The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [30]
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 round 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 which then compete with each other 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 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 animals have sex, but at how much recombination goes on among their genes. He measured this quite easily by counting the number of ‘cross-overs’ 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 animals do more genetic mixing regardless of their size or fecundity than short-lived, early-maturing animals. By Burt’s measure, man has thirty cross-overs, rabbits ten and mice three. Tangled bank theories would predict the opposite.17
The tangled bank also conflicted with the evidence from fossils. In the 1970s, evolutionary biologists realized that species do not change much. They stay exactly the same for thousands of generations, to be suddenly replaced by other forms of life. The tangled bank is a gradualist idea. If tangled banks were true, then species would gradually drift through the adaptive landscape, changing a little in every generation, instead of remaining true to type for millions of generations. A gradual drifting away of a species from its previous form happens on small islands, or in tiny populations, precisely because of effects somewhat analogous to Müller’s ratchet: the chance extinction of some forms and chance prosperity of other mutated forms. In larger populations, the process that hinders this is sex itself, for an innovation is donated to the rest of the species and quickly lost in the crowd. In island populations, sex cannot do this precisely because the population is so inbred.18
It was Williams