Canalisation (genetics)

Norms of reaction for two genotypes. Genotype B shows a strongly bimodal distribution indicating differentiation into distinct phenotypes. Each phenotype that results from genotype B is buffered against environmental variation—it is canalised.

Canalisation is a measure of the ability of a population to produce the same phenotype regardless of variability of its environment or genotype. It is a form of evolutionary robustness. The term was coined in 1942 by C. H. Waddington to capture the fact that "developmental reactions, as they occur in organisms submitted to natural selection...are adjusted so as to bring about one definite end-result regardless of minor variations in conditions during the course of the reaction".[1] He used this word rather than robustness to take into account that biological systems are not robust in quite the same way as, for example, engineered systems.

Biological robustness or canalisation comes about when developmental pathways are shaped by evolution. Waddington introduced the concept of the epigenetic landscape, in which the state of an organism rolls "downhill" during development. In this metaphor, a canalised trait is illustrated as a valley (which he called a creode) enclosed by high ridges, safely guiding the phenotype to its "fate". Waddington claimed that canals form in the epigenetic landscape during evolution, and that this heuristic is useful for understanding the unique qualities of biological robustness.[2]

Genetic assimilation

Waddington used the concept of canalisation to explain his experiments on genetic assimilation.[3] In these experiments, he exposed Drosophila pupae to heat shock. This environmental disturbance caused some flies to develop a crossveinless phenotype. He then selected for crossveinless. Eventually, the crossveinless phenotype appeared even without heat shock. Through this process of genetic assimilation, an environmentally induced phenotype had become inherited. Waddington explained this as the formation of a new canal in the epigenetic landscape.

It is, however, possible to explain genetic assimilation using only quantitative genetics and a threshold model, with no reference to the concept of canalisation.[4][5][6][7] However, theoretical models that incorporate a complex genotype–phenotype map have found evidence for the evolution of phenotypic robustness[8] contributing to genetic assimilation,[9] even when selection is only for developmental stability and not for a particular phenotype, and so the quantitative genetics models do not apply. These studies suggest that the canalisation heuristic may still be useful, beyond the more simple concept of robustness.