Breaking down barriers in morning glories

Abstract

One of the most striking and consistent results in speciation genomics is the heterogeneous divergence observed across the genomes of closely related species. This pattern was initially attributed to different levels of gene exchange—with divergence preserved at loci generating a barrier to gene flow but homogenized at unlinked neutral loci. Although there is evidence to support this model, it is now recognized that interpreting patterns of divergence across genomes is not so straightforward. One problem is that heterogenous divergence between populations can also be generated by other processes (e.g. recurrent selective sweeps or background selection) without any involvement of differential gene flow. Thus, integrated studies that identify which loci are likely subject to divergent selection are required to shed light on the interplay between selection and gene flow during the early phases of speciation. In this issue of Molecular Ecology, Rifkin et al. (2019) confront this challenge using a pair of sister morning glory species. They wisely design their sampling to take the geographic context of individuals into account, including geographically isolated (allopatric) and co‐occurring (sympatric) populations. This enabled them to show that individuals are phenotypically less differentiated in sympatry. They also found that the loci that resist introgression are enriched for those most differentiated in allopatry and loci that exhibit signals of divergent selection. One great strength of the study is the combination of methods from population genetics and molecular evolution, including the development of a model to simultaneously infer admixture proportions and selfing rates.