© 2008 Society of Systematic Biologists
Phylogeny of the Beaked Whale Genus Mesoplodon (Ziphiidae: Cetacea) Revealed by Nuclear Introns: Implications for the Evolution of Male Tusks
1 School of Biological, Earth, and Environmental Sciences (BEES), University of New South Wales Kensington, NSW 2052, Australia; E-mail: m.dalebout{at}unsw.edu.au (M.L.D.)
2 School of Biological Sciences, University of Auckland Private Bag 92019, Auckland, New Zealand
3 Marine Mammal Institute and Department of Fisheries and Wildlife, Oregon State University 2030 SE Marine Science Drive, Newport, Oregon 97365, USA
Edited by Michael Charleston
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Abstract |
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With 14 species currently recognized, the beaked whale genus Mesoplodon (family Ziphiidae) is the most speciose in the order Cetacea. Beaked whales are widely distributed but are rarely seen at sea due to their oceanic distribution, deep-diving capacity, and apparent low abundance. Morphological differentiation among Mesoplodon species is relatively limited, with the exception of tooth form in adult males. Based on scarring patterns, males appear to use their tusk-like teeth as weapons in aggressive encounters with other males. Females are effectively toothless. We used sequences from seven nuclear introns (3348 base pairs) to construct a robust and highly resolved phylogeny, which was then used as a framework to test predictions from four hypotheses seeking to explain patterns of Mesoplodon tusk morphology and/or the processes that have driven the diversification of this genus: (1) linear progression of tusk form; (2) allopatric speciation through isolation in adjacent deep-sea canyons; (3) sympatric speciation through sexual selection on tusks; and (4) selection for species-recognition cues. Maximum-likelihood and Bayesian reconstructions confirmed the monophyly of the genus and revealed that what were considered ancestral and derived tusk forms have in fact arisen independently on several occasions, contrary to predictions from the linear-progression hypothesis. Further, none of the three well-supported species clades was confined to a single ocean basin, as might have been expected from the deep-sea canyon-isolation or sexual-selection hypotheses, and some species with similar tusks have overlapping distributions, contrary to predictions from the species-recognition hypothesis. However, the divergent tusk forms and sympatric distributions of three of the four sister-species pairs identified suggest that sexual selection on male tusks has likely played an important role in this unique radiation, although other forces are clearly also involved. To our knowledge, this is the first time that sexual selection has been explicitly implicated in the radiation of a mammalian group outside terrestrial ungulates.
Keywords: Cetacean; nuclear introns; phylogeny; sexual selection; teeth; weapons
Received February 21, 2008; Revised May 28, 2008; Accepted September 11, 2008
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