Mullet's Ratchet

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 Muller, one of the fathers of the Vicar of Bray theory. Muller, who spent much of his career at the University of Indiana, 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; it has come to be known as "Mullet'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 re-created 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 re-created 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 from turning is for the perfect flea to have sex and pass its defect-free genes to other fleas before it dies.

Without sex, mutations would ratchet up. An infusion of good genes from another source keeps them clean.