Phylogenetic Patterns in Northern Hemisphere Plant Geography

Geological and climatological processes that have impacted the biota of the Northern Hemisphere during the Tertiary are expected to yield little resolution when area cladograms are compared without taking the timing of diversification into account. In an attempt to establish a set of appropriate phylogenetic comparisons, we distinguished between a Pacific track involving (minimally) China, Japan, and eastern North America but not Europe, and an Atlantic track involving China, Europe, and eastern North America but not Japan (or, in most cases, western North America). Within the two Atlantic-track taxa considered here—Liquidambar and Cercis—European and North American species are more closely related to one another than they are to the Asian species. Within a set of five Pacific-track taxa—Hamamelis, Weigela-Diervilla, Triosteum, Buckleya, and Torreya—we see all possible relationships involving China, Japan, and eastern North America. Estimates of minimum divergence times between Old World and New World lineages, based on molecular and fossil evidence, differ markedly between the two Atlantic-track clades. Among the Pacific-track taxa, we find no correlation between pattern of area relationships and estimated divergence times of the Old World–New World disjuncts. Instead, we see a wide range in the timing of these splitting events among and within phylogenetic patterns. Despite the existence of a variety of patterns, inferred ancestral areas and divergence times can be explained by assuming initial diversification within Asia in a number of lineages, followed by iterative transBeringian dispersion and vicariance.