Progress on canalization

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Abstract
In the 19th century, when evolutionary biologists focused on whole organisms, development played a central role in evolutionary theory. For much of the 20th century, genetic models and explanations replaced development at the center of evolutionary thought. Connections to development resurfaced at mid-century (1, 2) and accelerated after 1975, fueled by influential books (3–5) and by a resurgence of interest in the role of phenotypes in evolution and in the tension between recent selection and historical constraint in the design of organisms. By the turn of the new millennium, development had again become a major evolutionary theme in two quite different but interestingly connected ways. Under the label of Evo–Devo, the tools of molecular genetics are used to explore deeply conserved developmental mechanisms that are active early in development and thought to shape the regulatory systems responsible for the conservation of basic body plans (6). Under the label of the Genotype–Phenotype Map, a variety of theoretical and experimental programs explore the mechanisms and processes that shape the expression of genetic variation in phenotypes in nonlinear ways (7). Is there a natural bridge between these two research fields? Do the deeply conserved developmental systems that produce adult morphology also influence the expression of genetic variation in the traits they shape? Such a connection would point to part of one of evolution's long-sought Rosetta Stones: the mechanisms connecting macro- to microevolution. Building on work by A. Wagner (8), Siegal and Bergman (9) provide one of the first demonstrations of the plausibility of such a connection for a major component of the genotype–phenotype map: canalization. Canalization, now a classic idea, was suggested independently by Waddington (1) and Schmalhausen (2). Schmalhausen argued that canalization resulted from stabilizing selection shaping developmental mechanisms to buffer the expression of traits, holding them near their optimal …