Selective sweeps

In genetics, a selective sweep is the reduction or elimination of variation among the nucleotides near a mutation in DNA. It results from a beneficial allele's having recently reached fixation due to strong positive natural selection.

A selective sweep can occur when a rare or previously non-existing allele that increases the fitness of the carrier (relative to other members of the population) increases rapidly in frequency due to natural selection. As the prevalence of such a beneficial allele increases, genetic variants that happen to be present on the genomic background (the DNA neighborhood) of the beneficial allele will also become more prevalent. This phenomenon is called genetic hitchhiking. A selective sweep due to a strongly selected allele, which arose on a single genomic background therefore results in a region of the genome with a large reduction of genetic variation in that chromosome region. The idea that strong positive selection could reduce nearby genetic variation due to hitchhiking was proposed by Maynard-Smith and Haigh in 1974.

It is now recognized that not all sweeps reduce genetic variation in the same way. Sweeps can be categorized in three main categories.

Whether or not a selective sweep has occurred can be investigated in various ways. One method is to measure linkage disequilibrium, i.e., whether a given haplotype is overrepresented in the population. Under neutral evolution, genetic recombination will result in the reshuffling of the different alleles within a haplotype, and no single haplotype will dominate the population. However, during a selective sweep, selection for a positively selected gene variant will also result in selection of neighbouring alleles and less opportunity for recombination. Therefore, the presence of strong linkage disequilibrium might indicate that there has been a recent selective sweep, and can be used to identify sites recently under selection.

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