page_title: Selective sweep
section_title: Overview
hierarchical_section_title: Selective sweep / Overview
caption_attribution_description: English: This is a cartoon drawing of a hard selective sweep. It shows the different steps (a beneficial mutation occurs, increases in frequency and fixes in a population) and the effect on nearby genetic variation.
context_page_description: In genetics, a selective sweep is the process through which a new beneficial mutation that increases its frequency and becomes fixed in the population leads to the reduction or elimination of genetic variation among nucleotide sequences that are near the mutation. In selective sweep, positive selection causes the new mutation to reach fixation so quickly that linked alleles can "hitchhike" and also become fixed.
context_section_description: 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 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 John Maynard-Smith and John Haigh in 1974.
  Not all sweeps reduce genetic variation in the same way. Sweeps can be placed into three main categories: 
  The "classic selective sweep" or "hard selective sweep" is expected to occur when beneficial mutations are rare, but once a beneficial mutation has occurred it increases in frequency rapidly, thereby drastically reducing genetic variation in the population.
  A so-called "soft sweep from standing genetic variation" occurs when a previously neutral mutation that was present in a population becomes beneficial because of an environmental change. Such a mutation may be present on several genomic backgrounds so that when it rapidly increases in frequency, it doesn't erase all genetic variation in the population.
  Finally, a "multiple origin soft sweep" occurs when mutations are common (for example in a large population) so that the same or similar beneficial mutations occurs on different genomic backgrounds such that no single genomic background can hitchhike to high frequency.
  Sweeps do not occur when selection simultaneously causes very small shifts in allele frequencies at many loci each with standing variation (polygenic adaptation).
caption: This is a cartoon drawing of a hard selective sweep. It shows the different steps (a beneficial mutation occurs, increases in frequency and fixes in a population) and the effect on nearby genetic variation.
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