Beringia: Intercontinental exchange and diversification of high latitude mammals and their parasites during the Pliocene and Quaternary
Beringia is the region spanning eastern Asia and northwestern North America that remained ice-free during the full glacial events of the Pleistocene. Numerous questions persist regarding the importance of this region in the evolution of northern faunas. Beringia has been implicated as both a high latitude refugium and as the crossroads (Bering Land Bridge) of the northern continents for boreal mammals. The Beringian Coevolution Project (BCP) is an international collaboration that has provided material to assess the pattern and timing of faunal exchange across the crossroads of the northern continents and the potential impact of past climatic events on differentiation. Mammals and associated parasite specimens have been collected and preserved from more than 200 field sites in eastern Russia, Alaska and northwestern Canada since 1999. Previously, fossils and taxonomic comparisons between Asia and North America mammals have shed light on these events. Molecular phylogenetics based on BCP specimens is now being used to trace the history of faunal exchange and diversification. We have found substantial phylogeographic structure in the Arctic and in Beringia in mustelid carnivores, arvicoline rodents, arctic hares and soricine shrews, including spatially concordant clades and contact zones across taxa that correspond to the edges of Beringia. Among the tapeworms of these mammalian hosts, new perspectives on diversity have also been developed. Arostrilepis horrida (Hymenolepididae) was considered to represent a single widespread and morphologically variable species occurring in a diversity of voles and lemmings in eastern and western Beringia and more broadly across the Holarctic region. The BCP has demonstrated a complex of at least 10 species that are poorly differentiated morphologically. The diversity of Paranoplocephala spp. and Anolocephaloides spp. (Anoplocephalidae) in Beringia included relatively few widespread and morphologically variable species in arvicolines. BCP collections have changed this perspective, allowing the recognition of a series of highly endemic species of Paranoplocephala that demonstrate very narrow host specificity, and additional species complexes among arvicolines. Thus, extensive, previously unrecognized, diversity for tapeworms of 2 major families characterizes the Beringian fauna. By elucidating evolutionary relationships and phylogeographic variation among populations, species and assemblages, refined views of the sequence and timing of biotic expansion, geographic colonization and impact of episodic climate change have been developed for Beringia. Ultimately, Beringia was a determining factor in the structure and biogeography of terrestrial faunas across the Nearctic and Neotropical regions during the Pliocene and Quaternary.