DELETERIOUS MUTATIONS IN ANIMAL MITOCHONDRIAL-DNA

A simple neutral model predicts that the ratio of non-synonymous to sy nonymous fixed differences between species will be the same as the rat io of non-synonymous to synonymous polymorphisms within species. This prediction is tested with existing mitochondrial datasets from 25 anim al species. In slightly over half of the studies, the ratio of replace ment to silent polymorphisms within species is significantly greater t han the ratio of replacement to silent fixed differences between speci es. These observations are best explained by a substantial number of m ildly deleterious amino acid mutations that contribute to heterozygosi ty but rarely become fixed.