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Background : The genus Draba (Brassicaceae) has an arctic-alpine and montane distribution, and contains 355 accepted species that encompass a broad range of ploidy levels.

Aims : We aimed (1) to explore the relationship between ploidy level and species richness throughout the geographical distribution of Draba, and (2) to estimate rates of speciation and polyploidisation in the genus.

Methods : Species and their ploidy levels, obtained from Floras, herbaria and checklists, were mapped and analysed with ArcView 9.1. Speciation and polyploidisation rates were calculated.

Results : We found that: (1) areas of high species richness have a high number of polyploid species; (2) diploid and polyploid distributions overlap; (3) aneuploids, and uneven and lower polyploid levels (≤ 6x) are strongly associated with the North American Cordillera; (4) species with high ploidy levels (> 10x) occur mainly in the …