We follow Smith (1996) and Lieberman et al. Developmental stability refers to the suppression of phenotypic variation within individuals, and morphological integration refers to how variability is structured by the under-lying developmental and genetic connections between traits. Canalization refers to “the suppression of phenotypic variation” among individuals ( Wagner et al., 1997). These three components of variability describe how the tendency to vary is structured. By variability, we mean the tendency or propensity to vary and not variation itself ( Wagner and Altenberg, 1996). Such modules would produce covariation that would need to be overcome by selection for divergence in hind- and forelimb morphology.Ĭanalization, developmental stability, and morphological integration are three related components of phenotypic variability. This is evidence for the existence of developmental modules that link structures between the limbs. Finally, we report that the covariation structure of fore- and hindlimb skeletal elements shows evidence for morphological integration between serially homologous structures between the limbs. This supports the idea that the mechanisms underlying canalization and developmental stability are related. We also find that heritability and FA are significantly correlated when different limb measurements are compared in fetal mice. We infer that the greater variability of more distal segments in macaques is due to postnatal mechanical effects. We find that environmental variances and fluctuating asymmetries (FA) increase distally along the limb in adult macaques but not in fetal mice. We then test three developmentally motivated hypotheses about the patterning of variability components in the mammalian limb. In this paper, we review the topics of canalization, developmental stability, and morphological integration and their relevance to primate and human evolution. All three phenomena are emergent properties of developmental systems that can affect the interaction of development and evolution. Morphological integration refers to the tendency for structures to show correlated variation because they develop in response to shared developmental processes or function in concert with other structures. Canalization is the tendency for development of a specific genotype to follow the same trajectory under different conditions (different environments or different genetic backgrounds), while developmental stability is the tendency for the development of a specific genotype to follow the same trajectory under the same conditions. Canalization and developmental stability refer to the tendency of developmental processes to follow particular trajectories, despite external or internal perturbation.

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