title: Quantifying the slightly deleterious mutation model of molecular evolution.
abstract: We have attempted to quantify the frequency and effects of slightly deleterious mutations (SDMs), those that have selective effects close to the reciprocal of the effective population size of a species, by comparing the level of selective constraint in protein-coding genes of related species that have different present-day effective population sizes. In our two comparisons, the species with the smaller effective population size showed lower constraint, implying that SDMs had become fixed. The fixation of SDMs was supported by the observation of a higher fraction of radical to conservative amino acid substitutions in species with smaller effective population sizes. The fraction of strongly deleterious mutations (which rarely become fixed) is >70% in most species. Only approximately 10% or fewer of mutations seem to behave as SDMs, but SDMs could comprise a substantial fraction of mutations in protein-coding genes that have a chance of becoming fixed between species.
authors.affiliation: Centre for the Study of Evolution & School of Biological Sciences, University of Sussex, Brighton BN1 9QG, UK. a.c.eyre-walker@sussex.ac.uk
authors.forename: Adam
authors.initials: A
authors.lastname: Eyre-Walker
authors.forename: Peter D
authors.initials: PD
authors.lastname: Keightley
authors.forename: Nick G C
authors.initials: NG
authors.lastname: Smith
authors.forename: Daniel
authors.initials: D
authors.lastname: Gaffney
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D019143:Evolution, Molecular; D006801:Humans; D008957:Models, Genetic; D009154:Mutation; D011156:Population Density