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Müller’s ratchet applies if you use a photocopier to make a copy of a copy of a copy of a document. With each successive copy the quality deteriorates. Only if you guard the unblemished original can you regenerate a clean copy. But suppose the original is stored with the copies in a file and more copies are made when there is only one left in the file. You are just as likely to send out the original as to send out a copy. Once the original is lost, the best copy you can make is less good than it was before. But you can always make a worse copy just by copying the worst copy you have.

Graham Bell of McGill University has disinterred a curious debate that raged among biologists at the turn of the century about whether sex had a rejuvenating effect. What intrigued these early biologists was if and why a population of protozoa kept in a tank with sufficient food but given no chance to have sex inevitably fell into a gradual decline in vigour, size and rate of (asexual) reproduction. Reanalysing the experiments, Bell found some clear examples of Müller’s ratchet at work. Bad mutations gradually accumulated in the protozoa deprived of sex. The process was accelerated by the habit of this one group of protozoa, the ciliates, of keeping its germ-line genes in one place and keeping copies of them elsewhere for everyday use. The method of reproducing the copies is hasty and inaccurate, so defects accumulate especially fast there. During sex, one of the things the creatures do is to throw away their copies and create new ones from the germ-line originals. Bell compares it with a chair maker who copies the last chair he made, errors and all, and only returns to his original design occasionally. Sex therefore does indeed have a rejuvenating effect. It enables these little animals to drop all the accumulated errors of an especially fast asexual ratchet whenever they have sex.37

Bell’s conclusion was a curious one. If a population is small (less than ten billion), or the number of genes in the creature is very large, the ratchet has a severe effect on an asexual lineage. This is because it is easier to lose the defect-free class in a smaller population. So those creatures with larger genomes and relatively smaller populations (10 billion is twice as many people as there are on earth) will be ratcheted into trouble fairly quickly. But those with few genes and vast populations are all right. Bell reckons that being sexual was a prerequisite for being big (and therefore few), or, conversely, sex is unnecessary if you stay small.38

Bell calculated the amount of sex, or rather of recombination, that is needed to halt the ratchet; for smaller creatures, less sex is necessary. Water fleas need have sex only once every several generations. Human beings need to have sex in every generation. Moreover, as James Crow at the University of Wisconsin in Madison has suggested, Müller’s ratchet may explain why budding is a relatively rare way of reproducing – especially among animals. Most asexual species still go to the trouble of growing their offspring from single cells (eggs). Why? Crow suggests that it is because defects that would be fatal in a single cell can be easily smuggled into a bud.39

If the ratchet is only a problem for big creatures, why do so many small ones have sex? Besides, to halt the ratchet requires only occasional episodes of sex; it does not require so many animals to abandon asexual reproduction altogether. Aware of these difficulties, in 1982 Alexey Kondrashov of the Research Computer Centre in Poschino, near Moscow, came up with a theory that is a sort of reverse Müller’s ratchet. He argued that in an asexual population, every time a creature dies because of a mutation it gets rid of that mutation, but no more. In a sexual population some of the creatures born have lots of mutations and some have few. If the ones with lots of mutations die, then sex keeps throwing the ratchet into reverse, purging mutations. Since most mutations are harmful, this gives sex a great advantage.40

But