Species Occurrence, Hybridization and Speciation in Postglacial East and South Asia

Abstract

Geological disjunction and the changing climate affect the distribution of plants and animals in East Asia. The species of postglacial East and South Asia have gone through the process of speciation. In the Northern Hemisphere, a disjunction of the distribution of closely related plants between eastern Asia and North America has been reported in many previous studies. These plants include Rhus (Yi et al. 2004), Cornus (Xiang et al. 2005), Berberis (Kim et al. 2004), Chamaecyparis (Wang et al. 2003), Panax (Wen and Zimmer, 1996), and Pontica (Milne 2004), Fraxinus (Jeandroz et al. 1997), etc. The modern distribution of plant species between eastern Asia and North America reflects migration, speciation, and extinction due to climatic exchange in past glaciating time periods (Milne 2004). Numerous case studies pertaining to eastern Asia and eastern North America have helped to reconstruct the relationships among morphologically similar species to explain the differences in species diversity among areas with similar environmental conditions (Wen et al., 1996, 1998; Wen, 1999, 2000, 2001; Qiu et al., 1995a, b; Soltis 2001). During the glacial and postglacial periods, climate and sea level changes affected the migration and distribution of plants and animals, which caused landbridge formations and a breakdown between islands and continents. The different species showed variant patterns of phytogeography in postglacial East and South Asia. For example, the Cycas revoluta, the insular species distributed from Kyushu to South Ryukyu, and C. taitungensis, the insular species restricted on South Taiwan, were allopatrically distributed but paraphyly due to ancient ancestral polymorphisms (Chiang et al., 2009). Likewise, Amentotaxus is restricted to Taiwan, to the west across southern China, to Assam in the eastern Himalaya and to the south in Vietnam. A. argotaenia, A. formosana, A. poilanei and A. yunnanensis are considered complex species and a result of speciation (Ge et al., 2005). In addition, most plant species in the Western Pacific island chain, including the islands from Ilyushin, Japan, and Taiwan to the Philippines, are derived from temperate and tropical Asia, particularly from the mainland of China. Due to the variable topography and the short distance from the continent of Asia, the island forest species are diverse and similar to their relatives that are found on continent of Asia. In addition, the tropical currents and the Kuroshio Current provide an ideal environment for long-distance colonization of seeds from Oceania to the circum-Pacific island arc. In the