Rapid radiation in the barley genus Hordeum (Poaceae) during the Pleistocene in the Americas

Abstract

Evidence was found for a rapid radiation of the grass genus Hordeum in the Americas during the last 2 million years, accumulating 23 species in South and North America, while only 10 Hordeum species occur in other regions of the world. The differences in species richness are caused by distinct evolutionary mechanisms in the Americas and Eurasia, as recovered by the integration of phylogenetic and phylogeographic analyses with modeling of ecological niches. The Eurasian region is mainly characterized by a loss of biodiversity during the Pleistocene glaciations, while vivid speciation took place in the Americas during this time period. Thus, speciation in Eurasia was mainly affected by severe genetic bottlenecks probably due to small populations surviving in ice-age refugia, while such restrictions in New World species groups seem less pronounced. Particularly in southern Patagonia, speciation was due to multiple geographical subdivisions of relatively large populations during the last million years, without measurable reduction of genetic diversity or population sizes. This together with long-distance colonization of remote areas was the main cause of species diversity in the New World.