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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 genotype to produce the same phenotype regardless of variability of its environment. 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".

A recent molecular example was given by Rutherford & Lindquist.^[1] Hsp90 is a chaperone protein, monitoring the correct folding of some polypeptides into proteins. Rutherford & Lindquist heat shocked drosophila embryos, therefore recruiting a vast proportion 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. One possible conclusion is that Hsp90 is buffering mutations: flies have accumulated many mutations, but their effect is suppressed 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 : the amount of accumulated mutations in these flies had overcome the buffering capacity of Hsp90 and these flies had changed their epigenetic valley.

1. ^ Rutherford SL, Lindquist S. (1998). "Hsp90 as a capacitor for morphological evolution". Nature (journal) 396: 336. 

• Developmental biology

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