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whereas the sea is predictable); among weeds that live in disturbed habitats; and in small creatures rather than large ones. 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.9

Lottery models predict that sex should be commonest 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 on Williams, whose ‘elm–oyster’ model had at least predicted that fierce competition between saplings for space was the reason elms were sexual. Michael Ghiselin developed this idea further in 1974 and made some telling 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.10

Graham Bell has called this the ‘tangled bank’ theory, after the famous last paragraph of Charles Darwin’s The 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.11

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 replacements 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.12

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 each other 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.13

The trouble is, all these results are also predicted by rival theories just as plausibly. Williams wrote: ‘Fortune will be benevolent indeed if the inference from one theory contradicts that of another.’14 This is an especially acute problem in the debate. One scientist gives the analogy of somebody trying to decide