Patterns of neutral diversity under general models of selective sweeps

TL;DR

This paper develops a general coalescent model for recurrent selective sweeps, including partial sweeps, revealing how they influence neutral genetic diversity and the frequency spectrum.

Contribution

It introduces a flexible model that extends the full sweep paradigm to partial sweeps, showing the initial rapid increase impacts genealogy more than the final allele fate.

Findings

Recurrent partial sweeps can significantly reduce diversity without altering the frequency spectrum.

The impact of low-frequency sweeps on diversity is similar to the neutral model under high sweep rates.

Only the initial phase of allele increase affects genealogies at linked neutral sites.

Abstract

Two major sources of stochasticity in the dynamics of neutral alleles result from resampling of finite populations (genetic drift) and the random genetic background of nearby selected alleles on which the neutral alleles are found (linked selection). There is now good evidence that linked selection plays an important role in shaping polymorphism levels in a number of species. One of the best investigated models of linked selection is the recurrent full sweep model, in which newly arisen selected alleles fix rapidly. However, the bulk of selected alleles that sweep into the population may not be destined for rapid fixation. Here we develop a general model of recurrent selective sweeps in a coalescent framework, one that generalizes the recurrent full sweep model to the case where selected alleles do not sweep to fixation. We show that in a large population, only the initial rapid increase of a selected allele affects the genealogy at partially linked sites, which under fairly general assumptions are unaffected by the subsequent fate of the selected allele. We also apply the theory to a simple model to investigate the impact of recurrent partial sweeps on levels of neutral diversity, and find that for a given reduction in diversity, the impact of recurrent partial sweeps on the frequency spectrum at neutral sites is determined primarily by the frequencies achieved by the selected alleles. Consequently, recurrent sweeps of selected alleles to low frequencies can have a profound effect on levels of diversity but can leave the frequency spectrum relatively unperturbed. In fact, the limiting coalescent model under a high rate of sweeps to low frequency is identical to the standard neutral model. The general model of selective sweeps we describe goes some way towards providing a more flexible framework to describe genomic patterns of diversity than is currently available.