The Inhibitory Cascade Model is Not a Good Predictor of Molar Size Covariation

Abstract

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.