Data from: 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 disproportionately account for
disparity; excluding them, we find compelling evidence for stasis of jaw
shape but not size. The primary optima of these lineages diverge little,
if at all over approximately 30 million years. Once their trophic
apparatus was assembled, their morphological specialization steepened the
slopes of their adaptive peak and constrained the position of the optima
without limiting niche breadth.