Museum of Natural Science and Department of Biological Sciences, Louisiana State University , Baton Rouge, LA , USA.
Proc Biol Sci. 2019 May 15;286(1902):20190672. doi: 10.1098/rspb.2019.0672.
Understanding the number of times a trait has evolved is a necessary foundation for comprehending its potential relationships with selective regimes, developmental constraints and evolutionary diversification. Rodents make up over 40% of extant mammalian species, and their ecological and evolutionary success has been partially attributed to the increase in biting efficiency that resulted from a forward shift of one or two portions of the masseter muscle from the zygomatic arch onto the rostrum. This forward shift has occurred in three discrete ways, but the number of times it has occurred has never been explicitly quantified. We estimated an ultrametric phylogeny, the first to include all rodent families, using thousands of ultraconserved elements. We examined support for evolutionary relationships among the five rodent suborders and then incorporated relevant fossils, fitted models of character evolution, and used stochastic character mapping to determine that a portion of the masseter muscle has moved forward onto the rostrum at least seven times (with one reversal) during the approximately 70 Myr history of rodents. Combined, the repeated evolution of this key innovation, its increasing prevalence through time, and the species diversity of clades with this character underscores the adaptive value of improved biting efficiency and the relative ease with which some advantageous traits arise.
了解一个特征进化的次数是理解其与选择制度、发育约束和进化多样化的潜在关系的必要基础。啮齿动物约占现存哺乳动物物种的 40%以上，它们的生态和进化成功部分归因于咀嚼肌的一部分或两部分从前颧骨向前转移到吻部，从而提高了咬合力。这种向前的转移以三种不同的方式发生，但它发生的次数从未被明确量化。我们使用数千个超保守元件估计了一个超度量系统发育，这是第一个包含所有啮齿动物科的系统发育。我们检查了五个啮齿动物亚目之间的进化关系的支持，并纳入了相关的化石，拟合了性状进化模型，并使用随机性状映射来确定咀嚼肌的一部分在大约 7000 万年的啮齿动物历史中至少向前移动了七次（一次逆转）。综上所述，这一关键创新的重复进化、随着时间的推移其越来越普遍的出现以及具有这种特征的进化枝的物种多样性，突显了提高咬合力的适应性价值以及某些有利特征出现的相对容易性。
