Meeting Abstract
If variational modularity explains how complex adaptations can evolve, it should predict the pattern of evolutionary modularity. The rodent mandible has long served as a classic example of variational modularity, but the prevailing hypothesis is of two “functional modules”: the tooth-bearing front and muscle-bearing back. That is difficult to reconcile with the idea that the mammalian mandible can be modeled as a beam because a beam is not divisible into two functional complexes along its length. In addition, it raises the specter of functionally incompatible changes occurring in different regions. This hypothesis is even more difficult to reconcile with the rodent mandible because the incisor runs through the mandible, well past the boundary between the modules, and the masseteric muscle overlaps the border in the other direction. These anatomical relationships lead to the question: why would the front/back model fit so well, if it does? Analyses of mandibular shape changes in four major lineages of squirrels reveal evolutionary modularity in all clades consistent with the Front-Back model, which is consistently one of the best for variational modularity, although it missed the integration along the beam. For evolutionary modularity, models derived from mechanical principles, incorporating that integration of the beam and uniting muscle-bearing processes in a module consistent with their functional coupling, provide evidence of an even stronger modular signal, indicating they improve upon the Front-Back model. Thus, modular patterns of evolutionary change are partially aligned with the Front-Back model, but lineages differ in their deviations from it. Also, the most disparate are the least modular in their evolutionary changes.
