Phylogenetic analysis of the perissodactylan family Tapiridae using mitochondrial cytochromec oxidase (COII) sequences
The four extant members of the family Tapiridae have a disjunct, relictual distribution, with three species being Neotropical (Tapirus bairdii, T. terrestris, andT. pinchaque) and one found in Southeast Asia (T. indicus). Little recent work on tapir systematics have appeared, and no molecular studies of this group have been published. A phylogenetic analysis was undertaken using sequences of the mitochondrial cytochromec oxidase subunit II gene (COII) from representatives of the four species of tapirs, as well as a representative outgroup,Equus caballus. Analyses of the COII sequences indicate a close relationship between the two South American species of tapirs,T. terrestris andT. pinchaque, and estimates of divergence dates using rates of COII evolution are compatible with migration of a single tapir lineage into South America following the emergence of the isthmus of Panama, about 3 million years bp. Various methods of analysis, including maximum parsimony, maximum likelihood, and neighbor-joining, provided poorer resolution of other tapir relationship. The COII data suggest that three distinct tapir mitochondrial lineages, a South American (represented byT. terrestris andT. pinchaque), a Central American (represented byT. bairdii), and an Asian (represented byT. indicus) diverged relatively rapidly, 20–30 million years bp. Another goal of this study was to calibrate the rate of COII evolution in a eutherian mammal group which has a good fossil record, such as perissodactyls, to estimate accurately the rate of COII evolution in a nonprimate mammalian group. The rate of COII evolution in equids and tapirs has been relatively constant and, using corrected distances, calibrated to be approximately 0.22% lineage/million years. This rate is three-to fourfold lower than that of hominoid primates.