Abstract
Despite the homogenizing effect of strong gene flow between two populations, adaptation under symmetric divergent selection pressures results in partial reproductive isolation: adaptive substitutions act as local barriers to gene flow, and if divergent selection continues unimpeded, this will result in complete reproductive isolation of the two populations, i.e. speciation. However, a key issue in framing the process of speciation as a tension between local adaptation and the homogenizing force of gene flow is that the mutation process is blind to changes in the environment and therefore tends to limit adaptation. Here we investigate how globally beneficial mutations (GBMs) affect divergent local adaptation and reproductive isolation. When phenotypic divergence is finite, we show that the presence of GBMs limits local adaptation, generating a persistent genetic load at the loci that contribute to the trait under divergent selection and reducing genome-wide divergence. Furthermore, we show that while GBMs cannot prohibit the process of continuous differentiation, they induce a substantial delay in the genome-wide shutdown of gene flow.This article is part of the theme issue ‘Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers’.
Highlights
Felsenstein [1] pointed out that by any measure there are many more niches than species and demonstrated that gene flow is a strong homogenizing force that will tend to prevent populations adapting to different niches
We study the impact of global selection by adding globally beneficial mutations (GBMs) to a multilocus model of divergent local adaptation similar to that studied by Yeaman & Whitlock [5] and Rafajlović et al [3]
We ask how GBMs affect genome-wide divergence measured in terms of the distribution of between-population coalescence times and consider their influence on clustering of locally beneficial mutations (LBMs)
Summary
Felsenstein [1] pointed out that by any measure there are many more niches than species and demonstrated that gene flow is a strong homogenizing force that will tend to prevent populations adapting to different niches. Other simulation studies of a pair of populations connected by gene flow have taken on the quantitative genetics point of view by modelling an explicit phenotype and studying local adaptation to a new set of fixed local optima [3,5] While these sweep-based and trait-based studies of divergent selection make different assumptions about the genetic basis of local adaptation (an eternal stream of local sweeps versus adaptation to a fixed set of local phenotypic optima), they share an important feature: only locally beneficial mutations (LBMs) that affect the trait(s) under divergent selection are considered. We extend the existing simulation frameworks of [2,3,5] in which divergence evolves under a constant high rate of mutational influx In these models, local adaptation involves many loci and the dynamics resulting from the selective interference of LBMs and GBMs cannot be captured by the analytic results that are available for the simpler case of a single introgressing locus. We (i) ask to what extent adaptation to locally divergent trait optima is impeded by selective sweep interference from GBMs, (ii) assess how the effect of GBMs depends on the assumptions about local adaptation (trait-based versus sweep- 2 based models) and (iii) consider how GBMs influence the evolution of reproductive isolation
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