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Rochester, Graham Bell of Montreal, Hans Bremermann of Berkeley, John Tooby of Harvard, and Bill Hamilton of Oxford:'
But it was Hamilton who pursued the connection between sex and disease most doggedly and became most associated with it.
In appearance, Hamilton was an almost implausibly perfect example of the absentminded professor as he stalked through the streets of Oxford, deep in thought, his spectacles attached umbilically to a string around his neck, his eyes fixed on the ground in front of him.
His unassuming manner and relaxed style of writing and storytelling were deceptive. Hamilton had a habit of being at the right place in biology at the right time. In the 196os he molded the theory of kin selection—the idea that much of animal cooperation and altruism is explained by the success of genes that cause animals to look after close relatives because they share many of the same genes. Then in 1967 he stumbled on the bizarre internecine warfare of the genes that we shall meet in chapter 4. By the 198os he was anticipating most of his colleagues in pronouncing reciprocity as the key to human cooperation: Again and again in this book we will find we are treading in Hamilton's footsteps! 8
With the help of two colleagues from the University of Michigan, Hamilton built a computer model of sex and disease, a slice of artificial life. It began with an imaginary population of two hundred creatures. They happened to be rather like humans—each began breeding at fourteen, continued until thirty-five or so, and had one offspring every year. But the computer then made some of them sexual—meaning two parents had to produce and rear each child— and some of them asexual: Death was random: As expected, the sexual race quickly became extinct every time they ran the computer. In a game between sex and asex, asex always won, other things being equal: ;9
Next, they introduced several species of parasites, 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, mostly 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 got more common, then virulence genes that undid those resistance genes got more common in turn, so those resistance genes grew rare again, followed by the virulence genes. As Hamilton put it, 'Antiparasite adaptations are in constant obsolescence: ' But instead of the unfavored 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:'°
When it 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 stays 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 there is pure chaos: The two lines just fill the cube with colored spaghetti. It is strangely alive.'
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 THE POWER OF PARASITES
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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 rainstorm, 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. In certain habitats, mutation is common and diseases rare—mountaintops, 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 more common on mountaintops. It is not. Alpine flowers are often among the most asexual of flowers. In some groups of flowers, the ones that live near the tops of mountains are asexual, while those that live lower down are sexual. In five species
There are all sorts of explanations of this that have little to do with parasites, of course: The higher you go, the colder it gets, and the less you can rely on insects to pollinate a sexual flower. But if Kondrashov were right, such factors should be overwhelmed by the need to fight mutation. And the altitude effect is mirrored by a latitude effect: In the words of one textbook: ' There are ticks and lice, bugs and flies, moths, beetles, grasshoppers, millipedes, and more, in all of which males disappear as one moves from the tropics toward the poles. ''
Another trend that fits the parasite theory is that most asexual plants are short-lived annuals. Long-lived trees face a particular problem because their parasites have time to adapt to their genetic defenses—to evolve. For
