Reproductive isolation and gene flow vary among contact zones between incipient species

The speciation process separates a single species into multiple lineages of independently evolving taxa, resulting in the diversification of the lineages into new species. However, it is not a unidirectional process that always results in speciation after it begins; secondary contact between partially-isolated lineages can either catalyze an increased rate of diversification via reinforcement or allow the lineages to merge back together into a single species. Studies comparing multiple natural contact zones between incipient species are necessary to understand what factors influence the outcome of secondary contact. In this study, I use the plant Campanula americana to test how consistent the outcome of secondary contact is across the range of incipient species. Campanula americana is divided into an Appalachian and a Western lineage that are separated by reproductive isolation, and are in contact in North Carolina, Pennsylvania, and Virginia. I found that in the North Carolina contact zone, there is low pre- and postzygotic reproductive isolation relative to allopatry, and a high level of gene flow between the lineages. Together, these findings indicate that these incipient lineages are merging together in North Carolina. By contrast, in the Pennsylvania and Virginia contact zones, gene flow between lineages is low. In Pennsylvania, postzygotic reproductive isolation is almost as high as it is in allopatry, and prezygotic isolation is higher than in allopatry, suggesting that reinforcement may be driving increased divergence in Pennsylvania. Together, this work demonstrates that even between the same two lineages, the outcome of secondary contact can vary among contact zones. Divergent outcomes are hypothesized to be driven by differences in initial levels of reproductive isolation between the lineages at time of contact and/or by the geographic structure of the contact zones. Secondary contact is a dynamic process whose outcome can be changed by factors that vary within a species.