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 , i.e. the class with the smallest number of mutations, is determined by the genome mutation rate and the average selective disadvantage 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 . It is argued that diploidy 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