natural selection

Fisherian runaway is a model of sexual selection, first proposed by R.A. Fisher in 1915, and expanded upon in his 1930 book ‘The Genetical Theory of Natural Selection,’ that suggests an explanation for sexual selection of traits that do not obviously increase fitness of survival, based upon a positive feedback ‘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 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.

Peter Frost has proposed a sexual selection thesis in respect to human skin color. 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, sociologist Pierre L. van den Berghe summarizes: ‘Although virtually all cultures express a marked preference for fair female skin, even those with little or no exposure to European imperialism, and even those whose members are heavily pigmented, many are indifferent to male pigmentation or even prefer men to be darker. The trend … in integrated societies has [been toward] increasing popularity for men of color, especially those [of] African descent. These trends have been recorded in areas such as South America, where in Brazil it was estimated that by 2009 black people of African descent will be the single most dominant ethnic group. In popular media in the western world ‘blacks’ have been repeatedly surrounded by advantageous stereotypes and myths that praise their athletic aptitudes amongst many other things, and often depict them as males of superior genetic inheritance.’

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. 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.

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