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.
Ring distributions leading to species formation: a global topographic analysis of geographic barriers associated with ring species