ON THE RELATIONSHIP BETWEEN THE MACROEVOLUTIONARY TRAJECTORIES OF PHENOTYPIC INTEGRATION AND MORPHOLOGICAL DISPARITY
Although morphospace analysis has been focused mostly on temporal variations in the extent of morphospace realized (the magnitude of disparity), morphological disparity itself also has an underlying structure. Disparity potentially reflects aspects of the hierarchical organization of phenotypes into quasi-independent units of evolutionary transformation. In this context, how phenotypic integration and modularity may actually relate with morphological disparity is an important but still largely unexplored question. For instance, might changes in morphological disparity characteristically result from the interplay of parcellation and integration of phenotype organization? Or might they result instead from intrinsic changes in the variational potential of a relatively constant number of modules? When are integration and disparity likely to correlate? Analogously, might disparity be operationally used as a meaningful proxy for modularity?
Here, we address such questions by explicitly quantifying temporal trajectories of clade-wide measures of phenotypic integration. We explore a few summary statistics of phenotypic correlation and covariance structure as a means of portraying macroevolutionary patterns of integration and their contrast with disparity profiles. These questions and methodological approaches are illustrated with analyses of a few major echinoderm clades with putatively distinct modular organizations. Our results suggest that qualitatively different, context-dependent relationships between disparity and integration can be envisioned, calling for a broader array of causal interpretations.