MILDLY DELETERIOUS MUTATIONS IN AVIAN MITOCHONDRIAL DNA: EVIDENCE FROM NEUTRALITY TESTS.

To determine whether mildly deleterious mutations (MDMs) are present in nonrecombining genomes such as avian mitochondrial DNA (mtDNA), I analyzed molecular data from 14 studies using the neutrality tests of Tajima (1989a) and McDonald and Kreitman (1991). The presence of MDMs in mtDNA is inferred from trends observed across species in estimates of heterozygosity (θ and π) and by comparisons of polymorphism and divergence using the neutrality index (NI). Assuming neutrality, θ equals π and NI equals one. In this study, however, θ is greater than π more often than expected by chance, which reflects an excess of low-frequency alleles, and NI values presented here and elsewhere are consistently greater than one, which suggests an excess of nonsynonymous mutations within species (polymorphism) relative to between species (divergence). These observations suggest that, within species, there is an excess of rare haplotypes and that these haplotypes are carrying MDMs. The excess rare haplotypes may need to be accounted for when estimating population genetic parameters that assume strict neutrality.