Weighing the evidence for adaptation at the molecular level.

The abundance of genome polymorphism and divergence data has provided unprecedented insight into how mutation, drift and natural selection shape genome evolution. Application of the McDonald-Kreitman (MK) test to such data indicates a pervasive influence of positive selection, particularly in Drosophila species. However, evidence for positive selection in other species ranging from yeast to humans is often weak or absent. Although evidence for positive selection could be obscured in some species, there is also reason to believe that the frequency of adaptive substitutions could be overestimated as a result of epistatic fitness effects or hitchhiking of deleterious mutations. Based on these considerations it is argued that the common assumption of independence among sites must be relaxed before abandoning the neutral theory of molecular evolution.