For many species, both local abundance and regional occupancy are highest near the center of their geographic distributions. One hypothesis for this pattern is that niche suitability declines with increasing distance from a species geographic center, such that populations near range margins are characterized by reduced density and increased patchiness. In these smaller edge populations, genetic drift is more powerful, leading to the loss of genetic diversity. This simple verbal model has been formalized as the central-marginal hypothesis, which predicts that core populations should have greater genetic diversity than edge populations. However, demographic shifts over time can generate a similar pattern. For example, in species with expanding ranges, populations at the range edge experience serial founder effects, creating a gradient of declining genetic diversity from the range core to edge. Testing the central-marginal hypothesis properly thus requires us to consider the confounding role of historical demography. Here, we account for the role of history in testing the central-marginal hypothesis using a genomic dataset of 25 species-level taxa of Australian skink lizards (genus: Ctenotus and Lerista). We found support for the central-marginal hypothesis in 16 of our 25 taxa, of which eight taxa recovered significant support. Unexpectedly, species with the strongest evidence for range expansion were the least likely to follow predictions of the central-marginal hypothesis. The majority of these species had range expansions that originated at the range edge, which led to lower genetic diversity at the range edge compared to the core, contrary to the central-marginal hypothesis.

In 1859, Charles Darwin proposed that species are not fundamentally different from subspecies or the varieties from which they evolve. A century later, Dobzhansky (1958) suggested that many such lineages are ephemeral and are likely to revert differentiation through introgression (Fig. 1A); only a few evolve complete reproductive isolation and persist in sympatry. In this issue of Molecular Ecology, Bouzid et al. (2021) show how new analytical methods, when applied to genome data, allow us to more precisely determine whether or not species formation follows the paths outlined by Darwin and Dobzhansky (Fig. 1B). The authors study the diversification of the lizard Sceloporus occidentalis, finding a continuum of genetic interactions between the preservation of genetic identity to genetic merger, analogous to what is exemplified by ring species. In doing so, they teach us two tales on species formation: that lineages are fractal byproducts of evolutionary processes such as genetic drift and selection, and that lineages are often ephemeral and do not always progress into species. Studying ephemeral lineages like those in S. occidentalis allows us to capture divergence at its earliest stages, and potentially to determine the factors that allow lineages to remain distinct despite pervasive gene flow. These lineages thus serve as a natural laboratory to address long standing hypotheses on species formation.