Not all species display substantial ecophenotypic response. Indeed, many species react to environmental shifts by merely tracking the preferred environment. But what is it about these low-plasticity species that is preventing the phenotype from responding viably to environmental change? One candidate mechanism is integrated phenotypic response. If, for instance, many anatomical features are developmentally constrained to respond together, then selective pressure on any one feature will force an integrated developmental response of the whole suite. Such an integrated phenotypic response will prove inviable if the correlated features are "dragged" to levels of reduced fitness. Thus the viable magnitude of the overall response ("feasible plasticity") will be governed by the degree of integration among features — or, to be more precise, among the responses of individual features. Two distinct attributes of phenotypic integration are both relevant in limiting feasible plasticity: the "level" of overall integration and the modular structure of the integration. The overall "level" must be high in order to exert substantial control, and the covariance structure must be highly integrated, with few independent modules, in order to narrowly limit viable phenotypic response.

This determination of feasible plasticity has striking evolutionary consequences. It is feasible plasticity that shapes phenotypic breadth — especially in unstable environments, where environmental change is so rapid it can only be dealt with on the timescale of the individual. As environments change, those individuals that are capable of a broad phenotypic range are more likely to survive. When this translates to survival at the species level, then taxon duration should correlate positively with the phenotypic response's feasible magnitude, and negatively with its integration.

Using morphometric data from ammonites and trilobites, I explore several measures of integration as potential predictors of these evolutionary consequences. In each case, at least one of the measures of integration shows a significant and consistent relationship with stratigraphic duration. Further study should help to elucidate the causal cascade from developmental integration to its apparent evolutionary consequences.