Some theorists, including Alan Grafen and W. D. Hamilton, prefer alternative kinds of theory in which choice made by a female really does have a beneficial effect on her progeny, in a utilitarian, eugenic sense. The theory they are together working on is that female birds act as diagnostic doctors, picking out those males who are least susceptible to parasites. Bright plumage, according to this characteristically ingenious theory of Hamilton, is a male's way of conspicuously advertising his health.
The theoretical importance of parasites would take too long to explain fully. Briefly, the problem with all 'eugenic' theories of female choice has always been as follows. If females really could successfully choose males with the best genes, their very success would reduce the range of choice available in the future: eventually, if there were only good genes around, there would be no point in choosing. Parasites remove this theoretical objection. The reason is that, according to Hamilton, parasites and hosts are running a never-ceasing cyclical arms race against one another. This in turn means that the 'best' genes in any one generation of birds are not the same as the best genes in future generations. What it takes to beat the current generation of parasites is no good against the next generation of evolving parasites. Therefore there will always be some males that happen to be genetically better equipped than others to beat the current crop of parasites. Females, therefore, can always benefit their offspring by choosing the healthiest of the current generation of males. The only general criteria that successive generations of females can use are the indicators that any vet might use - bright eyes, glossy plumage, and so on. Only genuinely healthy males can display these symptoms of health, so selection favours those males that display them to the full, and even exaggerate them into long tails and spreading fans. But the parasite theory, though it may well be right, is off the point of my 'explosions' chapter.
