Hitchhiking and Selective Sweeps

When a favorable allele increases in frequency, it alters the coalescent structure (the pattern of times back to a common ancestor) at linked sites relative to that under drift. This creates patterns of sequence polymorphism than can be used to potentially detect ongoing, or very recent, selection. This idea of a neutral allele hitchhiking up to high frequency when coupled to a favorable allele is the notion of a selective sweep, and this chapter reviews the considerable body of associated population-genetics theory on sweeps. Different types of sweeps leave different signatures, resulting in the very diverse collection of tests of selection discussed in Chapter 9. Either a history of recurrent sweeps, or of background selection, results in linked genomic regions of reduced effective population size. This implies that more mutations in sich regions are efficiently neutral, which can result in increased substitution rates and lower codon bias. Finally, the chapter examines the theory for when response is expected to start from existing variation, as opposed to waiting for the appearance of new mutations.