Barriers to dispersal and resulting biogeographic boundaries are responsible for much of life's diversity. Distinguishing the contribution of ecological, historical, and stochastic processes to the origin and maintenance of biogeographic boundaries, however, is a longstanding challenge. Taking advantage of newly available data and methods-including environmental niche models and associated comparative metrics-we develop a framework to test two possible ecological explanations for biogeographic boundaries: (1) sharp environmental gradients and (2) ribbons of unsuitable habitat dividing two highly suitable regions. We test each of these hypotheses against the null expectation that environmental variation across a given boundary is no greater than expected by chance. We apply this framework to a pair of Hispaniolan Anolis lizards (A. chlorocyanus and A. coelestinus) distributed on the either side of this island's most important biogeographic boundary. Integrating our results with historical biogeographic analysis, we find that a ribbon of particularly unsuitable habitat is acting to maintain a boundary between species that initially diverged on distinct paleo-islands, which merged to form present-day Hispaniola in the Miocene.