On the origins of morphological variation, canalization, robustness, and evolvability.

Canalization is a concept, introduced by Waddington that describes the reduced sensitivity of a phenotype to genetic and environmental perturbations. Some research in canalization assumes that lack of variation in a trait in one genotype with respect to another genotype in a population, is due to the existence of buffering mechanisms against environmental and/or genetic variation. This article criticizes this assumption and out points out other possible problems with the concepts of canalization, robustness, and evolvability. These involve: the neglect of alternative explanations for the lack of variation in a trait, the incompatibility with current understanding of development, the way the mutivariate nature of morphological variation is considered. In addition, this article tries to explain that these concepts implicitly assume, although not generally acknowledged, that without buffering any genetic or environmental variation should give rise to a distinct phenotypic outcome. This can be avoided by restricting the use of canalization to cases in which, as in hsp90, there is direct evidence of buffering. For the other cases it would be clearer to talk about variational properties or simply type of variation. The concept of evolvability is also biased towards univariate comparisons and is dependent on selective pressures. It is suggested that this can be replaced by "type of phenotypic variation" from a genotype or variational properties. Overall, this article proposes that the concepts of canalization and evolvability involve some assumptions that, in most situations, unnecessarily complicate the study of evolution and development.