Canalisation

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 (canalization in American English) is a measure of the ability of a population to produce the same phenotype regardless of variability of its environment or genotype. The term canalisation was coined by C. H. Waddington, who also helped explain its developmental mechanisms. He also introduced the epigenetic landscape, in which a canalised trait is illustrated as a valley enclosed by high ridges, safely guiding the phenotype to its "fate".

Canalisation is divided into genetic and environmental canalisation; genetic canalisation refers to distinct genotypes producing the same phenotype, while environmental canalisation refers to the same genotype producing the same phenotype in spite of environmental variation.

Genetic canalization could allow evolutionary capacitance, where genetic diversity accumulates in a population over time, but without changing the organisms' phenotype. This hidden diversity could then be unleashed by extreme changes in the environment, allowing a rapid burst of evolution.^[1]

A recent molecular example was given by Rutherford & Lindquist.^[2] Hsp90 is a chaperone protein, monitoring the correct folding of some polypeptides into proteins. Rutherford & Lindquist heat shocked Drosophila embryos, therefore presumably recruiting a portion of cytoplasmic Hsp90 to respond to the stress. The decrease in the normal monitoring activity of Hsp90 resulted in many morphological changes in the adult flies. These changes would disappear at the next generation in the absence of the stress. A genetic reduction in HSP90 function had similar effects. One possible conclusion is that Hsp90 is buffering mutations: flies have accumulated many mutations, but their effect is masked by Hsp90. To test this hypothesis, they crossed flies displaying morphological changes, mimicking natural selection during big environmental changes. The resulting flies displayed morphological changes even in the absence of heat shock or mutant alleles : the amount of accumulated mutations in these flies had overcome the buffering capacity of Hsp90 and these flies had changed their epigenetic valley. This, then, is an example of genetic canalisation.

See also

• Developmental biology

References

1. ^ Bergman A, Siegal ML (July 2003). "Evolutionary capacitance as a general feature of complex gene networks". Nature 424 (6948): 549–52. doi:10.1038/nature01765. PMID 12891357. 
2. ^ Rutherford SL, Lindquist S. (1998). "Hsp90 as a capacitor for morphological evolution". Nature 396: 336. doi:10.1038/24550. 

External links

• Abstract of A Population Genetic Theory of Canalization, by Günter P. Wagner, Ginger Booth, and Homayoun Bagheri-Chaichian

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