Muller's Ratchet

Muller's ratchet is a proposed mechanism explaining the long-term genetic decline of populations that reproduce asexually, describing how such populations irreversibly accumulate increasingly harmful mutations over successive generations because, without genetic recombination through sexual reproduction, an asexual lineage can never reconstruct a mutation-free genome once its least-mutated individuals are lost from the population by chance. Geneticist Hermann Joseph Muller first proposed the underlying concept in a 1964 paper, building on his earlier foundational work studying the effects of mutation, for which he had already been awarded the 1946 Nobel Prize in Physiology or Medicine. The mechanism works like a ratchet that can only turn one direction, since each time an asexual population's least-mutated class of individuals is lost, whether by chance or otherwise, that lower mutational load can never be recovered again in the absence of recombination to reshuffle and separate accumulated mutations. Muller's ratchet is widely cited as one of the leading proposed explanations for why sexual reproduction, despite its own substantial costs, is so evolutionarily widespread, since recombination in sexually reproducing populations provides a mechanism to purge accumulated harmful mutations that asexual populations lack.