Soft selective sweep

[9] The main difference between soft and hard selective sweeps lies in the expected number of different haplotypes carrying the beneficial mutation or mutations, and therefore in the expected number of haplotypes that hitchhike to considerable frequency during the selective sweep, and which remain in the population at the time of fixation.

[2] If hard sweeps facilitate evolutionary rescue, then just a single ancestor is responsible for the spread of the advantageous variants and so genetic diversity will be removed from the population as a consequence of adaptation as well as demographic decline.

On the other hand, a soft sweep, in which the beneficial allele is independently derived in multiple ancestors, will keep certain ancestral diversity that existed prior to the environmental shift that initiated the fitness changes.

[13][14] For a strong interpretation of selection versus neutrality, we need a test statistic with reliably high power for hard and soft sweeps.

Some studies and tests based on a combination of summary statistics have been developed by Peter, Huerta-Sanchez & Nielsen (2012)[13] and by Schrider & Kern (2016).

In addition, well-defined practical instances typically rely on other indications, go with footprint:[18] e.g., a source population is recognized with the selected allele in the SGV (e.g., marine and freshwater sticklebacks,[19] or identified and very recent selection pressure does not leave enough time for the allele to increase from a single copy to the frequency observed today (for example CCR5 adaptation to HIV in humans).