Species with U-shaped geographic ranges as an opportunity to study early stages of divergence: The case of Ipomoea microsepala.

PREMISE: The study of the early stages of divergence between populations is crucial to understanding how species and biodiversity originate. Geographic distributions that capture some aspects of the process of divergence, such as those that are markedly U-shaped, could offer a window to identify systems undergoing incipient speciation, which have traditionally been difficult to identify.

METHODS

We selected Ipomoea microsepala, a species of morning-glory with a markedly U-shaped distribution, as a candidate to evaluate potential incipient evolutionary divergence. We used a combination of principal component, phylogenetic, population, and correlation analyses (Mantel tests) of data derived from leaf morphometrics, climatic information, and genome-wide DNA variation (RADseq) that span the whole species range to assess potential divergence in genomic, climatic, and morphological dimensions.

RESULTS

We showed that Ipomoea microsepala achieved its current geographic distribution through historical range expansion by successively colonizing adjacent areas in a fashion that is consistent with a stepping-stone model and isolation by distance. Phylogenetic and population genetic models supported three present leading fronts through which I. microsepala could continue to expand its range and diverge. Plants in these fronts are already genetically distinct, do not exhibit introgression, and use marginally different climate spaces, but lack differences in leaf morphology.

CONCLUSIONS

A geographic perspective continues to be useful to identify and shed light on the process of divergence. Further ecological experimentation with a genomic perspective is necessary to assess the degree to which the differentiation we documented is congruent with early stages of speciation.