Fisherian runaway is a hypothesized mechanism by which sexual selection can lead to exaggerated and sometimes even maladaptive ornamentation and may play a role in the evolution of beauty. There is evidence that entire species have become extinct due to runaway sexual selection. The name derives from the originator of the concept, Ronald Fisher, the 20th century British statistician, geneticist, eugenicist and racialist.
Mechanism[edit | edit source]
Fisherian runaway is a feedback loop occurring over many generations, in which the one sex becomes more choosy about a heritable trait for the simple reason that others ﬁnd the trait attractive. As the the choosiness for the trait increases, the selective pressure to prefer the trait increases too, forming a positive feedback loop. In response to the increased choosiness, the other sex evolves to enlarge, overcomplicate or beautify that trait in efforts of becoming more attractive. The exponential nature of positive feedback loops exerts a strong selection pressure that can even overcome the selective pressure for survival itself and hence lead to extinction.
Such feedback loops can be initiated by arbitrary aesthetic selection, but also when a trait is slightly correlated with fitness (e.g. health), or when a trait is similar in appearance to attractive or otherwise valuable objects or body parts. For example, women's breasts may have evolved to mimic their buttocks because the latter was already a sexually attractive body part before humans developed upright posture, and then Fisherian runaway may have lead to breasts becoming increasingly larger and increasingly attractive to men.
In summary, beauty could have evolved simply by sexual selection, i.e. mate selection favoring objective beauty (simplicity). But feedback loops as mentioned above may have exaggerated aspects of it, leading to increased sexual dimorphism and amplified and narrowed the attraction to specific kinds of beautiful phenotypes. This also stabilizes the phenotype, making it harder to adapt to environmental changes. Any sort of sexual dimorphism, whether behavioral or ornamental likely tends to get reinforced by such feedback loops because sexual dimorphism is inherently attractive.
Since females are more choosy in many species throughout the animal kingdom (including humans), the males tend to be more ornamented.
Another mechanism that could explain exaggerated ornament and the immense sexual attraction to it, is Zahavi's handicap principle, also called good genes hypothesis which claims that exaggerated ornament is a costly and hence reliable signal of other desirable traits. For example, a peacock with a very large tail would be easy prey (which is costly), hence must likely have other good traits that make up for it enabling his survival despite his handicap (good genes). There is, however, little supporting scientific evidence. In humans in particular, beauty is not strongly correlated with health at all, and even less with cognitive ability. Neither do human sexually dimorphic traits seem to be particularly costly (except for tallness and large female breasts).
Animal examples[edit | edit source]
Peafowls[edit | edit source]
For example, in peafowls, the males evolved to have extremely large plumage, most likely due to persistent female preference for large plumage, strengthened by Fisherian runaway. The narrow preference the peahens have for large plumage caused the peacock's plumage to become absurdly large, hindering its mobility to evade predators, and also requiring a lot of energy to grow and maintain.
Sabre Tooth Tigers[edit | edit source]
It has been hypothesized that Fisherian runaway selection contributed to the demise of the Sabre-tooth Tiger, by which female Sabre-tooths preference for males with long fangs resulted in a Fisherian runaway occurring, which led to the tiger's teeth becoming so large they became a liability, hampering the beast's ability to feed itself, and most likely led to frequent infections from the brittle, comically large teeth breaking.
Irish Elk[edit | edit source]
The Irish elk is also thought to have gone extinct from female selection giving way to Fisherian runaway. The female Irish elk preferred male Irish elk with large horns. Eventually this preference created a feedback loop that resulted in Irish elk eventually evolving to carry 12 ft (3.6 m) wide horns, which is 3.3 ft (1 m) longer than its height. Energy requirements for Irish elk antler growth were 75% as large as the energy requirements for fat and protein deposition. The enormous amount of calcium required to grow the antlers resulted the calcium being depleted from most of the body causing a state like similar to osteoporosis. The adaptive trait in Irish elk became maladaptive and the species died out.
Humans[edit | edit source]
Even though human females are more choosy in accordance to Bateman's principle, both sexes are ornamented. Women have permanently swollen breasts, an hour glass shaped body etc., men have a V-shaped upper body, more toned muscles, beards, very large penises compared to other great apes, and various other features. Both sexes have clearly defined hairlines and very clear skin. None of these features serve a known biological purpose besides being good looking, so they're likely sexually selected, likely mostly by aesthetic selection common to many higher animals, and possibly by feedback loops like Fisherian runaway, leading to exaggeration and strengthened sexual dimorphism.
Women are likely mainly ornamented because throughout history, alpha males could freely choose among many women, could produce a large number of children and evidently repeatedly chose women with pronounced secondary sexual characteristics such as big boobs and booties. Men are of course ornamented because all women are choosy.
While it is not proven that feedback loops were involved in shaping human ornament, or just sexual selection without feedback, it is certainly plausible, especially considering the importance people place on good looks. It could explain why people undergo great risks and costs merely for improving their looks, e.g. by surgeries, diets or steroid intake (see Zyzz's photo). It could also explain the phenomenon of being stunned by someone's appearance, as well as "love on first sight" and oneitis.
Various members of the MRA hypothesized that the increasingly dimorphic beauty standards that men are expected to have just to get a date in a sexually liberated online dating environment may be the beginning of a Fisherian runaway or intensification of existing ones. Only a very small percentage of men are being chosen, and it's the men with the most sexually dimorphic traits: cartoonishly large muscles and frame, with no selective attention paid to traits like loyalty, intelligence, etc.
This also results in an even higher competitive environment among men without physically sexually dimorphic traits, and men with dark triad traits getting even more to win female attention, exaggerating a trait which arguably became maladaptive after civilization and the industrial revolution. Of course this possibility is a concerning trend even without Fisherian runaway possibly intensifying such selection patterns in future generations.
Men's rights activist Warren Farrell warned of such a thing in 1993, in his book, The Myth of Male Power, warning women that their preference for and encouragement of, "hunter-killer", "star quarterback", type men could cause the extinction of the human race with the arrival of nuclear technology. He also claimed that since civilization and the industrial revolution dark triad traits have become maladaptive, as the traits which foster a healthy society have switched from might-makes-right individual brutality, to cooperation, intelligence, empathy etc.
Sexy son hypothesis[edit | edit source]
The sexy son hypotheses was also proposed by Fisher and is merely a corollary of Fisherian runaway regarding men's beauty. It simply states that the positive feedback loop makes men's beauty so attractive that women are willing to choose a beautiful male regardless of other considerations such as morality or ability to provide, because the males' beauty—which is partly heritable—confers on their offspring a potential reproductive advantage. There is indeed compelling evidence of this behavior. Of course men also more readily copulate with beautiful women, but men have less parental investment and hence do not need to care as much about such considerations. Further, women's ability to provide does not matter nearly as much as men's and women also largely lack this ability (at least they did before the invention of modern welfare states). Hence, women's behavior of disregarding the ability to provide merely at the benefit of better looking offspring has much more drastic implications.
See Also[edit | edit source]
References[edit | edit source]
- Fuller, R. C., Houle, D., & Travis, J. 2005. Sensory Bias as an Explanation for the Evolution of Mate Preferences. [Abstract], p. 444