Genetic hitchhiking or genetic draft is the process by which an allele may increase in frequency by virtue of being linked to a gene that is positively selected. Proximity on a chromosome may allow genes to be dragged along with a selective sweep experienced by an advantageous gene nearby. More generally, genetic hitchhiking can refer to changes in an allele's frequency due to any form of selection operating upon linked genes, including background selection against deleterious mutations.
Whether a neutral allele becomes fixed is a matter of chance. The traditional view of this stochastic process is that it is dominated by sampling error, that is genetic drift. But it may instead be dominated by whether the allele is linked to a good genetic background: this is known as genetic draft.
Advantage to mutators 
Consider a hypothetical mutator allele that increases the mutation rate in the area around it.
On this chromosome the gene M is a mutator allele, increasing the rate of mutation in the surrounding area. A is an allele which is fixed in the population. Due to the increased mutation rate, the A allele may be mutated into a new, advantageous allele, A*.
The individual in which this chromosome lies will now have a selective advantage over other individuals of this species, so the allele A* will spread through the population by the normal processes of natural selection. M, due to its proximity to A*, will also increase in frequency. This hitchhiking only works when M is very close to the beneficial allele that it has created by mutation. 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, hitchhiking on beneficial mutations is most likely to affect the evolution of mutation rates in asexual species where recombination cannot disrupt linkage.
See also 
- Barton, N H (2000-11-29). "Genetic hitchhiking". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 355 (1403): 1553–1562. doi:10.1098/rstb.2000.0716. PMC 1692896. PMID 11127900. Retrieved 2009-09-23.
- Gillespie, John H. (2001). "Is the population size of a species relevant to its evolution?". Evolution 55 (11): 2161–2169. PMID 11794777.
Further reading 
- Maynard Smith, J. and Haigh, J. 1974. The hitch-hiking effect of a favourable gene. Genetical Research 23, 23-35.