Functional morphological innovation corresponds to shifting lines of genetic least resistance

Phenotypic traits are often constrained in their evolution by the genetic and developmental map underlying them. However, the extent to which these constraints themselves evolve has not been well-characterized. Traits that are integrated are generally thought to be constrained in their ability to evolve because their shared developmental and genetic architecture causes any change in one to be mirrored in the other. Nevertheless, it is not yet clear whether correlations can constrain major episodes of phenotypic diversification over longer timescales. We reconstruct patterns in integration within nine primate lineages and model the evolution of the resulting modules in a phylogenetic context. We find that patterns in integration are generally evolvable across lineages, with apes displaying particular lability in the composition of morphological modules. This increased turnover in module composition corresponds to both divergent evolution of skeletal shape and the formation of novel complexes of locomotor behaviors and postures. While integration may play a role in constraining evolutionary innovation, its effects are dynamic, shifting as the structure of integration itself evolves during episodes of exceptional ecomorphological diversification.