possible—asexually.
When the little world came to an end and the next generation of aphid or rotifer faced the uncertainty of finding a new home or waited for the old one to reappear, then it paid to produce a variety of different young in the hope that one would prove ideal.
Williams contrasted the 'aphid-rotifer model ' with two others: the strawberry-coral model and the elm-oyster model.
Strawberry plants and the animals that build coral reefs sit in the same place all their lives, but they send out runners or coral branches so that the individual and its clones gradually spread over the surrounding space: However, when they want to send their young much farther away, in search of a new, pristine habitat, the strawberries produce sexual seeds and the corals produce sexual larvae called 'planulae.' The seeds are carried away by birds; the planulae drift for many days on the ocean currents: To Williams, this looked like a spatial version of the lottery: Those who travel far-thest are most likely to encounter different conditions, so it is best that they vary in the hope that one or two of them will suit the place they reach. Elm trees and oysters, which are sexual, produce millions of tiny young that drift on breezes or ocean currents until a few are lucky enough to land in a suitable place and begin a new life. Why do they do this? Because, said Williams, both elms and oysters have saturated their living space already. There are few clearings in an elm forest and few vacancies on an oyster bed. Each vacancy will attract many thousands of applicants in the form of new seeds or larvae: Therefore, it does not matter that your young are good enough to survive. What matters is whether they are the very best. Sex gives variety, so sex makes a few of your offspring exceptional and a few abysmal, whereas asex makes them all average:'
THE TANGLED BANK
Williams's proposition has reappeared in many guises over the years, under many names and with many ingenious twists. In general, however, the mathematical models suggest that these lottery models only work if the prize that rewards the right lottery ticket is indeed a huge jackpot. Only if a very few of the dispersers survive and do spectacularly well does sex pay its way. In other cases, it does not.'
Because of this limitation, and because most species are not necessarily producing young that will migrate elsewhere, few ecolo-THE POWER OF PARASITES
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gists wholeheartedly adopted lottery theories. But it was not until Graham Bell in Montreal asked, like the apocryphal king and the goldfish, to see the actual evidence for the pattern the lottery model was designed to explain that the whole edifice tumbled down: Bell set out to catalog species according to their ecology and their sexuality: He was trying to find the correlation between ecological uncertainty and sexuality that Williams and Maynard Smith had more or less assumed existed. So he expected to find that animals and plants were more likely to be sexual at higher latitudes and altitudes (where weather is more variable and conditions harsher); in fresh water rather than the sea (because fresh water varies all the time, flooding, drying up, heating up in summer, freezing in winter, and so on, 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.
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 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.'°
Graham Bell has called this the 'tangled bank' theory, after the famous last paragraph of Charles Darwin's
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.'
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