Dispersal-vicariance Analyses of Intercontinental Disjuncts: Historical Biogeographical Implications for Angiosperms in the Northern Hemisphere

Ten North Temperate taxa representing diverse angiosperm lineages were analyzed for biogeographic histories using the dispersal-vicariance analysis method to gain insights into the origin and evolution of disjunct distributions in the Northern Hemisphere. Results indicate four general biogeographic patterns: (1) origin and speciation in eastern Asia with subsequent expansion into North America and/or Europe (e.g., Aralia sect. Aralia, Symplocarpus, and possibly Asarum, Aesculus, and Chrysosplenium); (2) origin in eastern Asia and western North America with subsequent spread into eastern North America (e.g., Calycanthus and Boykinia); (3) a disjunct origin in eastern Asia and eastern North America with subsequent dispersal from eastern Asia into eastern North America (e.g., Panax); and (4) a widespread origin in the Northern Hemisphere with subsequent fragmentation by intercontinental vicariance (e.g., Cornus and Trautvetteria). Although there are caveats, the results indicate that the disjunct distributions of angiosperm lineages in the Northern Hemisphere cannot be explained with a simple vicariance model. Most lineages may have been restricted ancestrally to one or two adjacent areas and then secondarily expanded their ranges via dispersal. A noteworthy finding was the one-way intercontinental plant exchange from the Old World to the New World and biased dispersal within each continent. There was more dispersal from the west to the east in North America but more dispersal from the east to the west in Eurasia. Such asymmetrical dispersal has also been documented in animals. The results also indicate that eastern Asia and western North America were the centers of origin for a majority of lineages examined, implying that these two areas were important sources of temperate angiosperm evolution in the Northern Hemisphere. The results further support a complex evolutionary history of angiosperms in the Northern Hemisphere and suggest pseudocongruence among lineages in phylogenetic relationships and distributional patterns.