The inevitable and irreversible accumulation of deleterious mutations that occurs in an asexually reproducing population. The mechanism is named after the US geneticist Hermann J. Muller (1890–1967), who likened each successive asexual generation to the one-directional movement of a ratchet. He observed that the evolution of sexual reproduction offered the great advantage of allowing the removal of mutations through recombination. However, factors such as large population size, low mutation rate, and the occurrence of beneficial mutations can all counteract Muller’s ratchet and its ultimate consequence—extinction.