Strength of natural selection relative to genetic drift as measured particularly in terms of in populations.

The dN/dS ratio is of nonsynonymous to synonymous substitutions. Since this is post fixation, mutations must survive within populations to be counted.

Synonymous substitions are assumed to be silent and therefore neutral alleles. If so, then genetic drift must have been operating for their fixation to have occurred. Nonsynonymous substitutions by contrast are assumed to impact phenotype. These therefore are less likely to be neutral and as a consequence are more likely than for synonymous substitutions to have become fixed within populations due to the action of directional selection.

This ratio serves as an approximation of the impact of selection on the sequence of protein-encoding genes relative to . The greater the ratio, that is, the more nonsynonymous substitutions relative to synonymous substitutions, the greater the impact of natural selection, particularly in terms of directional selection on .

As stabilizing selection has the effect of blocking to populations, the impact of stabilizing selection actually is essentially the opposite to diversifying selection, i.e., reductions in the ratio.