SWIDERSKI, D.L.: Independence of divergence and diversification in the evolution of scapula shape in squirrels: possible evidence of the lability of constraints.

Lack of morphological diversity in a clade is often taken as evidence of constraints that restrict the magnitude or direction of change. Liberation from constraints is expected to result in both divergence and diversification, i.e., a greater variety of morphologies, not just a new constrained morphology. Tree squirrels have a reputation for being morphologically conservative, a reputation supported by geometric morphometric analysis of scapula shapes in several species. The mean shapes are significantly different, but the distances among means are small relative to the variation around each mean. In contrast, chipmunks and ground squirrels appear to have broken out of the constraints limiting tree squirrels diversity: basal members of both terrestrial lineages are clearly outside of the range of tree squirrel variation. Further divergence, in a variety of directions, separates derived ground squirrels from the basal ground squirrel. In some cases, the distance between basal and derived ground squirrels is as great as the distance between the basal ground squirrel and tree squirrels. Consequently, the diversity of ground squirrels is significantly greater than the diversity of tree squirrels. Chipmunks also exhibit additional divergence from the basal chipmunk, in yet another direction, but there is little differentiation among the derived chipmunks. In contrast to ground squirrels, the diversity of chipmunks is not significantly higher than the diversity of tree squirrels. Because scapula shape has evolved in at least two different directions during the divergence of chipmunks, establishment of new constraints appears more likely than evolution within constraints inherited from tree squirrels.