Plants endemic to areas covered by ice sheets during the last glaciation represent model examples of rapid speciation in changing environments. Kolář et al. compare karyological and ecological variation of the Galium pusillum aggregate (Rubiaceae) in deglaciated northern and unglaciated central Europe. They find substantial differences in monoploid genome size between species from both geographic regions, suggesting independent evolutionary histories. The species’ integrity is most likely maintained by their spatio-ecological segregation. Among the northern endemics, the G. pusillum agg. is an unusual geographically vicariant and ecologically distinct diploid/tetraploid species complex, which enhances our understanding of the evolutionary patterns and dynamics in once-glaciated areas.