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Trait-scale-bimodal

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

Canalisation (or canalization) is a measure of the ability of a population to produce the same phenotype regardless of variability of its environment or genotype. In other words, it means robustness. The term canalisation was coined by C. H. Waddington, who used the word 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 the 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 epigenetic landscape, in which the state of an organism rolls "downhill" during development. In this metaphor, a canalised trait is illustrated as a valley 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 assimilationEdit

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 this observation of 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.

Congruence hypothesisEdit

Neither canalisation nor robustness are simple quantities to quantify: it is always necessary to specify which trait is canalised/robust to which perturbations. For example, perturbations can come either from the environment or from mutations. It has been suggested that different perturbations have congruent effects on development taking place on an epigenetic landscape.[10][11][12][13][14] This could, however, depend on the molecular mechanism responsible for robustness, and be different in different cases.[15]

Evolutionary capacitanceEdit

The canalisation metaphor suggests that phenotypes are very robust to small perturbations, for which development does not exit the canal, and rapidly returns back down, with little effect on the final outcome of development. But perturbations whose magnitude exceeds a certain threshold will break out of the canal, moving the developmental process into uncharted territory. Strong robustness up to a limit, with little robustness beyond, is a pattern that could increase evolvability in a fluctuating environment.[16] Genetic canalization could allow for evolutionary capacitance, where genetic diversity outside the canal accumulates in a population over time, sheltered from natural selection because it does not normally affect phenotypes. This hidden diversity could then be unleashed by extreme changes in the environment or by molecular switches, releasing previously cryptic genetic variation that can then contribute to a rapid burst of evolution.

See alsoEdit

ReferencesEdit

  1. Waddington CH (1942). Canalization of development and the inheritance of acquired characters. Nature 150 (3811): 563–565.
  2. Waddington CH (1957). The strategy of the genes, George Allen & Unwin.
  3. Waddington CH (1953). Genetic assimilation of an acquired character. Evolution 7 (2): 118–126.
  4. Stern C (1958). Selection for subthreshold differences and the origin of pseudoexogenous adaptations. American Naturalist 92 (866): 313–316.
  5. Bateman KG (1959). The genetic assimilation of the dumpy phenocopy. American Naturalist 56: 341–351.
  6. Scharloo W (1991). Canalization – genetic and developmental aspects. Annual Reviews in Ecology and Systematics 22: 65–93.
  7. Falconer DS, Mackay TFC (1996). Introduction to Quantitative Genetics, 309–310.
  8. Siegal ML, Bergman A (2002). Waddington's canalization revisited: Developmental stability and evolution. Proceedings of the National Academy of Sciences of the United States of America 99 (16): 10528–10532.
  9. Masel J (2004). Genetic assimilation can occur in the absence of selection for the assimilating phenotype, suggesting a role for the canalization heuristic. Journal of Evolutionary Biology 17 (5): 1106–1110.
  10. Meiklejohn CD, Hartl DL (2002). A single mode of canalization. Trends in Ecology & Evolution 17: e9035.
  11. Ancel LW, Fontana W (2000). Plasticity, evolvability, and modularity in RNA. Journal of Experimental Zoology 288 (3): 242–283.
  12. Szöllősi GJ, Derényi I (2009). Congruent Evolution of Genetic and Environmental Robustness in Micro-RNA. Molecular Biology & Evolution 26 (4): 867–874.
  13. Wagner GP, Booth G Bagheri-Chaichian H (1997). A population genetic theory of canalization. Evolution 51 (2): 329–347.
  14. Lehner B (2010). Genes Confer Similar Robustness to Environmental, Stochastic, and Genetic Perturbations in Yeast. PLoS ONE 5 (2): 468–473.
  15. Masel J Siegal ML (2009). Robustness: mechanisms and consequences. Trends in Genetics 25 (9): 395–403.
  16. Eshel,I. Matessi, C. (1998). Canalization, genetic assimilation and preadaptation: A quantitative genetic model. Genetics 4: 2119–2133.


The development of phenotype
Key concepts: Genotype-phenotype distinction | Norms of reaction | Gene-environment interaction | Heritability | Quantitative genetics
Genetic architecture: Dominance relationship | Epistasis | Polygenic inheritance | Pleiotropy | Plasticity | Canalisation | Fitness landscape
Non-genetic influences: Epigenetic inheritance | Epigenetics | Maternal effect | dual inheritance theory
Developmental architecture: Segmentation | Modularity
Evolution of genetic systems: Evolvability | Mutational robustness | Evolution of sex
Influential figures: C. H. Waddington | Richard Lewontin
Debates: Nature versus nurture
List of evolutionary biology topics
Basic topics in evolutionary biology(edit)
Processes of evolution: evidence - macroevolution - microevolution - speciation
Mechanisms: selection - genetic drift - gene flow - mutation - phenotypic plasticity
Modes: anagenesis - catagenesis - cladogenesis
History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis
Subfields: population genetics - ecological genetics - human evolution - molecular evolution - phylogenetics - systematics - evo-devo
List of evolutionary biology topics | Timeline of evolution | Timeline of human evolution
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