The trouble is, all these results are also predicted by rival theories just as plausibly. Williams wrote: 'Fortune will be benevo-lent indeed if the inference from one theory contradicts that of another.'' This is an especially acute problem in the debate. One scientist gives the analogy of somebody trying to decide what makes his driveway wet: rain, lawn sprinklers, or flooding from the local river. It is no good turning on the sprinkler and observing that it wets the drive or watching rain fall and seeing that it wets the drive.' To conclude anything from such observations would be to fall into the trap that philosophers call ' the fallacy of affirming the consequent. ' Because sprinklers can wet the drive does not prove that they
It is hard to find dedicated enthusiasts of tangled banks these days. Their main trouble is a familiar one: If it ain' t broke, why does sex need to fix it? An oyster that has grown large enough to breed is a great success, in oyster terms. Most of its siblings are dead: If, as tangled bankers assume, the genes had something to do with that, then why must we automatically assume that the combination of genes that won in this generation will be a flop in the next? There are ways around this difficulty for tangled bankers, but they sound a bit like special pleading: It is easy enough to identify an individual case where sex would have some advantage, but to raise it to a general principle for every habitat of every mammal and bird, for every coniferous tree, a principle that can give a big enough advantage to overcome the fact that asex is twice as fecund as sex—nobody can quite bring himself to do that: There is a more empirical objection to the tangled bank theory. Tangled banks predict a greater interest in sex in those animals and plants that have many small offspring that then compete with one another than among the plants and animals that have few large young. Superficially, the effort devoted to sex has little to do with how small the offspring are. Blue whales, the biggest animals, have huge young—each may weigh five tons or more. Giant sequoias, the biggest plants, have tiny seeds, so small that the ratio of their weight to the weight of the tree is the same as the ratio of
the tree to the planet Earth. 16 Yet both are sexual creatures. By contrast, an amoeba, which splits in half when it breeds, has an enormous ' young' as big as 'itself: ' Yet it never has sex.
A student of Graham Bell 's named Austin Burt went out and looked at the real world to see if the tangled bank fitted the facts. He looked not at whether mammals have sex but at how much recombination goes on among their genes. He measured this quite easily by counting the number of 'crossovers ' on a chromosome: These are spots where, quite literally, one chromosome swaps genes with another: What Burt found was that among mammals the amount of recombination bears no relation to the number of young, little relation to body size, and close relation to age at maturity: In other words, long-lived, late-maturing mammals do more genetic mixing regardless of their size or fecundity than short-lived, early maturing mammals: By Burt 's measure, man has thirty crossovers, rabbits ten, and mice three. Tangled-bank theories would predict the opposite:'
The tangled bank also conflicted with the evidence from fossils: In the 1970s evolutionary biologists realized that species do not change much.
It was Williams who first pointed out that a huge false assumption lay, and indeed still, lies, at the core of most popular treatments of evolution. The old concept of the ladder of progress THE POWER OF PARASITES
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still lingers on in the form of a teleology: Evolution is good for species, and so they strive to make it go faster. Yet it is stasis, not change, that is the hallmark of evolution. Sex and gene repair and the sophisticated screening mechanisms of higher animals to ensure that only defect-free eggs and sperm contribute to the next generation—all these are ways of preventing change. The coelacanth, not the human, is the triumph of genetic systems because it has remained faithfully true to
'Vicar of Bray ' model of sex, in which sex is an aid to faster evolution, implies that organisms would prefer to keep their mutation rate fairly high—since mutation is the source of all variety—and then do a good job of sieving out the bad ones: But, as Williams put it, there is no evidence yet found that any creature ever does anything other than try to keep its mutation rate as low as possible.
It strives for a mutation rate of zero: Evolution depends on the fact that it fails. 19
Tangled banks work mathematically only if there is a sufficient advantage in being odd: The gamble is that what paid off in one generation will not pay off in the next and that the longer the generation, the more this is so —which implies that conditions keep changing.
THE RED QUEEN
Enter, running, the Red Queen. This peculiar monarch became part of biological theory twenty years ago and has been growing ever more important in the years since then: Follow me if you will into a dark labyrinth of stacked shelves in an office at the University of Chicago, past ziggurats of balanced books and three-foot Babels of paper. Squeeze between two filing cabinets and emerge into a Sty-gian space the size of a broom cupboard, where sits an oldish man in a checked shirt and with a gray beard that is longer than God 's but not so long as Charles Darwin 's. This is the Red Queen's first prophet, Leigh Van Valen, a single-minded student of evolution.
One day in 1973, before his beard was so gray, Van Valen was searching his capacious mind for a phrase to express a new discovery he had made while studying marine fossils. The discovery was that the probability a family of animals would become extinct does not depend on how long that family has already existed. In other words, species do not get better at surviving (nor do they grow feeble with age, as individuals do). Their chances of extinction are random.
The significance of this discovery had not escaped Van Valen, for it represented a vital truth about evolution that Darwin had not wholly appreciated. The struggle for existence never gets easier. However well a species may adapt to its environment, it can never relax, because its competitors and its enemies are also adapting to their niches., Survival is a zero-sum game. Success only makes one species a more tempting target for a rival species. Van Valen's mind went back to his childhood and lit upon the living chess pieces that Alice encountered beyond the looking glass. The Red Queen is a formidable woman who runs like the wind but never seems to get anywhere:
' Well, in
