Glacial survival or tabula rasa?: The history of North Atlantic biota revisited

Up to the 1960s, there was nearly complete consensus that disjunctions and endemism in the North Atlantic cannot be explained without in situ survival during the glaciations (the nunatak hypothesis). The alternative tabula rasa hypothesis of posiglacial immigration was regarded to be of merely historical interest. Herein we review recent geological, molecular, taxonomic, and biogeographic data to re-examine this view. There is now strong geological evidence for some ice-free North Atlantic areas within the maximum limits of the Late Weichselian/Wisconsian ice sheets, but no fossils have been found to prove continuous in situ existence of life in these areas. Molecular data suggest that many plants and animals have migrated recently across the Atlantic, even if they lack mechanisms promoting long-distance dispersal. In other species, there are deep trans-oceanic phylogeographic splits suggesting survival in two or more refugia, but these refugia may have been located outside the ice sheets. For vascular plants, we provide an updated list of 77 north boreal, alpine, and arctic taxa accepted as North Atlantic endemics. The degree of endemism is very low (0.0-1.9% single-region endemism). Forty endemics occur in more than one of the isolated Atlantic regions, indicating extensive migration and complicating inferences on the location of refugia. Thirty-four endemics are probably not hardy enough for nunatak survival and are explained by postglacial immigration (or in situ evolution). Among the 43 hardy endemics, there is not a single outcrossing diploid that could suggest long-term evolution. Most of the hardy endemics are asexual or self-fertilizing polyploids, some of postglacial hybrid origin. Others are preglacial polyploids which immigrated postglacially or survived in situ. Some ice-free areas, such as the extensive Greenlandic ones, may have supported survival of some hardy organisms. The evidence accumulated since the 1960s suggests, however, that endemism and disjunctions in the North Atlantic can be explained without invoking in situ glacial survival.