Two sides of the same coin? Transient hybridization in refugia and rapid postglacial ecological divergence ensure the evolutionary persistence of sister Nothofagus.

This article investigates the evolutionary persistence of two sister cold-tolerant tree species, *Nothofagus antarctica* and *Nothofagus pumilio*, across Patagonia during glacial cycles. By integrating fossil pollen records of the shared *Nothofagus dombeyi* pollen type and multilocus genetic data (isozymes, microsatellites, nuclear and chloroplast DNA sequences) sampled along five latitudinal bands (35°S–55°S), the study tests the hypothesis that these species survived in multiple glacial refugia where transient hybridization occurred. Results indicate long-term local persistence with high genetic diversity and presence of unique alleles across latitudes, rejecting recolonization from northern refugia alone. Genetic analyses support synchronous divergence during the postglacial period accompanied by historical gene flow (introgression) between species, likely facilitated by overlapping flowering phenologies under colder climates. The authors propose that transient hybridization in refugia, followed by rapid ecological divergence, acted as a dynamic mechanism enabling local survival of these ecologically distinct but interfertile species through climatic oscillations.