Western Eurasian-western North American disjunct plant taxa: The dry-adapted ends of formerly widespread north temperate mesic lineages-and examples of long-distance dispersal

By reviewing 25 cases of western Eurasian western North American disjunct flowering plant taxa, we sought to improve understanding of the origin of this type of biogeographic pattern. In nine of the groups studied, phylogenetic and other evidence (often circumstantial) was found for parallel evolutionary shifts of widespread Northern Hemisphere lineages into dry environments in western Eurasia and western North America (Asteraceae-Cichorieae, Chenopodiaceae-Betoideae, Descurainia, Ericaceae-Arbutoideae, Papaver, Platanus, Antirrhineae-Maurandya group, Anemone, Styrax). Lotus s.l. also represents independent evolution of dryland taxa but is polyphyletic. Six taxa (Daucus, Erodium, Oligomeris, Plantago, Polycarpon Senecio) and probably four others (Chenopodiaceae-Camphorosmeae, California, Antirrhineae-Gambelia group, Antirrhineae-Antirrhinum group) are likely examples of long-distance dispersal between western Eurasia and western North America. In Asteraceae-Gnaphalieae either parallel evolution or long-distance dispersal might explain the disjunction. Migration across a Beringian or North Atlantic Land Bridge appears a possible explanation for only three of the 25 disjunctions (Datisca, Cicendia, Zeltnera/Exaculum/Schenkia) based on taxon ecology and divergence times. The disjunction in Cercis remains unexplained. In short, independent ecological shifts to dry environments and long-distance dispersal probably account for the majority of taxon-disjunctions studied; migration of lineages (pre)adapted to dry conditions across either the Beringian or the North Atlantic Land Bridges, much discussed in the literature, only rarely needs to be invoked. Based on the similar times of origin of disjunctions between western Eurasian and western North American taxa and those between East Asian and eastern North American plants, we conclude that these two patterns are best regarded as parts of an ecological, geographical, and temporal continuum.