The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [24]
It is almost the definition of sex that it breaks up combinations of genes. The great cry of the geneticists is that sex reduces ‘linkage disequilibria’. What they mean is that if it were not for recombination, genes that are linked together, like those for blue eyes and blond hair, would always be linked together and nobody would ever have blue eyes and brown hair, or blond hair and brown eyes. Thanks to sex, the moment the fabled synergy is found, it is lost again. Sex disobeys that great injunction: ‘If it ain’t broke, don’t fix it.’ Sex increases randomness.34
In the late 1980s there was one last revival of interest in theories of ‘good’ mutation. Mark Kirkpatrick and Cheryl Jenkins of the University of Texas were interested not in two separate inventions but in the ability to invent the same thing twice. Suppose, for example, that blue eyes double fertility, so that people with blue eyes have twice as many children as people with brown. But suppose that at first everybody has brown eyes. The first mutation in a brown-eyed person to blue eyes will have no effect because blue eyes are a recessive gene and the dominant brown-eye gene on the person’s other chromosome will mask it. Only when two of the descendants of the original mutant person marry and both their blue-eye genes come together will the great benefit of blue eyes be seen. Only sex would allow the people to marry and the genes to meet. This so-called segregation theory of sex is logical and uncontroversial. It is indeed one of the advantageous consequences of sex. Unfortunately, it is far too weak an effect to be the main explanation for sex’s prevalence. Mathematical models reveal that it would take five thousand generations to do its good work and asex would long ago have won the game by then.35
In recent years, the geneticists have turned away from good mutations and begun to think about bad ones. Sex, they suggest, is a way of getting rid of bad mutations. This idea also has its origins in the 1960s, with Hermann Müller, one of the fathers of the Vicar of Bray theory. Müller, who spent much of his career at the University of Indiana, had published his first scientific paper on genes in 1911 and a veritable flood of ideas and experiments followed in the succeeding decades. In 1964, he had one of his greatest insights, one that has come to be known as ‘Müller’s ratchet’. A simplified example of it goes like this: there are ten water fleas in a tank, only one of which is entirely free of mutations; the others all have one or several minor defects. On average only five of the water fleas in each generation manage to breed before they are eaten by a fish. The defect-free flea has a one-in-two chance of not breeding. So does the flea with the most defects, of course, but there is a difference. Once the defect-free flea is dead the only way for it to be recreated is for another mutation to correct the mutation in a flea with a defect – a very unlikely possibility. The one with two defects can be recreated easily, by a single mutation in a water flea with one defect anywhere among its genes. In other words, the random loss of certain lines of descent will mean that the average number of defects gradually increases. Just as a ratchet turns easily one way, but cannot turn back, so genetic defects inevitably accumulate. The only way to prevent the ratchet turning is for the perfect flea to have sex and pass its defect-free