M alone with any deleterious mutations it may have caused. For this reason, evolution of mutators is generally expected to happen largely in asexual species where recombination cannot disrupt linkage disequilibrium. Neutral theory of molecular evolution The neutral theory of molecular evolution assumes that most new mutations are either deleterious (and quickly purged by selection) or else neutral, with very few being adaptive. It also assumes that the behavior of neutral allele frequencies can be described by the mathematics of genetic drift. Genetic hitchhiking has therefore been viewed as a major challenge to neutral theory, and an explanation for why genome-wide versions of the McDonald-Kreitman test appear to indicate a high proportion of mutations becoming fixed for reasons connected to selection. References Population genetics Selection