Sweeps can be categorized in 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.