An evolutionary process by which action may cause the extinction of a population through the accumulation of deleterious mutations. Given a population that is polymorphic for (*i.e.* for which there is variation in) the number of deleterious mutations in individual genomes, there exists some discrete class of individuals that bear the fewest number. Members of this class do not necessarily share the same mutations despite having the same number.

Genetic drift in a finite population will cause the loss of this class of lowest number. In the absence of back mutation, this is an irreversible step and the next discrete class containing individuals with one more mutation becomes the new lowest numbered class. The ratchet has turned.

This process was first suggested by Nobel laureate H. J. Muller in 1964, and was subsequently studied numerically by J. Felsenstein in 1974. Legend has it that Felsenstein, then a graduate student, approached J. Maynard Smith at a conference and shared his exciting results. Maynard Smith then encouraged his own graduate student J. Haigh to subject the process to rigorous mathematical study. Haigh's analysis involves branching processes and is a very nice analysis of the problem. Muller's ratchet has since appeared many times in the literature as an extinction factor in small populations, and in current theoretical population genetics.