Dynamics of drift, gene flow, and selection during speciation in Silene.

The mechanics of speciation with gene flow are still unclear. Disparity among genes in population differentiation (FST) between diverging species is often interpreted as evidence for semipermeable species boundaries, with selection preventing key genes from introgressing despite ongoing gene flow. However, FST can remain high before it reaches equilibrium between the lineage sorting of species divergence and the homogenizing effects of gene flow (via secondary contact). Thus, when interpreting FST, the dynamics of drift, gene flow, and selection need to be taken into account. We illustrate this view with a multigenic analyses of gene flow and selection in three closely related Silene species, S. latifolia, S. dioica, and S. diclinis. We report that although S. diclinis appears to have evolved in allopatry, isolation with (bidirectional) gene flow between S. latifolia and S. dioica is likely, perhaps as a result of parapatric speciation followed by more extensive sympatry. Interestingly, we detected the signatures of apparently independent instances of positive selection at the same locus in S. latifolia and S. dioica. Despite gene flow between the species, the adaptive alleles have not crossed the species boundary, suggesting that this gene has independently undergone species-specific (diversifying or parallel) selection.