Front Cover

Cover: The cover image shows an Italian barred grass snake (Natrix helvetica sicula; female from the Ferchensee near Mittenwald, Bavaria; photo: M. Franzen). This taxon was only recently discovered in Germany and is restricted to a few river valleys in southernmost Bavaria. In the Holocene, the Italian barred grass snake crossed the Alps, most likely using the Brenner Pass and/or the Reschen Pass, underlining that this mountain range is not an impermeable biogeographic barrier. Asztalos et al. show that N. h. sicula hybridizes locally with the common grass snake (Natrix natrix) at the widening of the valleys into the pre-Alpine plains. In this bimodal hybrid zone, the combination of geographic setting, intrinsic genetic factors and population-density-dependent blocking of immigrants by the resident species (N. natrix) prevents the formation of a geographically more extended hybrid zone. Journal of Zoological Systematics and Evolutionary Research, Volume 59, Issue 5, July 2021, Pages 1136–1148, Marika Asztalos, Frank Glaw, Michael Franzen, Carolin Kindler and Uwe Fritz DOI: https://doi.org/10.1111/jzs.12471.Copyright of picture: © M. Franzen 2021.