The genetic architecture of polygenic local adaptation and its role in shaping barriers to gene flow

Abstract

We study the maintenance of polygenic local adaptation and its effects on reproductive isolation in a mainland-island model for populations with a general biphasic life cycle, encompassing haploid and diploid models as special cases. We quantify the strength of a multilocus barrier to gene flow due to divergent local adaptation at L unlinked and weakly selected loci, and obtain predictions for the equilibrium frequencies of locally adaptive alleles with arbitrary dominance and haploid-phase selection, accounting for genetic drift using a diffusion approximation. We extend classical single locus results on the role of dominance in the mainland-island model to the multilocus case, highlighting how linkage disequilibrium in multilocus barriers has rather different effects on differentiation and swamping thresholds for recessive alleles compared to dominant ones. Details about the biphasic life cycle can be captured through a set of effective parameters, and we show that for the same total strength of selection over the life cycle, increasing the relative intensity of selection in the haploid phase leads to stronger barriers to gene flow. We study the effect of heterogenous genetic architectures of local adaptation on the resulting barrier to gene flow, characterizing the realized genetic architecture at migration-selection balance for different distributions of fitness effects. Our results highlight the importance of barrier heterogeneity in shaping observable patterns of differentiation between populations under divergent selection pressures.