Over at the NYT, Oliva Judson’s third piece on mutations:
There are several reasons for this neglect of the benign [mutations]. One — dare I say it — is fashion. In the late 1960s, the geneticist Motoo Kimura proposed the neutral theory of molecular evolution. According to this idea, most mutations are either harmful (and will quickly disappear from the population because those bearing them die) or irrelevant. If this is the case, most genetic variation has no impact on fitness — the technical term for how good an organism is at surviving and reproducing. Kimura’s development of the neutral theory was enormously influential, and prompted a flurry of work investigating whether most genetic variation is irrelevant.
Then it was the turn of deleterious mutations, which became trendy in the late 1980s and early 1990s. Deleterious mutations have been hypothesized to play a central role in a variety of evolutionary phenomena, including (and most prominently) sex. The argument is that organisms with a deleterious mutation rate above a certain threshold must reproduce sexually.
The reason is that sex purges deleterious mutations from the population: sex generates new gene combinations, and thus in each generation it creates some individuals with relatively few deleterious mutations and some with lots.