Geographical and ecological allopatry effects on niche change in two sister species pairs of hummingbirds in western North America

Here, we explored how variations in the allopatric speciation scenario, specifically ecological vs. vicariant allopatry, relate to climatic niche change in sister species. We selected two sister species pairs of North American hummingbirds (Calypte anna, C. costae, Basilinna leucotis, B. xantusii) that diverged 2.5 and 3.6 million years ago, under ecological (arid climate tendency during Pliocene) and vicariant (Baja California peninsula separation) allopatric processes, respectively. We constructed the climatic niche of each species using occurrence records and estimated the distance, similarity, and unique fraction of climatic niche between sister species. Calypte species showed moderate niche divergence (Euclidean distance between centroids = 1.94, Sørensen index of similarity = 0.080, unique fraction of hypervolume in C. costae = 0.57, and C. anna = 0.95). However, contrary to expectations, Basilinna species, which diverged under a vicariant scenario, displayed clear niche divergence (Euclidean distance between centroids = 3.78, Sørensen index of similarity = 0.0001, and unique fraction of hypervolume in B. xantusii = 0.98, and B. leucotis = 0.99). We explained dissimilarity in climatic niches between Basilinna species by the ecological divergence between habitats of disjunct populations, which would have also been associated with increased aridity during the Pliocene.