Phylogenetics of Perissodactyla and Tests of the Molecular Clock |
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Authors: | Jane E Norman Mary V Ashley |
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Institution: | (1) Department of Biological Sciences, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607-7060, USA, US |
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Abstract: | Two mitochondrial genes, the protein-coding cytochrome c oxidase subunit II (COII) gene and a portion of the 12S rRNA gene, were used for phylogenetic investigation of the mammalian
order Perissodactyla. The primary objective of the study was to utilize the extensive fossil record of perissodactyls for
calibrating molecular clocks and comparing estimates of divergence times using both genes and two fossil calibration points.
Secondary objectives included clarification of previously unresolved relationships within Tapiridae and comparison of the
results of separate and combined analyses of two genes. Analyses included several perissodactyl lineages representing all
three families (Tapiridae, Equidae, and Rhinocerotidae), most extant genera, all four species of tapirs, two to four species
of rhinoceros, and two species of Equus. The application of a relatively recent fossil calibration point and a relatively ancient calibration point produced greatly
different estimates of evolutionary rates and divergence times for both genes, even though a relative rates test did not find
significant rate differences among taxa. A likelihood-ratio test, however, rejected a molecular clock for both genes. Neither
calibration point produced estimates of divergence times consistent with paleontological evidence over a range of perissodactyl
radiations. The combined analysis of both genes produces a well-resolved phylogeny with Perissodactyla that conforms to traditional
views of interfamilial relationships and supports monophyly of neotropical tapirs. Combining the data sets increases support
for most nodes but decreases the support for a neotropical tapir clade because the COII and 12S rRNA data sets are in conflict
for tapir relationships.
Received: 6 January 1999 / Accepted: 2 August 1999 |
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Keywords: | : Perissodactyl — Tapirs — Mitochondrial DNA — 12S rRNA — Cytochrome oxidase II — Evolutionary rates — Molecular clock |
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