Canalization, cryptic variation, and developmental buffering: A critical examination and analytical perspective

This chapter focuses on assessing canalization and cryptic genetic variation. Regardless of the concerns with the mechanistic explanation of genetic assimilation, the plausibility of the phenomena of genetic assimilation as well as the existence of cryptic genetic variation were established via some empirical experiments. Waddington demonstrated that traits that were invariant under most (normal) environmental circumstances could be sensitized so as to express phenotypic variation for these traits. The classic example of Waddington's was the use of a high-temperature "heat-shock" in Drosophila, which resulted in some flies having lost their wing cross-veins. Waddington demonstrated that the cross-veinless phenotype could be selected upon, suggesting considerable hidden (cryptic) genetic variation for this trait. In considering the origin of developmental variation, one must consider the contribution of both genetic (mutational) sources and sources of variation arising from developmental processes themselves (intrinsic sources of developmental variation). Phenotypic effects of intrinsic developmental variation are seen in phenomena ranging from fluctuating asymmetry to variable penetrance and expressivity of alleles. From evolutionary perspectives, it is possible to conceive that variation arose first as intrinsic variation and then was captured genetically by abundant genetic variation. Such a scenario provides an alternative to the "genes first" ideas of neo-Darwinism and provides additional avenues for rapid and saltational changes in the evolution of development as well as for gradual change. © 2005 Elsevier Inc. All rights reserved.