Genetic hitchhiking

Genetic hitchhiking, also called genetic draft or the hitchhiking effect, is when an allele changes frequency not because it itself is under natural selection, but because it is near another gene that is undergoing a selective sweep and that is on the same DNA chain. 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. Selective sweeps happen when newly appeared (and hence still rare) mutations are advantageous and increase in frequency. Neutral or even slightly deleterious alleles that happen to be close by on the chromosome 'hitchhike' along with the sweep. In contrast, effects on a neutral locus due to linkage disequilibrium with newly appeared deleterious mutations are called background selection. Both genetic hitchhiking and background selection are (random) evolutionary forces, like genetic drift.

Although the term hitchhiking was coined in 1974 by Maynard Smith and Haigh, the phenomenon it refers to remained little studied until the work of John H. Gillespie in 2000.

Hitchhiking occurs when a neutral polymorphism is in linkage disequilibrium with a second locus that is undergoing a selective sweep. The neutral allele that is linked to the adaptation will increase in frequency, in some cases until it becomes fixed in the population. The other neutral allele, which is linked to the non-advantageous version, will decrease in frequency, in some cases until extinction. Overall, hitchhiking reduces the amount of genetic variation.

Deleterious "passenger" mutations can also hitchhike, not just neutral mutations.

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