Rapid radiation events in the family Ursidae indicated by likelihood phylogenetic estimation from multiple fragments of mtDNA

The bear family (Ursidae) presents a number of phylogenetic ambiguities as the evolutionary relationships of the six youngest members (ursine bears) a re largely unresolved. Recent mitochondrial DNA analyses have produced conf licting results with respect to the phylogeny of ursine bears. In an attemp t to resolve these issues, we obtained 1916 nucleotides of mitochondrial DN A sequence data from six gene segments for all eight bear species and condu cted maximum likelihood and maximum parsimony analyses on all fragments sep arately and combined. All six single-region gene trees gave different phylo genetic estimates; however only for control region data was this significan tly incongruent with the results from the combined data. The optimal phylog eny for the combined data set suggests that the giant panda is most basal f ollowed by the spectacled bear. The sloth bear is the basal ursine bear, an d there is weak support for a sister taxon relationship of the American and Asiatic black bears. The sun bear is sister taxon to the youngest clade co ntaining brown bears and polar bears. Statistical analyses of alternate hyp otheses revealed a lack of strong support for many of the relationships. We suggest that the difficulties surrounding the resolution of the evolutiona ry relationships of the Ursidae are linked to the existence of sequential r apid radiation events in bear evolution. Thus, unresolved branching orders during these time periods may represent an accurate representation of the e volutionary history of bear species. (C) 1999 Academic Press.