polygenic adaptation
Novembre J & Han E 2012 Human population structure and the adaptive response to pathogen-induced selection pressures. Phil Trans R Soc Lond B 367:878-886.
- selection from standing variation would appear to be the rule in the response to AIDS
- the history of that standing variation has been an interesting question
- prior to the onset of AIDS, the Δ32 allele may have conferred a selective advantage to an ancestral pathogen
- it may have undergone a partial sweep due to resistance to an ancestral pathogen and now is getting further swept due to its protective role in responding to a novel pathogen
- the selective agent changes but the increase in allele frequency keeps moving onward
- one relatively novel selection signature method that may work well for discovering pathogen-resistance alleles is the approach of correlating allele frequencies with environmental variables that are indicative of selection pressure
- this method does not rely directly on the expectations of a hard sweep model and so may have more power in soft sweep scenarios
- an important feature of the methods they applied was that they account for background demography
- one aspect of human population genetics that we have not described in this review is the potential impact of allele surfing [69–74], which is the outcome of serial founder effects and can be misleading for selection scans as it can make neutral variants appear as if they have swept to high frequency
- during the initial spread of an allele that exhibits overdominance (a mechanism of balancing selection) there may be sweep-like signatures that can be detected
- distinguishing directional from balancing selection may be difficult
- an appreciation for the impact of soft sweeps and polygenic adaptation is relatively new in human evolutionary genetics
- researchers will inevitably step up to the challenge of finding methods that are more sensitive for detecting regions of the genome that have experienced such soft sweeps
