Sex as a Survival Strategy

In 1966, George Williams exposed the logical flaw at the heart of the textbook explanation of sex. He showed how it required animals to ignore short-term self-interest in order to "further the survival and evolution of their species, a form of self restraint that could have evolved only under very peculiar circumstances. He was very unsure what to put in its place. But he noticed that sex and dispersal often seem to be linked. Thus, grass grows asexual runners to propagate locally but commits its sexually produced seeds to the wind to travel farther. Sexual aphids grow wings; asexual ones do not. The suggestion that immediately follows is that if your young are going to have to travel abroad, then it is better that they vary because abroad may not be like home.'^

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Williams was especially intrigued by creatures such as aphids and monogonont rotifers, which have sex only once every few generations. Aphids multiply during the summer on a rosebush, 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 that 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 favorable and predictable, it paid to reproduce as fast as 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 corora 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 farthest 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 sex makes them all average.

Many species that are asexual become sexual when it is time to disperse over large distances.