When one gene goes through a selective sweep, any other nearby polymorphisms that are in linkage disequilibrium will tend to change their allele frequencies too.
[2] Selective sweeps happen when newly appeared (and hence still rare) mutations are advantageous and increase in frequency.
In contrast, effects on a neutral locus due to linkage disequilibrium with newly appeared deleterious mutations are called background selection.
Hitchhiking occurs when a polymorphism is in linkage disequilibrium with a second locus that is undergoing a selective sweep.
[3] Instead, the effective population size may depend on factors such as the recombination rate and the frequency and strength of beneficial mutations.
[10] The Y chromosome does not undergo recombination, making it particularly prone to the fixation of deleterious mutations via hitchhiking.
A greater distance would increase the chance of recombination separating M from A*, leaving 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.
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.