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.

中文翻译：

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核内含子揭示的喙鲸属 Mesoplodon (Ziphiidae: Cetacea) 的系统发育：对雄性象牙进化的影响

目前已知有 14 个物种，喙鲸属 Mesoplodon (Ziphiidae) 是鲸目中最特殊的物种。喙鲸分布广泛，但由于它们的海洋分布、深潜能力和明显的低丰度，在海上很少见到。Mesoplodon 物种之间的形态分化相对有限，但成年雄性的牙齿形式除外。根据疤痕模式，雄性似乎在与其他雄性的攻击性遭遇中使用它们象牙一样的牙齿作为武器。女性实际上是无牙的。我们使用来自七个核内含子（3348 个碱基对）的序列来构建强大且高度解析的系统发育，然后将其用作框架来测试来自四个假设的预测，这些假设试图解释 Mesoplodon 象牙形态的模式和/或推动该属多样化的过程：（1）象牙形式的线性进展；(2) 在相邻的深海峡谷中通过隔离形成异域物种；(3) 通过对象牙进行性选择的同域物种形成；(4) 物种识别线索的选择。最大似然和贝叶斯重建证实了该属的单系性，并揭示了被认为是祖先和衍生的象牙形式实际上已经在多个场合独立出现，这与线性进化假说的预测相反。此外，三个得到充分支持的物种进化枝都没有局限于一个海洋盆地，正如深海峡谷隔离或性别选择假设所预期的那样，一些具有相似象牙的物种具有重叠分布，这与物种识别假设的预测相反。然而，所确定的四个姐妹物种中的三个的不同的象牙形式和同域分布表明，雄性象牙的性选择可能在这种独特的辐射中发挥了重要作用，尽管其他力量显然也参与其中。据我们所知，这是第一次将性选择明确地与陆地有蹄类动物以外的哺乳动物群的辐射有关。确定的四个姐妹物种中的三个的不同的象牙形式和同域分布表明，雄性象牙的性选择可能在这种独特的辐射中发挥了重要作用，尽管其他力量显然也参与其中。据我们所知，这是第一次将性选择明确地与陆地有蹄类动物以外的哺乳动物群的辐射有关。确定的四个姐妹物种中的三个的不同的象牙形式和同域分布表明，雄性象牙的性选择可能在这种独特的辐射中发挥了重要作用，尽管其他力量显然也参与其中。据我们所知，这是第一次将性选择明确地与陆地有蹄类动物以外的哺乳动物群的辐射有关。