Evolution of the eastern Asian and eastern North American disjunct pattern : Insights from phylogenetic studies

Abstract

This study compared the available phylogenetic analyses on taxa with an eastern Asian and eastern North American disjunct distribution, taxa with a broad disjunction in the north temperate zone, and north temperate disjunct taxa with close relatives in the tropical and subtropical regions. Taxa examined include Aralia sect. Aralia, Aralia sect. Dimorphanthus, Boykinia, Calycanthus, Cornus, Corylus, Hamamelis, the Liquidambaroideae (or Altingioideae), Magnolia sect. Rytidospermum, Nyssa, Panax, Staphylea, Styrax, Symplocarpus, Tiarella, Trautvetteria, and Trillium. Although the floristic similarity between eastern Asia and eastern North America has been traditionally emphasized since Linnaeus era and numerous presumable intercontinental species pairs were proposed, phylogenetic studies showed a general lack of direct sister species relationships between eastern Asia and eastern North America. Instead a pattern of further diversification in one or both continents is common, which suggests the antiquity of this disjunct pattern. Examinations of the phylogenetic analyses of taxa with a broad north temperate disjunct pattern showed a close biogeographic relationship between eastern North America, western North America, and eastern Asia. This biogeographic pattern is best explained by the classical hypothesis of floristic exchanges between Asia and North America via the Bering land bridge. The phylogenetic pattern in Liquidambar and Staphylea, however, is compatible with the hypothesis of the North Atlantic land bridge. Furthermore the phylogenetic studies provide important insights into the tempo and mode of evolution of the north temperate disjuncts. In the Liquidambaroideae, the tropical and subtropical Altingia and Semiliquidambar are shown to have derived from the north temperate disjunct Liquidambar. Yet Altingia and Semiliquidambar are morphologically highly distinct from Liquidambar. The morphological cohesion among the evolutionarily