The Arctic - an evolutionary freezer?

Traditionally, the Arctic has been viewed as an 'evolutionary freezer' with low evolutionary rates caused by extreme environmental constraints and depauperated by repeated glaciations. This view has been modified in recent years. Here we summarise recent studies demonstrating a very high diversity relative to the age of the arctic ecosystem (2-3 million years). Arctic species' diversity is underestimated in the sense that most arctic plants are polyploids, combining the genomes of several ancestral species and that they may contain considerable hidden diversity as cryptic species. There are now many examples of arctic species that originated in the Pleistocene, via divergent evolution or successively higher-level polyploidizations, following hybridization. Apart from genetic diversity associated with polyploid origin, many arctic plants show considerable variation both within and among populations. Genetic diversity varies with breeding system, frequency of inter-and intraspecific hybridization, and history in terms of refugial isolation and range shifts. Many cold-adapted species probably proliferated in vast tundra populations during most of the Pleistocene, followed by broad-fronted postglacial recolonisation resulting in little loss of genetic diversity. Migration at the beginning and end of warm and cold periods is likely to be an important driver of diversification. Molecular data have now provided ample evidence for extreme long-distant colonisation ability of many arctic plants. Local variants may thus spread over vast distances and create multiple hotspots for evolutionary novelty, and it is possible that even small differences in immigration history may have a considerable effect on the evolutionary emergence of diversity.