Homage to Hutchinson, and the role of ecology in lineage divergence and speciation

Aim: 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. Location: Aridlands of North America including the Sonoran and Chihuahuan deserts and Baja California. Methods: Ecological niche models were constructed for avian sister phylogroups on either side of two biogeographical barriers: the Vizcaino Desert in Baja California (c. 28°-30° N), and the Cochise Filter Barrier, which divides the Sonoran and Chihuahuan deserts. Each phylogroup pair was tested for differences in niche identity and niche divergence (using the background test). Results: Niche identity can be rejected for each phylogroup, although tests of niche divergence were insignificant. Main conclusions: Allopatric sister phylogroups exist in and respond to aspects of different environments, as shown by significant niche identity tests, leading to geographical variation that is often recognized taxonomically, supporting a role for ecology in lineage divergence. However, sister phylogroups also use shared ancestral components of the climatic niche more often than expected by chance, which might prevent selection from yielding divergence in traits related to resource use. Climatic niche divergence does not explain allopatric ranges of phylogroups. Allopatry is maintained because the phylogroups are too similar morphologically, lacking the necessary Hutchinsonian ratios for coexistence. The role of ecology in speciation differs between lineage and biological species concepts owing to their different ranking criteria.