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There are three ways for animals to defend their bodies against parasites. One is to grow and divide fast enough to leave them behind. This is well known to plant breeders, for example: the tip of the growing shoot, into which the plant is putting all its resources, is generally free of parasites. Indeed, one ingenious theory holds that sperm are small specifically so that they have no room to carry bacteria with them to infect eggs.33 A human embryo indulges in a frenzy of cell division soon after it is fertilized, perhaps to leave behind any viruses and bacteria stuck in one of the compartments. The second defence is sex, of which more anon. The third is an immune system, used only by the descendants of reptiles. Plants and many insects and amphibians have an additional method, chemical defence: they produce chemicals that are toxic to their pests; some species of pest then evolve ways of breaking down the toxins, and so on; an arms race has begun.

Antibiotics are chemicals produced naturally by fungi to kill their rivals: bacteria. But when man began to use antibiotics he found that, with disappointing speed, the bacteria were evolving the ability to resist the antibiotics. There were two startling things about antibiotic resistance in pathogenic bacteria. One, the genes for resistance seemed to jump from one species to another, from harmless gut bacteria to pathogens, by a form of gene transfer not unlike sex. And two, many of the bugs seemed to have the resistance genes already on their chromosomes; it was just a matter of re-inventing the trick of switching them on. The arms race between bacteria and fungi has left many bacteria with the ability to fight antibiotics, an ability they no longer ‘thought they would need’ when inside a human gut.

Because they are so short-lived compared with their hosts, parasites can be quicker to evolve and adapt. In about ten years, the genes of the AIDS virus change as much as human genes change in ten million years. For bacteria, thirty minutes can be lifetime. Human beings, whose generations are an eternal thirty years long, are evolutionary tortoises.

Picking DNA’s Locks

Evolutionary tortoises none the less do more genetic mixing than evolutionary hares. Austin Burt’s discovery of a correlation between generation length and amount of recombination is evidence of the Red Queen at work. The longer your generation time, the more genetic mixing you need to combat your parasites.34 Bell and Burt also discovered that the mere presence of a rogue parasitic chromosome called a B-chromosome is enough to induce extra recombination (more genetic mixing) in a species.35 Sex seems to be an essential part of combating parasites. But how?

Leaving aside for the moment such things as fleas and mosquitoes, let us concentrate on viruses, bacteria and fungi, the causes of most diseases. They specialize in breaking into cells, either to eat them, as fungi and bacteria do, or, like viruses, to subvert their genetic machinery for the purpose of making new viruses. Either way, they must get into cells. To do that they employ protein molecules that fit into other molecules on cell surfaces: in the jargon, they bind. The arms races between parasites and their hosts are all about these binding proteins. Parasites invent new keys; hosts change the locks. There is an obvious group-selectionist argument here for sex: at any one time a sexual species will have lots of different locks; members of an asexual one will all have the same locks. So a parasite with the right key will quickly exterminate the asexual species but not the sexual one. Hence the well-known fact that by turning our fields over to monocultures of increasingly inbred strains of wheat and maize we are inviting the very epidemics of disease that can only be fought by the pesticides we are forced to use in ever larger quantities.36

The Red Queen’s case is both subtler and stronger than that, though. It is that an individual, by having sex, can produce offspring more likely to