The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [40]
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 towards the tropics from the poles.’54
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 defences – to evolve. For example, among Douglas firs infested by scale insects (which are amorphous blobs of insectness that barely even look like animals), the older trees are more heavily infested than the younger ones. By transplanting scale insects from one tree to another, two scientists were able to show that this is an effect of better adapted insects, not weaker old trees. Such trees would do their offspring no favours by having identical young, on whom the well-adapted insects would immediately descend. Instead, the trees are sexual and have different young.55
Disease might almost put a sort of limit on longevity: there is little point in living much longer than it takes your parasites to adapt to you. Quite how yew trees, bristlecone pines and giant sequoias get away with living for thousands of years is not clear, but what is clear is that, by virtue of chemicals in their bark and wood, they are remarkably resistant to decay. In the Sierra Nevada of California lie the trunks of fallen sequoias, partly covered by the roots of huge pine trees that are hundreds of years old – yet the wood of the sequoia stumps is hard and true.56
In the same vein, it is tempting to speculate that the peculiar synchronized flowering of bamboo might have something to do with sex and disease. Some bamboos flower only once every 121 years, and they do so at exactly the same moment all over the world, then die. This gives their young all sorts of advantages: they do not have living parents to compete with and the parasites are wiped out when the bamboo parent plants die (their predators have problems, too: flowering causes a crisis for pandas).57
Moreover, it is a curious fact that parasites themselves are often sexual, despite the enormous inconvenience this causes. A bilharzia worm inside a human vein cannot travel abroad to seek a mate. But if it encounters a genetically different worm, infected on a separate occasion, they have sex. To compete with their sexual hosts, parasites, too, need sex.
Sexless Snails
But these are all hints from natural history, not careful scientific experiments. There is also a small amount of more direct evidence in favour of the parasite theory of sex. By far the most thorough study of the Red Queen was done in New Zealand by a soft-spoken American biologist named Curtis Lively, who became intrigued by the evolution of sex when told to write an essay on the subject as a student. He soon abandoned his other research, determined to solve the problem of sex. He went to New Zealand and examined water snails from streams and lakes and found that in many populations there are no males and the females give birth as virgins, but in other populations the females mate with males and produce sexual offspring. So he was able to sample the snails, count the males and get a rough-and-ready measure of the predominance of sex. His prediction was that