Ecology predicts parapatric distributions in two closely related Antirrhinum majus subspecies

Using a species distribution model, we reconstructed the environmental niches of Antirrhinum majus pseudomajus and Antirrhinum majus striatum, two closely related species with parapatric distributions. We tested whether retention of ancestral environmental niche (i.e. niche conservatism) or adaptation to different ecological conditions (i.e. niche divergence) could explain the maintenance of their non-overlapping geographic ranges. We found that the environmental niche of A. m. pseudomajus is almost twice as large as that of A. m. striatum, with substantial overlap indicating that A. m. pseudomajus and A. m. striatum should co-occur frequently within the geographic range of A. m. striatum. By analysing contact zones where both subspecies are geographically close, we found that the presence of one subspecies instead of the other was significantly influenced by particular combinations of climatic factors. Since independent genetic evidence indicates that the two subspecies have experienced phases of range overlap at or near contact zones over the course of their evolutionary history, we propose that ecological niche displacement might be an important factor in explaining the absence of current range overlap between A. majus subspecies.