The McDonald-Kreitman test is an important tool in molecular evolutionary analys

The test compares synonymous and nonsynonymous sites in gene-coding regions. If the nonsynonymous : synonymous substitution ratio within species is the same as that between species, then the substitutions are likely to be neutral. If the ratios differ, natural selection is likely to have been responsible by favoring certain substitutions over others. Insights the test provides include identifying genes on which natural selection may be acting for further study, and testing for neutrality for population genetic analyses that assume or require neutrality.