The inhibitory cascade (IC) model is a widely used evolutionary developmental explanation of among-species differences in relative molar tooth size. The IC model posits that, as molars develop from front to back, the relative strength of activating and inhibiting influences establishes a “ratcheting” mechanism leading to predictable relative molar sizes. Such a constraint on molar covariation would lead to strong variational biases on the evolutionary paths that the molar row can traverse through phenotypic space. These constraints manifest themselves in characteristic patterns of variation among species that loosely match observed macroevolutionary patterns. In this paper, we write out the predictions of the IC model for within-species covariation in molar size in a framework that unifies evolutionary developmental biological and quantitative genetic perspectives on the evolution of complex traits. We then evaluate these predictions about aspects of molar covariation in eight anthropoid primate species. We find that the IC model tends to over-predict aspects of within-species covariation by substantial margins. Only macaques exhibit covariation in and among individual teeth consistent with the IC model, but they do not show signs of the strong evolutionary constraint predicted by the model. Gorillas meet none of the predictions. While we cannot rule out an IC-like process as a contributor, causes of molar size covariation other than those described in the IC model must be major contributors to covariation in molar teeth within populations.

中文翻译：

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抑制级联模型不是良好的预测摩尔大小的协方差

抑制级联（IC）模型是相对摩尔齿大小的种间差异的广泛使用的进化发展解释。IC模型假定，随着磨牙从前向后发展，活化和抑制作用的相对强度建立了一种“棘齿”机制，导致可预测的相对磨牙大小。这种对臼齿共变的约束将导致在磨牙行可以穿越表型空间的进化路径上产生强烈的变化偏见。这些约束条件以与观测到的宏观进化模式大致匹配的物种间变异的特征模式表现出来。在本文中，我们在统一复杂特征进化的进化发育生物学和定量遗传学观点的框架内，写出了IC模型对臼齿大小内协变的预测。然后，我们评估了关于八种类人猿灵长类动物的摩尔协变方面的这些预测。我们发现，IC模型倾向于大量预测物种内协变的各个方面。仅猕猴在单个牙齿内和牙齿之间显示出与IC模型一致的协变，但它们没有显示出该模型所预测的强大进化约束的迹象。大猩猩没有满足任何预测。尽管我们不能排除像贡献者一样的类似IC的流程，