The Cytogeography of the Genus Eupatorium (Compositae)—A Review *—

Abstract The evolutionary history of Eupatorium is interpreted on the basis of the cytogeo graphical data from polyploid complexes. In North America, agamospermous polyploid complexes have their centers of distribution in the southeastern United States, Polyploids have a greater geographical range and occur more abundantly than their corresponding diploids. The diploids, which are related to polyploids, are confined either along the Coastal Plain, or in the Ouachita, Ozark and Appalachian Mountains, which are regarded as their refuges during the last lee Age. Speciation in Asian eupatoria has been induced predominantly by geographical isolation at the diploid level in the islands along the continental are. The polyploids of Asian taxa are confined to the rather limited habitats created by recent human activities. Both North American and Japanese eupatoria appear to have diminished their distribution to the southern refuges at some stage during the last Ice Age and since have spread northwards as the climate has improved. During this period, the range extension has been accelerated by human activities, and the agamospermous polyploids have colonized preferentially into the newly available, drier and more open habitats than those which diploids inhabited previously. This range extension has permitted many secondary contacts between previously separated species and has given rise to the new polyploid hybrids. The moist inhabiting species have stayed strictly or largely on the diploid level throughout their widespread geographical range up to the present time.