Interaction of selection and drift in molecular evolution

The nearly neutral theory proposes that interaction of selection and drift is important in molecular evolution. The theory predicts that evolutionary rate is negatively correlated with the species population size, for those genes whose function has been fixed long time ago. The prediction is tested through analysis of sequence data of 17 mammalian genes by estimating divergence among genes separately for synonymous substitutions and nonsynonymous substitutions. Star phylogenies composed of rodentia, artiodactyla and primates are examined. The generation-time effect is found to be more conspicuous for synonymous substitutions than for nonsynonymous substitutions. This result supports the nearly neutral theory.