Epigenetic canalization and phenotypic change: a minimax model.

A selectionist model of phylogenic behaviour is presented in which epigenetic canalization fulfills the pivotal rôle. It is shown that directional selection will result in stress induce creodal destabilization and in genetic unmasking. Both processes will in turn result in a rapid increase in phenotypic variance for natural selection to act on, and hence a sudden increase in the rate of phylogenic change. Acting on the epigenome via internal selection and on phenotypic peripherals via stabilizing selection, natural selection will also assure phenotypic stasis by means of increased genetic concealment, deepened canalization and developmental constraints. The latter two factors allow for the application of a minimax rule of phenotypic change: that part of an organism with the minimum potential for phenotypic change (hence the maximum canalization) will govern the direction of the maximum phenotypic change of the other parts. The hierarchialization of phenotypic change predicted by the minimax rule explains the coexistence of gradualistic and punctuational changes within lineages of the same species.