Evolution of eastern Asian and eastern North American disjunct distributions in flowering plants

The disjunct distributions of morphologically similar plants between eastern Asia and eastern North America have fascinated botanists and biogeographers since the Linnaean era. This biogeographic pattern is currently recognized by the disjunct distributions of some species, approximately 65 genera, and a few closely related genera in these two widely separated areas. Early workers treated many disjuncts as conspecific, but most were later recognized as intercontinental species pairs. Recent phylogenetic studies confirm affinities between many of the disjunct taxa but also indicate that the disjunct pairs of species are rarely each other's closest relatives. Instead, a pattern of further diversification of species on one or both continents is commonly found. Phylogenetic, molecular, geologic, and fossil data all support the hypothesis that the eastern Asian and eastern North American disjunct distributions are relicts of the maximum development of temperate forests in the northern hemisphere during the Tertiary. Fossil and geologic evidence supports multiple origins of this pattern in the Tertiary, with both the North Atlantic and the Bering land bridges involved. In many genera of flowering plants, current estimates of divergence times using molecular and fossil data suggest that the disjunct patterns were established during the Miocene. Morphological stasis, evidenced by the minimal morphological divergence of species after a long time of separation, must have occurred in some of the disjunct groups in the north temperate zone.