Concepts of Canalization and Genetic Assimilation in Developmental Biology: Current Approaches and Studies

The two related parts of the Conrad Waddington’s proposal, the concepts of canalization and genetic assimilation, formalized yet in the 1940s continue to arouse great interest among professionals, representing one of the most impressive examples of the transdisciplinary development of ideas. Individual development of any organism proceeds in the context of permanent and unavoidable changes in the factors of the external environment and internal perturbations of molecular and physiological processes. Considering that in any population organisms differ genetically, the implementation of the genetic program must therefore be relatively resistant to genetic variability. According to Waddington, individual development is canalized, i.e. occurs within a certain canal of environmental conditions that limits the variability in the developmental trajectory. However, strong environmental changes and considerable internal perturbations are able to “throw” some trajectories outside the walls of this canal. As a result, aberrant phenotypes may emerge, with some of them being able to get involved in subsequent selection. If the conditions that systematically lead to such an aberrant development persist, the features of these phenotypes can be fixed genetically by selection after a few generations. In other words, selection leads to the emergence of phenotype variants, most genetically suitable to the current circumstances, in which developmental trajectories are altered appropriately. Populations of organisms with altered trajectories and different genotypes continue to exist even when the impact of perturbing factors ceases. Waddington called the mechanism underlying the “same phenotype but different genotypes” evolutionary scenario genetic assimilation. Recent outcomes of evolutionary systems biology have provided a quantitative basis for Waddington’s classical concepts on the robustness of individual development and genetic assimilation. It has become possible to further develop these concepts in the light of new experimental results and theoretical ideas. Particular progress has been achieved in analyzing the molecular machinery of canalization. This paper aims to discuss the results obtained in this area of systems biology by computer modeling in comparison with experimental data.