Diploidy and Muller's ratchet

Abstract

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 (no), 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 no. If no 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 no. The argument suggests Muller's ratchet is more powerful than previously accepted in asexual species derived from diploid sexual ancestors.