At all taxonomic levels, from Baupläne to morphospecies, morphologic variation is neither randomly nor evenly distributed. This pattern is the product of adaptation, constraint, and plasticity acting on populations in ecological time and on lineages in evolutionary time. Because lineages can maintain their morphologic identity over millions of years and numerous speciation and extinction events, constraint is critical to macroevolution.
The Corbulidae (Bivalvia) is a morphologically diverse clade containing several morphologically conservative subclades that provide an opportunity to examine the role of constraint in evolutionary history. For the Caryocorbula+ (Bothrocorbula+ Hexacorbula) clade, all from Caribbean Neogene deposits, each genus is morphologically distinct both in shape and size. In addition, Caryocorbula morphospecies exhibit strong interspecific allometry that persists despite repeated speciation and extinction, as well as the vagaries of geographic and temporal sample coverage. In spite of the conservative morphology within this basal clade, more derived corbulid clades subsequently occupied new areas of morphospace.
Paleontologic data allow morphology to be examined in a geographic by temporal matrix over evolutionary time scales. For extinct taxa, whose diversity far exceeds that of extant members in many molluscan clades, the fossil record is the only source of information on evolutionary patterns. Thus, although it is often not possible to identify causes of morphologic constraint (e.g., pleiotropy vs. stabilizing selection), the fossil record provides the opportunity to examine morphologic response to changing environmental conditions over evolutionary time both within and among species.