The McDonald–Kreitman test is a statistical method used in evolutionary biology to assess the role of natural selection versus neutral evolution in shaping genetic variation within a population. Developed by biologists Brian McDonald and David Kreitman in the 1990s, the test compares the ratio of synonymous to nonsynonymous substitutions in a particular gene or set of genes.
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A comparison of nonsynonymous and synonymous polymorphism and divergence. Under a simple neutral model their ratios match. An excess of nonsynonymous divergence can support adaptive fixation, but mildly deleterious polymorphism, changing demography and site-class assumptions affect interpretation.[ref]