Some evolutionary consequences of deleterious mutations.

Most mutations with observable phenotypic effects are deleterious. Studies of Drosophila and inbred plant populations suggest that a new individual may have a mean number of new deleterious mutations that exceeds one-half. Most of these have relatively small homozygous effects and reduce fitness by 1-2% when heterozygous. Several striking features of present-day organisms have apparently evolved in response to the constant input of deleterious alleles by recurrent mutation. For example, the adaptations of hermaphroditic organisms for outcrossing have been widely interpreted in terms of the benefits of avoiding the reduced fitness of inbred progeny, which is partly due to deleterious mutations. Population genetic models of modifiers of the breeding system in the presence of genome-wide deleterious mutation are reviewed and their predictions related to genetic and comparative data. The evolution of degenerate Y chromosomes is a phenomenon that may be caused by the accumulation of deleterious mutations. The population genetic mechanisms that can drive this degenerations are reviewed and their significance assessed in the light of available data.