He found exactly the opposite. Asexual species tend to be small and live at high latitudes and high altitudes, in fresh water or disturbed ground. They live in unsaturated habitats where harsh, unpredictable conditions keep populations from reaching full capacity.
Indeed, even the association between sex and hard times in aphids and rotifers turns out to be a myth. Aphids and monogonont rotifers both turn sexual not when winter or drought threaten but when overcrowding affects the food supply. You can make them turn sexual in the laboratory just by letting them get too crowded.
Bell's verdict on the lottery model was scathing: "Accepted, at least as a conceptual foundation, by the best minds which have contemplated the function of sexuality, it seems utterly to fail the test of comparative analysis. "'
Lottery models predict that sex should be most common where in fact it is rarest—among highly fecund, small creatures in changeable environments. On the contrary, here sex is the exception; but in big, long-lived,: slow-breeding creatures in stable environments sex is the rule:
This was a bit unfair toward Williams, whose "elm-oyster model " had at least predicted that fierce competition between saplings for space was the reason elms were sexuaclass="underline" Michael Ghiselin developed this idea further in 1974 and made some telling
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analogies with economic trends. As Ghiselin put it, "In a saturated economy, it pays to diversify. " Ghiselin suggested that most creatures compete with their brothers and sisters, so if everybody is a little different from their brothers and sisters, then more can survive. The fact that your parents thrived doing one thing means that it will probably pay to do something else because the local habitat might well be full already with your parents ' friends or relatives doing their thing.'°
Graham Bell has called this the "tangled bank" theory, after the famous last paragraph of Charles Darwin's Origin of Species: "It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other and dependent upon each other in so complex a manner, have all been produced by laws acting around us:"
Bell used the analogy of a button maker who has no competitors and has already supplied buttons to most of the local market. What does he do? He could either continue selling replace-ments for buttons or he could diversify the range of his buttons and try to expand the market by encouraging his customers to buy all sorts of different kinds of buttons. Likewise, sexual organisms in saturated environments, rather than churning out more of the same offspring, would be better off varying them a bit in the hope of producing offspring that could avoid the competition by adapting to a new niche: Bell concluded from his exhaustive survey of sex and asex in the animal kingdom that the tangled bank was the most promising of the ecological theories for sex.' Z
The tangled bankers had some circumstantial evidence for their idea, which came from crops of wheat and barley. Mixtures of different varieties generally yield more than a single variety does; plants transplanted to different sites generally do worse than in their home patches, as if genetically suited to their home ground; if allowed to compete with one another in a new site, plants derived from cuttings or tillers generally do worse than plants derived from sexual seed, as if sex provides some sort of variable advantage."
THE POWER OF PARASITES
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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 did wet the drive. Because the tangled bank is consistent with the facts does not prove it is the cause of the facts.
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
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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. 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 Muller ' s ratchet: the chance extinction of some forms and the 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