• climate change;
  • colonization;
  • fixed heterozygosity;
  • frequency of polyploidy;
  • gene duplications;
  • genetic drift;
  • glaciation;
  • inbreeding;
  • levels of polyploidy;
  • recurrent formation

The Arctic is an excellent model system for the study of polyploidy. It is one the Earth's most polyploid-rich areas, in particular of high-level and recently evolved polyploids. Here we re-address previous hypotheses on arctic polyploidy based on a new analysis of the circumarctic flora, and review recent molecular, cytological and reproductive studies. The frequency and level of polyploidy strongly increase northwards within the Arctic. We found no clear-cut association between polyploidy and the degree of glaciation for the arctic flora as a whole, which contains many widespread species. However, for ‘arctic specialist’ taxa with restricted distributions, the frequency of diploids is much higher in the Beringian area, which remained largely unglaciated during the last ice age, than in the heavily glaciated Atlantic area. This result supports the hypothesis that polyploids are more successful than diploids in colonizing after deglaciation. There is abundant molecular evidence for recurrent formation of arctic polyploids at different scales in time and space. Examples are given of low-level polyploids formed after the last glaciation and of repeated and successively more high-level polyploidizations throughout the Quaternary. Recurrent polyploid origins, followed by interbreeding within and across ploidal levels, provide a major explanation for the taxonomic complexity of the arctic flora. In the well-studied, recently deglaciated archipelago of Svalbard, most species are mainly self-fertilizing or clonal. All Svalbard polyploids examined so far are genetic allopolyploids with fixed heterozygosity at isozyme loci. The level of heterozygosity in 65 taxa increases dramatically from diploids to high-level polyploids. In the circumarctic area, there is evidence for numerous recently evolved sibling species within diploid taxonomic species. Rapid evolution of crossing barriers at the diploid level promotes further diversification after expansion from different refugia, and may provide new raw materials for allopolyploid formation. We conclude that the evolutionary success of polyploids in the Arctic may be based on their fixed-heterozygous genomes, which buffer against inbreeding and genetic drift through periods of dramatic climate change. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82, 521–536.