SOME EVOLUTIONARY CONSEQUENCES OF DELETERIOUS MUTATIONS

Most mutations with observable phenotypic effects are deleterious. Stu dies of Drosophila and inbred plant populations suggest that a new ind ividual may have a mean number of new deleterious mutations that excee ds one-half. Most of these have relatively small homozygous effects an d reduce fitness by 1-2% when heterozygous. Several striking features of present-day organisms have apparently evolved in response to the co nstant input of deleterious alleles by recurrent mutation. For example , the adaptations of hermaphroditic organisms for outcrossing have bee n widely interpreted in terms of the benefits of avoiding the reduced fitness of inbred progeny, which is partly due to deleterious mutation s. Population genetic models of modifiers of the breeding system in th e 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 acc umulation of deleterious mutations. The population genetic mechanisms that can drive this degeneration are reviewed and their significance a ssessed in the light of available data.