The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [42]
The topminnow study beautifully illustrates the way in which sex enables hosts to impale their parasites on the horns of a dilemma. As John Tooby has pointed out, parasites simply cannot keep their options open. They must always ‘choose’. In competition with each other they must be continually chasing the commonest kind of host and so poisoning their own well by encouraging the less common type of host. The better their keys fit the locks of the host, the quicker the host is induced to change its locks.62
Sex keeps the parasite guessing. In Chile, where introduced European bramble plants became a pest, rust fungus was introduced to control them: it worked against an asexual species of bramble and failed against a sexual species. And when mixtures of different varieties of barley or wheat do better than pure crops of one variety (as they do), roughly two-thirds of the advantage can be accounted for by the fact that mildew spreads less easily through the mixture than through a pure crop.63
The Search for Instability
The history of the Red Queen explanation of sex is an excellent example of how science works by synthesizing different approaches to a problem. Hamilton and others did not pluck the idea of parasites and sex from thin air. They are the beneficiaries of three separate lines of research that have only now converged. The first was the discovery that parasites can control populations and cause them to go in cycles: this was hinted at by Alfred Lotka and Vito Volterra, in the 1920s and fleshed out by Robert May and Roy Anderson in London in the 1970s. The second was the discovery of abundant polymorphism by J. B. S. Haldane and others in the 1940s: the curious phenomenon that for almost every gene there seemed to be several different versions, and something was keeping one from driving out all the others. The third was the discovery by Walter Bodmer and other medical scientists of how defence against parasites works: the notion of genes for resistance providing a sort of lock-and-key system. Hamilton put all three lines of inquiry together and said: parasites are in a constant battle with hosts, a battle that is fought by switching from one resistance gene to another; hence the battery of different versions of genes. None of this would work without sex.64
In all three fields, the breakthrough was to abandon notions of stability. Lotka and Volterra were interested in knowing whether parasites could stably control populations of hosts; Haldane was interested in what kept polymorphisms stable for so long. Hamilton was different. ‘Where others seemed to want stability I always hope to find, for the benefit of my idea of sex, as much change and motion … as I can get.’65
The main weakness of the theory remains the fact that it seems to require cycles of susceptibility and resistance: the advantage should always be swinging back and forth like a pendulum though not necessarily with such regularity.66 There are some examples of regular cycles in nature: lemmings and other rodents often grow abundant every three years and rare in between. Grouse on Scottish moors go through regular cycles of abundance and scarcity, with about four years between peaks, and this is caused by a parasitic worm. But chaotic surges, like locust plagues, or much more steady growth or decline, like human beings, are more normal. It remains possible that versions of the genes for resistance to disease do indeed show cycles of abundance and scarcity. But nobody has looked.67
The Riddle of the Rotifer
Having explained why sex exists, I must now return to the case of the bdelloid rotifers, the tiny fresh-water creatures that never have sex at all – a fact that John Maynard Smith