Developmental constraints, modules, and evolvability

This chapter discusses the developmental origins and evolutionary implications of covariation between traits. These are important factors influencing the evolutionary potential of morphological traits. Strong covariation can constitute an evolutionary constraint because some character combinations are more likely to evolve than others. Modularity is a widespread feature of organismal organization: Groups of traits covary with each other but are relatively independent of other groups of traits. This modularity results from a similar organization of developmental systems, where signaling interactions primarily take place within spatially distinct fields. The covariation among morphological traits can result from direct interactions of the developmental pathways that produce the traits, which take place within developmental modules, or from parallel variation of separate pathways in response to the simultaneous influence of an external factor. These two origins of covariation among traits have different implications for pleiotropy of genes, the evolution of pleiotropy, the total genetic covariance structure, and the resulting evolutionary constraints. © 2005 Elsevier Inc. All rights reserved.