Box 1 | Muller's ratchet

From the following article:

Hermann Joseph Muller, Evolutionist

James F. Crow

Nature Reviews Genetics 6, 941-945 (December 2005)

doi:10.1038/nrg1728

This theory describes the accumulation of deleterious mutations in a small population in the absence of sexual reproduction.

Every population — sexual or asexual — contains deleterious mutations, and individuals totally lacking such mutations are uncommon. In a small population, every individual might have at least one deleterious mutation in any one generation. This is not a problem if the species is sexual, as an individual with no deleterious mutations can be created by recombination.

In an asexual population, however, deleterious mutations cannot be removed (except by reverse mutation, which would be improbable, especially in a small population). In the following generation the best individual in the population has one mutation. In a later generation there may be no individual that has only one deleterious mutation, so the best individual in the population has two mutations. And so on. The number of mutations increases in a stepwise manner and there is no turning back. And so, in a ratchet-like manner, a small asexual population gradually accumulates an increasing number of deleterious mutations, and may become extinct. Muller's ratchet constitutes an important argument for the evolution of sex, although the importance of the effect depends mainly on the population size and the mutation rate.