Genetic hitchhiking is the process by which an evolutionarily neutral or in some cases deleterious allele or mutation may spread through the gene pool by virtue of being linked to a beneficial mutation. More generally, genetic hitchhiking can refer to the process by which a gene's frequency changes due to selection operating upon linked genes. Proximity on a chromosome may allow genes to be dragged through the selection process due to an advantageous gene nearby. The most often cited example of this is one of a mutator which increases the general 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 at fixation in the population - that is that all individuals possess this allele in the homozygous state. 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 be dragged through into the general population. This process only works when M is very close to the allele it has mutated. 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 the two alleles are generally directly next to each other, or present in asexual species where recombination cannot disrupt linkage.
The M allele will generally have this benefit in species under strong selection. The large number of deleterious mutations produced would have too much of an effect on any species in evolutionary equilibrium, and so the M allele would generally not succeed. In a pressured species, however, the huge potential advantage of a mutated A allele may outweigh the effect of several deleterious mutations. eg. Bacteria evolving resistance to antibiotics - no matter how many deleterious mutations are created, when defence against antibiotics is developed that individual will see huge success and indeed all of the population will probably soon be the offspring of that individual. It is important to remember here however that much as we often describe genes as "wanting to spread", the mutator allele isn't predictively causing mutations due to some knowledge that the potential benefits it could create will outweigh any problems - the mutator simply ends up being selected for.
- Maynard Smith, J. and Haigh, J. 1974. The hitch-hiking effect of a favourable gene. Genetical Research 23, 23-35.