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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 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
References
- ^
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.
- ^
Rutherford SL, Lindquist S. (1998).
"Hsp90 as a capacitor for morphological evolution". Nature
396: 336. doi:10.1038/24550.
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