Женетикалық серуендеу және таңдаудың салдары

Incidental selection of non harmful genes which are close to a beneficial gene on the same DNA chain

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 stochastic (random) evolutionary forces, like genetic drift.

The term hitchhiking was coined in 1974 by Maynard Smith and John Haigh.

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. Overall, hitchhiking reduces the amount of genetic variation. A hitchhiker mutation (or passenger mutation in cancer biology) may itself be neutral, advantageous, or deleterious. Recombination can interrupt the process of genetic hitchhiking, ending it before the hitchhiking neutral or deleterious allele becomes fixed or goes extinct. 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. 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 .

The Y chromosome does not undergo recombination, making it particularly prone to the fixation of deleterious mutations via hitchhiking. This has been proposed as an explanation as to why there are so few functional genes on the Y chromosome.

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.