Coalescence, genetic diversity in sexual populations under selection

TL;DR

This paper develops a scaling analysis to understand how linked selection influences genetic diversity in sexual populations, introducing the concept of a genomic linkage block as the key unit of selection.

Contribution

It presents a novel linkage block model that accurately describes genetic diversity and coalescence under various selection regimes, bridging asexual and sexual population dynamics.

Findings

A linkage block length is identified where recombination balances fitness differences.

A simple asexual model with tuned selection strength effectively predicts diversity patterns.

The model accurately describes the infinitesimal limit with many weakly selected loci.

Abstract

In sexual populations, selection operates neither on the whole genome, which is repeatedly taken apart and reassembled by recombination, nor on individual alleles that are tightly linked to the chromosomal neighborhood. The resulting interference between linked alleles reduces the efficiency of selection and distorts patterns of genetic diversity. Inference of evolutionary history from diversity shaped by linked selection requires an understanding of these patterns. Here, we present a simple but powerful scaling analysis identifying the unit of selection as the genomic "linkage block" with a characteristic length determined in a self-consistent manner by the condition that the rate of recombination within the block is comparable to the fitness differences between different alleles of the block. We find that an asexual model with the strength of selection tuned to that of the linkage block provides an excellent description of genetic diversity and the site frequency spectra when compared to computer simulations. This linkage block approximation is accurate for the entire spectrum of strength of selection and is particularly powerful in scenarios with many weakly selected loci. The latter limit allows us to characterize coalescence, genetic diversity, and the speed of adaptation in the infinitesimal model of quantitative genetics.