Exponential Growth in Female Preference
In species where intersexual selection is active, as in many polygamous birds, sexual selection acts by accelerating the preference that specific "fashion" ornaments attract, causing the preferred trait and female preference for it to increase together, explosively. While Darwin had been criticised for simply accepting female whims as given, Ronald Fisher grasped the underlying mechanism in The Genetical Theory of Natural Selection, in a remark that was not widely understood for another 50 years:
| “ | ... plumage development in the male, and sexual preference for such developments in the female, must thus advance together, and so long as the process is unchecked by severe counterselection, will advance with ever-increasing speed. In the total absence of such checks, it is easy to see that the speed of development will be proportional to the development already attained, which will therefore increase with time exponentially, or in geometric progression. —Ronald Fisher, 1930 | ” |
Fisher's runaway process causes a dramatic increase in both the male's conspicuous feature and in female preference for it, until practical, physical constraints halt further exaggeration. A positive feedback loop is created, producing extravagant physical structures in the non-limiting sex, such as in the male African Long-Tailed Widow Bird (left). While males have long tails, female tastes in tail length are still more extreme. Fisher understood that female preference for long tails may be passed on genetically, in conjunction with genes for the long tail itself. Long-Tailed Widow Bird offspring of both sexes will inherit both sets of genes, with females expressing their genetic preference for long tails, and males showing off the coveted long tail itself.
Richard Dawkins presents a non-mathematical explanation of the runaway sexual selection process in his book The Blind Watchmaker. Females who prefer long tailed males tend to have mothers that chose long-tailed fathers. As a result, they carry both sets of genes in their bodies. That is, genes for long tails and for preferring long tails become linked. The taste for long tails and tail length itself may therefore become correlated, tending to increase together. The more tails lengthen, the more long tails are desired. Any slight initial imbalance between taste and tails may set off an explosion in tail lengths. Fisher corresponded that:
| “ | The exponential element, which is the kernel of the thing, arises from the rate of change in hen taste being proportional to the absolute average degree of taste. —Ronald Fisher, 1932 | ” |
The female widow bird will desire to mate with the most attractive long-tailed male so that her progeny, if male, will themselves be attractive to females of the next generation - thereby fathering many offspring who will carry the female's genes. Since the rate of change in preference is proportional to the average taste amongst females, and as females desire to secure the services of the most sexually attractive males, an additive effect is created that, if unchecked, can yield exponential increases in a given taste and in the corresponding desired sexual attribute.
| “ | It is important to notice that the conditions of relative stability brought about by these or other means, will be far longer duration than the process in which the ornaments are evolved. In most existing species the runaway process must have been already checked, and we should expect that the more extraordinary developments of sexual plumage are not due like most characters to a long and even course of evolutionary progress, but to sudden spurts of change. —Ronald Fisher, 1930 | ” |
Since Fisher's initial conceptual model of the 'runaway' process, Russell Lande and Peter O'Donald have provided detailed mathematical proofs that define the circumstances under which runaway sexual selection can take place.
Read more about this topic: Sexual Selection
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