Sampling site distribution and their grouping to predictor variables. Circles represent sampling points. The colour gradient from light grey (extreme climate) to black (average climate) represents environmental marginality (marg) regarding climate variation as inferred from PCA analysis (see Additional File 3). The convex polygon around all sampling points indicates the species range limits considered. Populations grouped to different predictors are indicated by differentially hatched lines. The Holocene expansion area (hol) comprises populations neither situated in the refugial nor in the recent range expansion area.