But there is another odd thing about bdelloids besides their feats of endurance and fecundity. No male bdelloid rotifer has ever been seen. As far as biologists can tell, every single member of every one of all five hundred species of bdelloid in the world is a female. Sex is simply not in the bdelloid repertoire.
It is possible that bdelloid rotifers mix others ' genes with their own by eating their dead comrades and absorbing some of their genes, or something bizarre like that,' but recent research by Matthew Meselson and David Welch suggests that they just never
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The Red Queen
do have sex. They have found that the same gene in two different individuals can be up to 30 percent different at points that do not affect its function—a level of difference that implies bdelloids gave up sex between 40 million and 80 million years ago.'
There are many other species in the world that never have sex, from dandelions and lizards to bacteria and amoebas, but the bdelloids are the only example of a whole order of animal that entirely lacks the sexual habit: Perhaps as a result the bdelloids all look rather alike, whereas their relatives, the monogonont rotifers, tend to be much more varied; they cover the whole range of shapes of punctuation marks: Nonetheless, the bdelloids are a living rebuke to the conventional wisdom of biology textbooks—that without sex, evolution can barely happen and species cannot adapt to change: The existence of the bdelloid rotifers is, in the words of John Maynard Smith, " an evolutionary scandaclass="underline" "'
THE ART OF BEING SLIGHTLY DIFFERENT
Unless a genetic mistake happens, a baby bdelloid rotifer is identical to its mother: A human baby is not identical CO its mother: That is the first consequence of sex: Indeed, according to most ecologists, it is the purpose of sex:
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.'
Elaborating on that idea was the main activity throughout THE POWER OF PARASITES
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the 1970s of ecologists interested in sex. In 1971, in his first attack on the problem, John Maynard Smith suggested that sex was needed for those cases in which two different creatures migrate into a new habitat in which it helps to combine both their characters.' Two years later Williams returned to the fray and suggested that if most of the young are going to die, as most who try their luck as travelers will, then it may be the very fittest ones that will survive. It therefore matters not one bit how many young of average quality a creature has. What counts is having a handful of young that are exceptional. If you want your son to become pope, the best way to achieve this is not to have lots of identical sons but to have lots of different sons in the hope that one is good, clever, and religious enough.'
The common analogy for what Williams was describing is a lottery. Breeding asexually is like having lots of lottery tickets all with the same number. To stand a chance of winning the lottery, you need lots of different tickets. Therefore, sex is useful to the individual rather than the species when the offspring are likely to face changed or unusual conditions.
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 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 sexuaclass="underline" 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.
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The Red Queen
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.