" It is often observed in RNA virus lineages since many of these lack mechanisms for horizontal gene flow; as a major such mechanism is crossing over during sexual reproduction, the idea of this process is a common explanation for the existence of sex.
This process named for its postulator can be foiled by epistatic selection since synergistic fitness effects can cause an increase in the frequency of the best available genotype. An instance of the Hill-Robertson effect, it was first introduced in 1932's "Some Genetic Aspects of Sex." It is often observed in RNA virus lineages since many of these lack mechanisms for horizontal gene flow; as a major such mechanism is crossing over during sexual reproduction, the idea of this process is a common explanation for the existence of sex. FTP, name this process whereby pure asexual reproduction leads to irreversible accumulation of harmful mutations, a so-called "ratchet" named for a German-American geneticist.
This process named for its postulator can be foiled by epistatic selection since synergistic fitness effects can cause an increase in the frequency of the best available genotype.
Muller's ratchet (accept just Muller's after "ratchet")