Canalization describes the phenomenon whereby particular genotypes exhibit reduced phenotypic sensitivity or variation (i.e., increased robustness) in response to mutations and/or to environmental changes relative to other genotypes. Canalization is a variational property of genotypes: it implies a reduced potential or propensity of the phenotype, produced by this genotype, to vary in response to genetic or environmental change. The terms “canalization,” “robustness” and “buffering” are typically used interchangeably; today, “robustness” is perhaps more commonly used than “canalization.” The concept of canalization was first introduced by Conrad Hal Waddington in the 1940s; around the same time, Ivan Ivanovich Schmalhausen came up with essentially the same concept (see Books and Early History of the Canalization Concept). Their main conjecture was the existence of a special kind of stabilizing selection, so-called canalizing selection, which favors genotypes that deviate least from the trait optimum (e.g., the fitness optimum), by selecting for genetic mechanisms that suppress phenotypic variation caused by mutations (genetic canalization) or by environmental perturbations or changes (environmental canalization). The concept of canalization is closely related to the phenomenon of genetic assimilation, that is, the idea that previously hidden, cryptic genetic variants can become phenotypically expressed following an environmental or genetic perturbation and increase in frequency by selection.
Early experimental evidence for the existence of canalization and genetic assimilation is reviewed in depth by Scharloo 1991, the first comprehensive review paper in the field. The review by Flatt 2005 focuses on work done after 1991. Gibson and Wagner 2000 connects canalization to the concept of variability, the propensity of a genotype to vary in response to genetic or environmental change and discuss how canalization can be detected experimentally. De Visser, et al. 2003 reviews genetic canalization and emphasizes the diverse mechanisms whereby this phenomenon can arise. Based on previous work, Meiklejohn and Hartl 2002 suggests that genetic canalization likely evolves as a correlated response to selection for increased environmental canalization. Siegal and Leu 2014 gives a modern treatment of mutational robustness and discuss its relationship with “cryptic” genetic variation, that is, genetic variation that is typically not phenotypically expressed but that can be revealed under certain conditions. Félix and Barkoulas 2015 discusses robustness from a contemporary systems biology point of view. The excellent book by Andreas Wagner (Wagner 2005) represents the most comprehensive treatment of robustness available to date; it provides a very good point of entry into the literature.
de Visser, J. Arjan G. M., Joachim Hermisson, Günter P. Wagner, et al. 2003. Perspective: Evolution and detection of genetic robustness. Evolution 57:1959–1972.
A review of genetic robustness, focusing on its evolutionary origin, proximate causes, and how to measure it.
Félix, Marie-Anne, and Michalis Barkoulas. 2015. Pervasive robustness in biological systems. Nature Reviews Genetics 16:483–496.
A modern treatment of robustness in biological systems; in particular, this paper discusses robustness from a systems biology point of view, that is, how robustness results from nonlinearities in molecular pathways and the genotype–phenotype map.
Flatt, Thomas. 2005. The evolutionary genetics of canalization. Quarterly Review of Biology 80:287–316.
A comprehensive general review of canalization; especially strong coverage of the theoretical and empirical literature that appeared after Scharloo 1991.
Gibson, Greg, and Günter Wagner. 2000. Canalization in evolutionary genetics: A stabilizing theory? BioEssays 22:372–380.
This review essay defines the process of canalization as a genetic change that results in the reduction of variability, the propensity to vary in response to genetic or environmental change; variability is contrasted with variation, which describes the actually realized differences among individuals in a population or an experiment; provides a succinct discussion of how canalization can be empirically detected.
Meiklejohn, Colin D., and Daniel L. Hartl. 2002. A single mode of canalization. Trends in Ecology and Evolution 17:468–473.
An opinion article discussing the notion that biological systems should evolve to be optimally buffered against both genetic and environmental perturbations; in particular, the authors argue that genetic canalization evolves as a correlated response to selection for environmental canalization (a phenomenon called “plastogenetic congruence”; see Computational Models of the Evolution of Robustness).
Scharloo, Willem. 1991. Canalization: Genetic and developmental aspects. Annual Review of Ecology and Systematics 22:65–93.
The first “modern” review of canalization and the phenomenon of genetic assimilation; provides a detailed discussion of Waddington’s early experiments with Drosophila.
Siegal, Mark L., and Jun-Yi Leu. 2014. On the nature and evolutionary impact of phenotypic robustness mechanisms. Annual Review of Ecology, Evolution, and Systematics 45:495–517.
An up-to-date review of mutational robustness and cryptic genetic variation.
Wagner, Andreas. 2005. Robustness and evolvability in living systems. Princeton, NJ: Princeton Univ. Press.
The most comprehensive treatment of robustness currently available. A special emphasis is placed on the mechanistic underpinnings of robustness as well as on the connection between neutrality and robustness.
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