Very slightly deleterious mutations and the molecular clock.

The model of very slightly deleterious mutations was examined from the standpoint of population genetics in relation to the molecular evolutionary clock. The distribution of selection coefficients of mutants (in terms of amino acid changes) with small effect is thought to be continuous around zero, with an average negative value. The variance of selection coefficients depends upon environmental diversity and hence on total population size of a species. By considering various examples of amino acid substitutions, the average and standard error of selection coefficients and the reciprocal of population size are assumed to have similar values. The model predicts negative correlation between evolutionary rate and population size. This effect is expected to be partially cancelled with the generation time effect of intrinsic mutation rate. Implications of this prediction on the molecular evolutionary clock are discussed.