Despite the pervasiveness of the world’s biodiversity, no single species has a truly global distribution. In fact, most species have very restricted distributions. What limits species from expanding beyond their current geographic ranges? This has been classically treated by ecologists as an ecological problem and by evolutionary biologists as an evolutionary problem. Such a dichotomy is false—the problem of species’ ranges sits firmly within the realm of evolutionary ecology. In support of this view, Polechová presents new theory that explains species’ range limits with reference to two key factors central to both ecological and evolutionary theory—migration and population size. This new model sets the scene for empirical tests of range limit theory and builds the case for assisted gene flow as a key management tool for threatened species.