A fundamental goal of biogeography is to explain why sister taxa are allopatric. Potential explanations include insufficient time for range expansion, adaptation to allopatrically distributed environments, or ecological similarity that prevents coexistence. Many sister species have been isolated for hundreds of thousands of years, providing ample time to attain sympatry. This focuses attention on the role of ecology. Allopatric phylogroups and phylogenetic sister species often exhibit morphological differences that are apparent responses to differing ecological conditions, revealing a role for ecology in lineage divergence and speciation. However, as Hutchinson proposed five decades ago, a certain level of morphological divergence is typically required before two taxa can partition resources and coexist. This paper uses ecological niche modelling and phylogeography to explore the role of niche divergence in shaping the ranges of sister taxa and in the speciation process.