Morphological integration is defined variously as an emergent property of individual ontogenies, populations, and clades. Integration is a pattern of relationships among morphological characters at the level of the organism, but the topology and variation of this pattern at different hierarchical levels may dictate whether integration promotes or constrains evolution. We examine this problem by describing morphological integration in Chione, a clade of Neogene venerid bivalves. We archive the shape of individual valves using geometric morphometric landmarks, and extract from this archive estimates of developmental integration and phenotypic integration. Patterns of integration are estimated for several species from the western Atlantic and eastern Pacific, and sampling of each species includes multiple populations spanning geographic and geologic ranges.
Landmarks or traits within a shell are considered to be integrated if they are mathematically correlated. The correlations are generally associated with developmental or genetically homologous (pleiotropic) relationships. Developmental integration can be estimated as the correlations of characters with a common growth vector. This estimate of integration is an emergent group property, and may vary between populations and species. Independent of developmental integration however, are the residual correlations among sets of landmarks within a population. Both this latter phenotypic integration and developmental integration determine the responses of populations and species to agents of micro- and macroevolution, and may explain the phenotypic variability of molluscan shells. We estimate the patterns of integration using two alternative methods: (1) empirical multivariate estimates of partial correlations among landmarks, and (2) Bayesian modeling of developmental and phenotypic correlations.