Body-size evolution in Cretaceous molluscs and the status of Cope's rule

Cope's rule, the tendency for lineages to evolve to larger body size, is widely seen as a pervasive evolutionary pattern1–4. However, only a few studies5–8 have gone beyond enumerating isolated examples to assess its overall frequency relative to body-size decrease or stasis. Thus, although size is clearly an important parameter for microevolution and ecology9–14, including conservation biology10, its impact on large-scale patterns remains poorly understood. The prevalence of Cope's rule is even more uncertain, as some reported cases of evolutionary size increase may actually represent an expansion of a clade's size range (a pattern generally termed an 'Increase in variance', although not necessarily in the formal statistical sense) rather than a phyletic, directional trend15–18. I have performed a comprehensive census of body-size changes in a large fauna of Cretaceous bivalve and gastropod genera. A directional net increase in body size (including the loss of small-sized species and thus representing Cope's rule in the strict sense) is no more frequent than an increase in size range among species or a net evolutionary size decrease. Thus the undisputed ecological importance of body size does not translate into a preferred macroevolutionary pattern.