The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [28]
Williams was especially intrigued by creatures like aphids and monogonont rotifers, which have sex only once every few generations. Aphids multiply during the summer on a rose bush and monogonont rotifers multiply in a street puddle. But when the summer comes to an end, the last generation of aphids or of monogonont rotifers is entirely sexual: it produces males and females which seek each other out, mate and produce tough little young that spend the winter or the drought as hardened cysts awaiting the return of better conditions. To Williams this looked like the operation of his lottery. While conditions were favourable and predictable, it paid to reproduce as fast as possible – asexually. When the little world came to an end and the next generation of aphids or rotifers faced the uncertainty of finding a new home, or waiting 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 further 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 furthest 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 vacancies on an oyster bed and few clearings in an elm forest. Each vacancy will attract many thousands of applicants in the form of new larvae or seeds. 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.7
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, but they do spectacularly well, does sex pay its way. In other cases, it does not.8
Because of this limitation, and because most species are not necessarily producing young that will migrate elsewhere, few ecologists 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 catalogue 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 to exist. 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,