Diversification in North American arid lands: Niche conservatism, divergence and expansion of habitat explain speciation in the genus Ephedra

Abstract

A lineage of 12 arid land shrubby species in the gymnosperm genus Ephedra (Gnetales) from North America is used to evaluate the influence of climate on speciation. With a long evolutionary history, and a well documented fossil record this lineage is an ideal model for understanding the process of speciation under a niche conservatism scenario. Using seven DNA molecular markers, Bayesian inference is carried out to uncover sister species and to estimate time of divergence of the lineages. Ecological niche models are generated for four parapatric and sympatric sister species and two analyses of niche evolution are performed, one based on ecological niche models and another using raw data and multivariate analysis. As previous analyses suggest, the diversification of North America Ephedra species may be the result of a recent secondary radiation. Both parapatric and sympatric species diverged mostly in a scenario of climatic niche conservatism. However, we also found strong evidence for niche divergence for one of the sister species pairs (E. californica–E. trifurca). Moreover, the multivariate analysis found environmental differences for some variables between sister species. The estimated divergence time of three pairs of sister species distributed in southwestern North America (E. cutleri–E. aspera, E. californica–E. trifurca and E. torreyana–E. viridis) is inferred to have occurred in the Late Miocene to Pliocene and for the sister species pair E. antisyphilitica–E. coryi distributed in the southern United States and northeastern Mexico, it was inferred from the Pliocene to Pleistocene. The orogenetic and climatic changes documented for these regions related to expansion of arid lands, may have contributed to the diversification in North American Ephedra, rather than adaptations to new climatic conditions.