Tale 1: The fractal nature of species
The first tale told by S. occidentalis is the fractal nature of lineage divergence. As populations spread across the landscape, they diverge due to adaptation to local conditions and due to genetic drift across distance, geographic barriers, and low-density areas. These two processes reinforce and facilitate each other, resulting in discontinuities in genetic variation perceived as evolutionary lineages. To understand how such evolutionary lineages form, species likeSceloporus occidentalis are ideal, because they are abundant throughout a large range that transverses a wide range of elevations, habitats, and climatic regimes. In this study, Bouzid et al. sample comprehensively throughout the S. occidentalis range. In doing so, they identify two major lineages within the species: one ranges along the coast up to the Sierra Nevada Mountains and the second extends east of the Sierra Nevada mountains into the arid Great Basin region (Fig. 1B). Within these lineages, they further identify genetic discontinuities that are coincident with previously glaciated mountain ranges and with steep ecological gradients in temperature and aridity. Concordance of genetic breaks with climatic and topographic transitions likely results from two, non-mutually exclusive processes. First, as inferred through modelling of historical demography, S. occidentalis occurs in a dynamic landscape that has been reshaped through repeated glacial cycles. As Bouzid et al. hypothesize, populations in the species persisted in isolated patches of habitat, which reflect current ecological boundaries. Second, gene flow across ecological transitions is often reduced (Endler 1977), both in cases of primary and secondary contact. This can lead to population breaks falling across ecological breaks. As recognized by Darwin and Dobzhansky, this tale of Sceloporus occidentalis shows that even cohesive species contain population substructure at multiple hierarchical levels, and that the formation of the genetic discontinuities observed today are a byproduct of drift and selection. Whether that divergence presages the formation of new species remains an open question addressed in Tale 2.