Echoes of the whispering land: interacting roles of vicariance and selection in shaping the evolutionary divergence of two Calceolaria (Calceolariaceae) species from Patagonia and Malvinas/Falkland Islands

A key to understanding the origin and identity of young species lays on the knowledge of the Quaternary climatic oscillations' effect on gene flow and vicariance. Even though the effect of climatic fluctuations is relatively well understood for southern hemisphere plant species, little is known about their effect on the evolutionary histories of species from mainland and islands. Thus, we investigated whether Quaternary climate-driven fluctuations translated into lineage divergence and speciation, followed or not by climatic niche differentiation, in two allopatric plant species, Calceolaria uniflora and C. fothergillii from Patagonia and Malvinas/Falkland islands, respectively. We sampled the range of both species, and sequenced two chloroplastic (cpDNA; trnS-trnG and trnH-psbA), and one single copy anonymous non-coding nuclear region (nDNA). We performed phylogeographic and dating analyses, and adjusted spatio-temporal diffusion models. We complemented molecular evidence with climatic niche differentiation analyses and species paleo-distribution projections. A species coalescent reconstruction based on multi-locus data retrieved both species as monophyletic. Estimates from cpDNA indicated the species diverged during the Great Patagonian Glaciation. Chloroplast and nuclear DNA showed east-west distribution of the main genetic groups but with contrasting spatial genetic diversity. The spatio-temporal diffusion analyses showed that between 1-0.8 Mya and 570 Kya the lineage leading to C. fothergillii diverged from C. uniflora and arrived to the islands. Climatic niche projections hindcasted range expansions during glaciations, and contractions during the interglacial periods. Comparisons of climatic niches between the two study species indicated that temperature variables show evidence of niche conservatism while precipitation regimes supported niche divergence, even when considering the background environmental divergence. Our study indicates that glacial fluctuations affected the mainland/islands connections favouring speciation mediated not only by isolation, but also by climatic niche differentiation.