The historical biogeography of Fagaceae: Tracking the tertiary history of temperate and subtropical forests of the Northern Hemisphere
The angiosperm family Fagaceae is a central element of several distinct community types throughout the Northern Hemisphere and a prime candidate for modern biogeographic analysis. The rich fossil record for the family provides an unparalleled source to compare with modern distributions and evaluate hypotheses of origin, migration, and vicariance. We conducted separate phylogenetic analyses on genera with intercontinentally disjunct distributions using various noncoding regions of chloroplast and nuclear ribosomal DNA sequences. Analyses generally supported the (North America - (Europe + Asia)) pattern of area relationships. Divergence times between disjunct species were estimated to aid in the development of a comparative synthesis of historical biogeography across the family. Fully resolved phylogenies were analyzed biogeographically using three distinct approaches: dispersal-vicariance analysis (DIVA), strict vicariance, and dispersal analysis using Fitch parsimony. Ancestral area reconstructions based on DIVA were preferred because event-based inferences generally were consistent with fossil evidence for migration and estimates of divergence times. Reconstructions suggested an Asian origin for the genus Fagus with bidirectional migration to Europe and North America, consistent with a paraphyletic assemblage of Asian species and intercontinental exchange via the Bering Land Bridge (BLB). Reconstructions within Quercus generally were more ambiguous in determining a center of origin; however, one optimization pathway was consistent with the vicariance of an ancestrally widespread distribution and the initial divergence between largely North American and Asian clades. Within the North American clade, dispersal to Eurasia is inferred for section Quercus (white oaks). Bidirectional floristic exchange via the BLB is supported for these temperate taxa, followed by intercontinental disjunction by the mid-Miocene. In contrast, disjunctions based on living and fossil distributions within evergreen Fagaceae (e.g., Castanopsis, Lithocarpus, Trigonobalanus) suggest older, temporally distinct biogeographic histories involving both the North Atlantic and Bering Land Bridges.
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