Meeting Abstract
100.4 Thursday, Jan. 7 Integration of squirrel mandibles ZELDITCH, M.L.*; SWIDERSKI, D.L.; Univ of Michigan, Ann Arbor; Univ of Michigan, Ann Arbor zelditch@umich.edu
Understanding how developmental modules are assembled into functionally integrated morphologies is central to our understanding of biological organization. Morphological integration, a key part of that organization, coordinates variation among parts, maintaining relationships among them despite their individual variation. Integration can arise by direct interactions between developmental pathways or by genes expressed independently along two or more pathways, and both of these, or correlational selection, can coordinate evolutionary changes in form. To determine whether evolutionary integration might be directed by development, we examine integration of the mandible in a representative tree squirrel, Sciurus niger, analyzing phenotypic integration and that due to direct interactions and compare these components to evolutionary integration in a lineage of tree squirrels. To analyze integration, we subdivide the mandible into 11 parts, dissecting structures that arise from a single mesenchymal condensation to assess correlations within as well as between them. Evolutionary integration is higher than phenotypic integration or direct interactions because there are more connections between parts, and those connections are stronger, on average. It is also differently structured; evolutionary correlations include some unique to one of the two developmental patterns and those not found in either. The high level of evolutionary integration is not peculiar to the functionally and morphologically homogeneous tree squirrels; even higher evolutionary integration is seen in the more diverse group of ground squirrels and chipmunks, which have a greater density of connections to the incisors. These different patterns of evolutionary integration, neither of which resembles phenotypic integration, suggest that selection rather than development integrates mandibular evolution.
