Phenotypic differentiation in a common garden reflects the phylogeography of a widespread Alpine plant

1. Glacial history has affected the phylogeographic structure of numerous Alpine plant species, but its impact on phenotypic differentiation has been little studied. Therefore, we asked whether phenotypic differentiation in a common garden reflects the phylogeographic structure of the widespread Alpine plant Geum reptans L. 2. We combined a molecular investigation with a common garden experiment and investigated genets from 16 populations of G. reptans sampled from the European Alps. Using neutral molecular markers (RAPDs) and Bayesian cluster analysis, we analysed the species' genetic differentiation and phylogeographic structure. In the common garden, we measured the differentiation of phenotypic traits related to growth, reproduction and leaf morphology. 3. Molecular analysis partitioned the populations into three genetic groups, indicating pronounced phylogeographic structure. Regional molecular variation was correlated with regional phenotypic differentiation. 4. Quantitative trait differentiation (Q(ST)) differed from neutral molecular differentiation (G(ST)) in 10 of 11 traits, indicating that selection has contributed to phenotypic differentiation. Significant negative correlations between biomass and precipitation records for site of origin are a further indication of adaptation. 5. Synthesis. The current study compared regional molecular variation and phenotypic differentiation among populations of a widespread species in the context of extreme range changes during glaciations in the Alps. Because the common garden phenotypic differentiation of G. reptans reflects its phylogeographic structure, we conclude that glacial history affected both genotypes and phenotypes. The results suggest that the present-day phenotypic differentiation was caused by genetic drift and limited gene flow between populations in glacial refugia and during post-glacial recolonization, as well as by adaptation to current climatic conditions. Our findings are relevant for understanding the adaptive potential of Alpine plants and predicting potential range shifts in response to future climate change.