Evolution of Eastern Asian–Eastern North American Biogeographic Disjunctions: A Few Additional Issues

Abstract

The reality of the eastern Asian–eastern North American disjunct biogeographic pattern is supported by (1) the occurrence of closely related species in many plant genera in these two widely distributed areas, (2) the high level of morphological similarities of species within these disjunct genera, and (3) the degree of morphological similarity between some corresponding species in more widely distributed genera of the Northern Hemisphere. Most disjunct genera are supported to be monophyletic, although some are found to be paraphyletic and, less commonly, polyphyletic. The disjunct pattern shown by many genera largely represents relict distributions of the Tertiary and is the product of complex processes such as migration/dispersal, extinction, speciation, vicariance, and stasis. The retention of morphological similarities among the many corresponding disjunct taxa may have arisen by morphological stasis and/or low rates of morphological evolution. Furthermore, accelerated rates of morphological evolution may have occurred in close relatives living in subtropical/tropical regions as well as in western North America. This may have highlighted plesiomorphic morphological similarities between eastern Asia and eastern North America, making them more detectable and more heavily emphasized by systematists in the past. Molecular and phylogenetic results support the importance of the Middle to Late Tertiary in the formation of the disjunct pattern for many temperate taxa. There is phylogenetic and molecular evidence that thermophilic and/or evergreen groups may have diverged earlier, corresponding to the Early Eocene pattern that has been inferred based on fossil evidence.