The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [83]
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Andrew Pomiankowski accepts much of what Ryan and Kirkpatrick say, but parts company with them on the matter of female choice. He says that what they are considering is merely a constraint that channels the male’s trait into the preferred direction of the female’s sensory bias. But that does not mean that the exaggeration happens without the female’s preference changing. It is almost impossible to see how females could avoid the Fisher effect as the male’s ornament gets more exaggerated, generation by generation: the female who is most discriminating picks the most sexy male and so has the most sexy sons, the one who has the most sexy sons has the most granddaughters, so females get more and more discriminating, more and more difficult to seduce or hypnotize. ‘The crucial question,’ wrote Pomiankowksi, ‘is not whether sensory exploitation has been involved, but why females have allowed themselves to be exploited.’ Besides, it is an impoverished view of selection to believe that it can tune a frog’s ear for detecting predators and not tune it simultaneously and differently for choosing males.80
Thus it is possible to argue with Ryan and Kirkpatrick that male courtship extravagances reflect the innate tastes of females, without abandoning the idea that those tastes are of use to the females in that they select the best genes for the next generation. A peacock’s tail is, simultaneously, a testament to naturally selected female preferences for eye-like objects, a runaway product of despotic fashion among peahens, and a handicap that reveals its possessor’s condition. Such tolerant pluralism is not to everybody’s taste, but Pomiankowski insists it does not stem from a misguided desire to please everybody. On a paper napkin in an Indian restaurant one day he sketched out for me a plausible account of all the sexual selection theories working in concert.
Each male trait begins as a chance mutation. If it happens to hit a sensory bias of the female it starts to spread. As it spreads, the Fisher effect takes over and both the trait and the preference are exaggerated. Eventually, the point is reached where the trait has spread to all males and there is no point in females following the fashion any more. It starts to fade again, under pressure from the fact that there is now a cost to female choice: if nothing else it is a waste of females’ time and effort comparing different males. The Fisher effect fades more slowly when that cost is small, for example in lekking species where the males can all be viewed at once. But some traits do not fade, because it so happens that they reflect the underlying health of their possessors. They change colour if the male is infected with parasites, for example. And therefore females do not stop choosing the best males at all. They keep picking (or being seduced by) the most fancy male because if they do so they will have disease-resistant offspring. In other words, condition-reflecting traits will not be the only ones brought to an exaggerated state, but they will be the ones that persist the longest. And in lekking species, all the Fisher-exaggerated traits remain as well because the cost of choosing is so small. The most promiscuous species end up a collage of different handicaps, ornaments and gaudy blotches. Pomiankowski has since begun to confirm his intuition (based on the symmetry idea discussed earlier) that multiple traits on polygamous birds, like the many adornments of a peacock, are Fisher ornaments, while single features on monogamous birds, like the swallow’s forked tail, are Good-gene ornaments, or condition-revealing