What stops a species adapting to an ever-wider range of conditions, continuously expanding its geographic range? The biomathematician Jitka Polechová, an Elise Richter Fellow at the University of Vienna, has published a paper in PLoS Biology which explains the formation of species’ range margins. The theory shows that just two compound parameters, important for both ecology and evolution of species, are fundamental to the stability of their range: the environmental heterogeneity and the size of the local population.