Diploidy and Muller's ratchet

Summary

Under the influence of deleterious mutation and selection a population will reach equilibrium and contain individuals with [0, 1, 2 - - mutations.] This deterministic equilibrium distribution is exactly the same for asexual and sexual populations. The size of the optimal class (n _o), i.e. the class with the smallest number of mutations, is determined by the genome mutation rate (U) and the average selective disadvantage (s) of the mutations. A large U and small s gives a very small n _o. If n _o is small in an asexual population it will be lost by drift and this causes a reduction in the mean fitness of the population (by the ratchet mechanism). It is argued that diploidy (when compared to haploidy) increases U and reduces s. Values of U and s observed from Drosophila indicate a diploid would have a vanishingly small n _o. The argument suggests Muller's ratchet is more powerful than previously accepted in asexual species derived from diploid sexual ancestors.