Selective sweep

A selective sweep is the reduction or elimination of variation among the nucleotides in neighbouring DNA of a mutation as the result of recent and strong natural selection.

A selective sweep can occur when a new mutation occurs in a gene that increases the fitness of the carrier relative to other members of the population. Natural selection will favour individuals that have a higher fitness and with time the newly mutated variant of the gene will increase in frequency relative to other variants (alleles) of the gene. As its prevalence increases, the neutral and even slightly deleterious genetic variation linked to the new mutation will also become more prevalent. This phenomenon is called genetic hitchhiking. A strong selective sweep results in a region of the genome where the positively selected haplotype (the mutated allele and its neighbours) are essentially the only ones that exist in the population, resulting in a large reduction of the total genetic variation in that chromosome region.

Whether a selective sweep has occurred or not can be investigated by measuring 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.

A study of genetic variation among 269 humans found evidence for selective sweeps on chromosomes 1, 2, 4, 8, 12, and 22.

In maize, a recent comparison of yellow and white corn genotypes surrounding Y1 - the phytoene synthetase gene responsible for the yellow endosperm color, shows strong evidence for a selective sweep in yellow germplasm reducing diversity at this locus and linkage disequilibrium in surrounding regions. White maize lines had increased diversity and no evidence of linkage disequilibrium associated with a selective sweep.