Stasis of functionally versatile specialists

A classic hypothesis posits that lineages exhibiting long-term stasis are broadly adapted generalists that remain well-adapted despite environmental change. However, lacking constraints that steepen adaptive peaks and stabilize the optimum, generalists’ phenotypes might drift around a broad adaptive plateau. We propose that stasis would be likely for morphological specialists that behave as ecological generalists much of the time because specialists’ functional constraints stabilize the optimum, but those with a broad niche can, like generalists, persist despite environmental change. Tree squirrels (Callosciurinae and Sciurini) exemplify ecologically versatile specialists, being extreme in adaptations for forceful biting that expand rather than limit niche breadth. Here, we examine the structure of disparity and the evolutionary dynamics of their trophic morphology (mandible size and shape) to determine if they exhibit stasis. In both lineages, a few dietary specialists disproportionate...