Matt Ridley

slowly. Pitted against a clone of asexual individuals, a sexual population must inevitably be driven extinct by the clone 's greater productivity unless the clone 's genetic drawbacks can appear in time. It is a race against time. For how long? Curtis Lively of the University of Indiana has calculated that for every tenfold increase in population size, the advantage of sex is granted six more generations to show its effects or sex will lose the game. If there are a million individuals, sex has forty generations before it goes extinct; if a billion, it has eighty. Yet the genetic repair theories all require thousands of generations to do their work. Kondrashov's is certainly the fastest theory, but it is probably not fast enough.'

There is still no purely genetic theory to explain sex that attracts wide support. An increasing number of students of evolution believe that the solution to the great enigma of sex lies in ecology, not genetics.

Chapter 3

THE POWER OF

PARASITES

The chessboard is the world; the pieces are the phenomena of the universe; the rules of the game are what we call the laws of Nature: The player on the other side is hidden from us: We know that his play is always fair, just, and patient: But also we know, to our cost, that he never overlooks a mistake or makes the small-est allowance for ignorance:

—Thomas Henry Huxley

Even for microscopic animals, the bdelloid rotifers are peculiar.

They live in any kind of fresh water, from puddles in your gutter to hot springs by the Dead Sea and ephemeral ponds on the Antarctic continent. They look like animated commas driven by what appear to be small waterwheels at the front of the body, and when their watery home dries up or freezes, they adopt the shape of an apostrophe and go to sleep. This apostrophe is known as a 'tun,' and it is astonishingly resistant to abuse. You can boil it for an hour or freeze it to within I degree of absolute zero—that is, to -272

degrees Centigrade—for a whole hour: Not only does it fail to dis-integrate, it does not even die: Tuns blow about the globe as dust so easily that rotifers are thought to travel regularly between Africa and America: Once thawed out, the tun quickly turns back into a rotifer, paddles its way about the pond with its bow wheels, eating bacteria as it goes, and within a few hours starts producing eggs that hatch into other rotifers. A bdelloid rotifer can fill a medium-sized lake with its progeny in just two months.

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 scandal: ''

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 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