The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [39]
Next they introduced several species of parasite, two hundred of each, whose power depended on ‘virulence genes’ matched by ‘resistance genes’ in the hosts. The least resistant hosts and the least virulent parasites were killed in each generation. Now the asexual race no longer had an automatic advantage. Sex often won the game. It won most often if there were lots of genes that determined resistance and virulence in each creature.
What kept happening in the model, as expected, was that resistance genes that worked would get commoner, then virulence genes that undid those resistance genes would get commoner in turn, so those resistance genes would grow rare again, followed by the virulence genes. As Hamilton put it, ‘antiparasite adaptations are in constant obsolescence’. But instead of the unfavoured genes being driven to extinction, as happened to the asexual species, once rare, they stopped getting rarer; they could therefore be brought back. ‘The essence of sex in our theory,’ wrote Hamilton, ‘is that it stores genes that are currently bad but have promise for reuse. It continually tries them in combination, waiting for the time when the focus of disadvantage has moved elsewhere.’ There is no permanent ideal of disease resistance, merely the shifting sands of impermanent obsolescence.50
When he runs the simulations, Hamilton’s computer screen fills with a red, transparent cube inside which two lines, one green and one blue, chase each other like fireworks on a slow-exposure photograph. What is happening is that the parasite is pursuing the host through genetic ‘space’, or to put it more precisely, each axis of the cube represents different versions of the same gene, and the host and the parasite keep changing their gene combinations. About half the time, the host eventually ends up in one corner of the cube, having run out of variety in its genes, and sticks there. Mutation mistakes are especially good at preventing it from doing that, but even without them it will do so spontaneously. What happens is entirely unpredictable, even though the starting conditions are ruthlessly ‘deterministic’ – there is no element of chance. Sometimes, the two lines pursue each other on exactly the same steady course around the edge of the cube, gradually changing one gene for fifty generations, then another and so on. Sometimes, strange waves and cycles appear. Sometimes, pure chaos, where the two lines just fill the cube with coloured spaghetti. It is strangely alive.51
Of course, the model is hardly the real world: it no more clinches the argument than building a model of a battleship proves that a real battleship will float. But it helps identify the conditions under which the Red Queen is running forever: a hugely simplified version of a human being and a grotesquely simplified version of a parasite will continually change their genes in cyclical and random ways, never settling, always running but never going anywhere, eventually coming back to where they started – as long as they both have sex.52
Sex at Altitude
Hamilton’s disease theory makes many of the same predictions as Alexey Kondrashov’s mutation theory, which we met in the last chapter. To return to the analogy of the lawn sprinkler and the rain storm, both can explain how the driveway got wet. But which is correct? In recent years, ecological evidence has begun to tip the scales Hamilton’s way. Mutation is common and diseases are rare in certain habitats: mountain tops, for example, where there is much more ultraviolet light of the type that damages genes and causes mutations. So, if Kondrashov is right, sex should be commoner on mountain tops. It is not. Alpine flowers are often among the most