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.