Canalisation (genetics) - AbsoluteAstronomy.com

Canalisation is a measure of the ability of a population

Population

A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

to produce the same phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

regardless of variability of its environment or genotype

Genotype

The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...

. In other words, it means robustness. The term canalisation was coined by C. H. Waddington. 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

Epigenetic landscape

Epigenetic landscape is a metaphor for biological development. Its originator, Conrad Hal Waddington, said that cell fates were established in development much like a marble rolls down to the point of lowest local elevation...

, 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.

Genetic assimilation

Waddington used the concept of canalisation to explain his experiments on genetic assimilation

Genetic assimilation

Note: Genetic assimilation is sometimes used to describe "eventual extinction of a natural species as massive pollen flow occurs from another related species and the older crop becomes more like the new crop." This usage is unrelated to the usage below....

. In these experiments, he exposed Drosophila

Drosophila melanogaster

Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

pupa

Pupa

A pupa is the life stage of some insects undergoing transformation. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, going through four life stages; embryo, larva, pupa and imago...

e to heat shock. This environmental disturbance caused some flies to develop a crossveinless phenotype. He then selected

Artificial selection

Artificial selection describes intentional breeding for certain traits, or combination of traits. The term was utilized by Charles Darwin in contrast to natural selection, in which the differential reproduction of organisms with certain traits is attributed to improved survival or reproductive...

for crossveinless. Eventually, the crossveinless phenotype appeared even without heat shock. Through this process of genetic assimilation

Genetic assimilation

, an environmental 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

Genetic assimilation

using only quantitative genetics

Quantitative genetics

Quantitative genetics is the study of continuous traits and their underlying mechanisms. It is effectively an extension of simple Mendelian inheritance in that the combined effects of one or more genes and the environments in which they are expressed give rise to continuous distributions of...

and a threshold model, with no reference to the concept of canalisation. However, theoretical models that incorporate a complex genotype-phenotype map have found evidence for the evolution of phenotypic robustness contributing to genetic assimilation, 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 hypothesis

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

Mutational robustness

Mutational robustness describes the extent to which an organism’s phenotype remains constant in spite of mutation. Natural selection can directly induce the evolution of mutational robustness only when mutation rates are high and population sizes are large...

. It has been suggested that different perturbations have congruent effects on development taking place on an epigenetic landscape. This could, however, depend on the molecular mechanism responsible for robustness, and be different in different cases.

Evolutionary capacitance

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

Evolvability

Evolvability is defined as the capacity of a system for adaptive evolution. Evolvability is the ability of a population of organisms to not merely generate genetic diversity, but to generate adaptive genetic diversity, and thereby evolve through natural selection.In order for a biological organism...

in a fluctuating environment. Genetic canalization could allow for evolutionary capacitance

Evolutionary capacitance

Just as electric capacitors store and release charge, by analogy evolutionary capacitance is the storage and release of variation. Living systems are robust to mutations. This means that living systems accumulate genetic variation without the variation having a phenotypic effect...

, 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.

Genetic assimilation

Congruence hypothesis

Evolutionary capacitance

See also