What is the evolutionary process described by the "Muller's ratchet" model?
The vast majority of mutations are deleterious and are eliminated by purifying selection. Yet in finite populations, purifying selection cannot completely prevent the accumulation of deleterious mutations. Muller's ratchet is a paradigmatic model for the accumulation of deleterious mutations in a population of finite size, due to genetic drift. Muller's ratchet suggests that population bottlenecks, which may constrain the genetic diversity of a population, could lead to extinction of all individuals with the least number of deleterious mutations, due to a stochastic fluctuation. When the most-fit class of genotypes is lost, it is lost irreversibly.