Geraldine A. Allen, Kendrick L. Marr, Laurie J. McCormick and Richard J. Hebda
Northern Hemisphere, especially North America.
We sampled Sibbaldia from 176 localities, including 168 for S. pro-cumbens. We analysed sequence variation in three plastid DNA non-coding regions (the atpI–atpH and trnL–trnF intergenic spacers and the trnL intron), performed Bayesian phylogenetic analyses and statistical parsimony analyses on the combined sequences, and analysed the geographical patterns of haplotype distribution and genetic diversity using data from all populations.
Sibbaldia procumbens probably originated in the mountains of South and East Asia. We identified highly distinct clades in Europe and North America, which overlapped on oceanic islands of the North Atlantic indicating long-distance dispersal capability. The North American clade included two lineages, one in California and the other widely distributed across the continent and North Atlantic. Haplotype diversity in the latter lineage was markedly higher to the south, suggesting mid–late Pleistocene southward displacement of North American populations with subsequent migration northwards into previously glaciated regions. In Europe, disjunct geographical regions generally harboured distinct haplotypes.
Multiple Pleistocene refugia for S. procumbens occurred in both North America and Europe. North American refugia existed in California and in the southern Rocky Mountains, but in contrast with most widespread arctic–alpine species we found no evidence for a Beringian refugium. Cryptic refugia may have existed within the Cordilleran Ice Sheet. Episodes of range expansion and contraction and long-distance dispersal have all contributed to the genetic structure and widespread but fragmented distribution of this species.