Allopatric Speciation Drives Diversification of Ecological Specialists on Sandhills

Premise of research.The evolution of lineages on continental edaphic islands has been an important determinant of diversification. To understand the complexities of continental edaphic diversification, the historical, biogeographic, and ecological aspects of the diversification process must be studied, even when the relative contributions of ecological and nonecological processes are difficult to differentiate. To decipher the ecological determinants of diversification on continental edaphic islands, we examined the diversification of the plant genus Paronychia (Caryophyllaceae) that occurs on sandhills across the three ecological axes that follow: habitat, soil chemistry, and bioclimate.Methodology.A phylogenetic hypothesis of Paronychia was reconstructed, and ecological variables were measured in natural populations to determine the key axes of diversification. We tested an allopatric model that made predictions based on new habitat formation as sea levels lowered and sand ridges formed, as well as an ecological divergence hypothesis. Ecological variation was summarized with principal component analyses and subjected to several phylogenetic diversification models. Ecological niche modeling approaches were applied to determine whether species were ecologically diversifying across a bioclimate axis.Pivotal results.Although Paronychia is ecologically conservative in occurring on sandy, dry, and open soils, we identified substantial ecological variation, in which individuals occupied greater ecological space within rather than between species. Despite substantial variation, species had not partitioned into unique ecological space. Instead, we found that allopatric speciation based on the geological appearance of habitats was supported and is a key determinant of diversity.Conclusions.Our findings highlight the role of phylogenetic niche conservatism and allopatric speciation for similar, yet disjunct, habitats as species diversified across continental edaphic islands. Despite species occupying similar niche space, our results suggest that niche envelopes can be quite broad and that diversity is dependent on the extent of habitat isolation.