Evolutionary modularity, integration and disparity in an accretionary skeleton: Analysis of venerid Bivalvia

Modular evolution, the relatively independent evolution of body parts, may promote high morphological disparity in a clade. Conversely, integrated evolution via the stronger covariation of parts may limit disparity. However, integration can also promote high disparity by channeling morphological evolution along lines of least resistance—a process that may be particularly important in the accumulation of disparity among organisms with accretionary growth, as in many invertebrate systems. We use a time-calibrated phylogenetic hypothesis and high-density, 3D semilandmarking to analyze the relationship between modularity, integration, and disparity in the most diverse extant bivalve family: the Veneridae. In general, venerids have a simple, two-module parcellation of body features that is divided into features of the calcium carbonate shell and features of the internal soft anatomy. This division falls more along developmental than functional lines when placed in the context of bivalve evolutionary-development and biomechanics. While the venerid body is tightly integrated in absolute terms, increasing modularity strength among subclades and ecologies appears to correlate with increasing disparity, indicating that shifts towards more mosaic evolution beget higher morphologic variance in this speciose family.