Genetic hitchhiking

The term hitchhiking was coined in 1974 by Maynard Smith and John Haigh.[1] Subsequently the phenomenon was studied by John H. Gillespie and others.[4]

Hitchhiking occurs when a polymorphism is in linkage disequilibrium with a second locus that is undergoing a selective sweep. The allele that is linked to the adaptation will increase in frequency, in some cases until it becomes fixed in the population. The other allele, which is linked to the non-advantageous version, will decrease in frequency, in some cases until extinction.[5][6] Overall, hitchhiking reduces the amount of genetic variation. A hitchhiker mutation (or passenger mutation in cancer biology) may itself be neutral, advantageous, or deleterious.[7]

Recombination can interrupt the process of genetic hitchhiking, ending it before the hitchhiking neutral or deleterious allele becomes fixed or goes extinct.[6] The closer a hitchhiking polymorphism is to the gene under selection, the less opportunity there is for recombination to occur. This leads to a reduction in genetic variation near a selective sweep that is closer to the selected site.[8] This pattern is useful for using population data to detect selective sweeps, and hence to detect which genes have been under very recent selection.

Both genetic drift and genetic draft are random evolutionary processes, i.e. they act stochastically and in a way that is not correlated with selection at the gene in question. Drift is the change in the frequency of an allele in a population due to random sampling in each generation.[9] Draft is the change in the frequency of an allele due to the randomness of what other non-neutral alleles it happens to be found in association with.

Assuming genetic drift is the only evolutionary force acting on an allele, after one generation in many replicated idealised populations each of size N, each starting with allele frequencies of p and q, the newly added variance in allele frequency across those populations (i.e. the degree of randomness of the outcome) is ${\displaystyle {\frac {pq}{2N}}}$.[3] This equation shows that the effect of genetic drift is heavily dependent on population size, defined as the actual number of individuals in an idealised population. Genetic draft results in similar behavior to the equation above, but with an effective population size that may have no relationship to the actual number of individuals in the population.[3] Instead, the effective population size may depend on factors such as the recombination rate and the frequency and strength of beneficial mutations. The increase in variance between replicate populations due to drift is independent, whereas with draft it is autocorrelated, i.e. if an allele frequency goes up because of genetic drift, that contains no information about the next generation, whereas if it goes up because of genetic draft, it is more likely to go up than down in the next generation.[9] Genetic draft generates a different allele frequency spectrum to genetic drift.[10]