Canalization: Genetic and Developmental Aspects

The concept of canalization was used by Waddington (66) in the context of developmental biology. He emphasized two important points in the develop­ ment of higher organisms. First, the end products of development, i.e. the adult tissues, are of sharply distinct types without intergradation. Second, the normal course of development is in his view a preferred path. Deviations of this path due to disturbances in the internal or external environment are corrected by regulatory processes. This occurs not only in the development of distinct types of tissue, but also on the organismic level in the realization of morphological patterns, in size and shape of organs and in matters of growth and determination of size of whole organisms. He depicted these phenomena in what he called the epigenetic landscape. Development starts in the egg. From there numerous developmental pathways branch out, leading to a great variety of distinct end results. These pathways are represented as a system of branching valleys in a descending slope. The developmental process is visual­ ized as balls rolling through the valleys to their end point. The steeper the valley and the larger the ridges separating the valleys, the stronger the tendency of the ball, when it is pushed from its course along the valley bottom by internal or external disturbances, to go back to its original course. This