Heterogeneous evolutionary history defines the rear edge of the North American herb Campanula americana

AimWarmer range limits of species distributions offer a window into the ecological and evolutionary processes associated with long-term warming, as they often harbor relicts of refugial populations from last glaciation. However, warm-edge populations vary in their refugial, evolutionary, and climatic histories, complicating their use as models. Here, we characterize this variation at the warmer range limit of the North American herb Campanula americana. LocationEastern North America TaxonCampanula americana MethodsWe evaluated the warmer range limit using species distribution model hindcasting, range-wide analyses of population structure, effective migration and phylogenetic relationships. ResultsWarm-edge populations vary in their refugial history; most are relicts of historic refugial populations (i.e. rear edge), but some result from southward postglacial colonization. Within the rear edge, populations exhibit pronounced genetic structure, forming highly isolated clusters despite contiguous geography and only modest landscape barriers. Distinct rear-edge clusters also varied in climate history and gene flow among population, with the more southern populations experiencing higher habitat decline, fragmentation and isolation. Finally, only rear-edge populations from northern refugial areas contributed to postglacial range expansion. Main conclusionsOur study highlights the challenges of using warm-edge populations as ecological and evolutionary models of response to climate change. Even in a geographically simple setting, warm-edge populations exhibit refugial, genetic, and climatic heterogeneity. Characterizing this variation is essential for accurately identifying rear-edge populations and inferring their evolutionary history. Knowledge of the contribution of warm-edge populations to range-wide genetic variation is key when considering how these populations may inform studies of adaptation to future climate change.