The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [74]
The vindication of Zahavi played a critical role in reigniting the debate between the Fisherians and Good-geners. Until that happened, Good-gene theories could work only if the ornaments they resulted in were not encumbrances to the males. Thus a male might advertise the quality of his genes, but to do so at a high cost to himself would be counterproductive unless there were a sexy-son effect.
Lousy Males
The handicap theory now comes face to face with the central conundrum of sexual selection. This is the lek paradox: that peahens are constantly skimming off the cream of the genetic cream by choosing only the very few best males to mate with and as a result, within a very few generations, no variety is left to choose from. The Good-gene assertion that mutations are likely to make ornaments and displays less effective provides a partial answer, but it is not a persuasive one. After all, it argues only for not choosing the worst, rather than for choosing the best.
Only the Red Queen can solve our dilemma. For what sexual selection theory seems to have concluded is that females are constantly running (by being so selective), yet staying in the same place (having no variety to select from). When we find that, we should be on the look-out for some ever-changing enemy, some arms-race rival. It is here that we meet Bill Hamilton again. We last encountered him when discussing the idea that sex itself is an essential part of the battle against disease. If the main purpose of sex is to grant your descendants immunity from parasites, then it follows directly that it makes sense to seek a mate with parasite-resistance genes. AIDS has reminded us all too forcibly of the value of choosing a healthy sexual partner, but similar logic applies to all diseases and parasites. In 1982, Hamilton and a colleague, Marlene Zuk (now at the University of California at Riverside), suggested that parasites might hold the key to the lek paradox, and so to gaudy colours and peacocks’ tails, for parasites and their hosts are continually changing their genetic spots to outwit each other. The commoner a particular strain of host is in one generation, the commoner is the strain of parasite that can overcome its defences in the next. And vice versa: whatever strain of host is most resistant to the prevalent strain of parasite will itself be the prevalent strain of host in the next generation. Thus, the most disease-resistant male might often turn out to be the descendant of the least resistant one in a previous generation. The lek paradox is thus solved at a stroke. By choosing the healthiest male in each generation, females will be picking a different set of genes each time and never run out of genetic variety to select from.41
The Hamilton–Zuk parasite theory was bold enough, but the two scientists did not stop there. They looked up the data for one hundred and nine species of bird and found that the most brightly coloured species were also the ones most troubled by blood parasites. That claim has been challenged and much debated, but it seems to hold up. Zuk found the same in a survey of five hundred and twenty-six tropical birds, and others found it to be true of birds of paradise and some species42 of fresh-water fish: the more parasites, the more showy the species. Even among human beings, the more polygamous a society is, the greater its parasite burden, though it is not clear if this means anything.43