question: What is the evolutionary process described by the "Muller's ratchet" model?
answer: 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.