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Genetic Assimilation

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Synonyms: Assimilation, Genetic

Genetic Assimilation is an evolutionary concept developed by Conrad Hal Waddington. It is explains with Darwinian principles how acquired characters become inheritable. Probably because of his leftist political views his ideas never became widely accepted.

General confusion

On genetic assimilation exists a not so instructive instructive wikipedia page. I didn't read the original publication. I have no access. I found many quotations that leave the matter more or less dubious.

Among those who tried to summarize it is Mary Jane West-Eberhard[1],

"

Genetic assimilation treated environmentally induced traits as polygenic threshold traits, which could spread if positive selection lowered the threshold for their expression.

"

Jonathan M W Slack [2], and

"

This is a Darwinian mechanism that mimics the inheritance of acquired characters.
...
His key idea was that a qualitative morphological change can be produced by an external treatment, but the competence to respond to the treatment is a quantitative variable that depends on many
loci. If the treatment is applied, and a proportion of individuals show the morphological effect, then selective breeding from this group can eventually produce a population in which the morphological change arises spontaneously without the treatment.

"

Christian Braendle and Thomas Flatt [3].

"

Genetic assimilation is the evolutionary process by which a phenotype produced specifically in response to some environmental stimulus, such as a stressor, becomes stably expressed independently of the evoking environmental effect. How does this process of assimilation work?

First, in the absence of an environmental stimulus, a particular threshold trait is stably expressed, and phenotypic deviants remain cryptic because the environmental threshold for their expression is too high.

Second, in the presence of an environmental stimulus, previously cryptic genetic variation for the threshold trait is uncovered and the threshold for the expression of deviant phenotypes not seen under normal conditions is lowered.

Third, selection in the presence of the environmental factor enriches the previously cryptic alleles determining the trait.

Eventually, these alleles become so frequent that the expression of the trait overcomes the higher threshold in the absence of the environmental stimulus.

Thus, genetic assimilation transforms an environmentally induced (phenotypically plastic) trait into a phenotype which is stably expressed without the eliciting environmental stimulus: the genetically assimilated phenotype is no longer plastic, but exhibits a genetically fixed response independent of the environmental conditions, a phenomenon called canalization.

"

If anyone can make something reasonable out of it, I can't. Therefore let's start with the phenomenon Waddington tried to explain.

Waddington Experiment

Conrad Hal Waddington performed several experiments to provide evidence in support of genetic assimilation, a Darwinian concept to explain inheritance of acquired characters. The one described by Jonathan M W Slack is illustrated here [1].

Fruit flies Drosophila have a small vein crossing the middle their wings. This crossvein can be made to disappear in some individuals by brief hight temperature exposure during pupation. If individuals that show the crossveinless phenotype are selected and the heat treatment and selection steps repeated several times, we end up with fruit flies that eventually show the crossveinless phenotype without any heat treatment.

Waddington Experiment

The crossing vein is symbolized by a red line on the wings of the fly left. If their pupae are exposed to heat a generation results in which some of the individuals show a crossveinless phenotype. If these individuals are selected and their pupae exposed exposed to heat again after a few generations a some individuals show the crossveinless phenotype even if their pupae have not been exposed to heat (green arrows).

References

1. Jonathan M W Slack, Conrad Hal Waddington: the last Renaissance biologist?, Nature Reviews. Genetics 3, Nr. 11 (November 2002): 889-895.

References

1. Mary Jane West-Eberhard, Toward a Modern Revival of Darwin’s Theory of Evolutionary Novelty, Philosophy of Science 75, Nr. 5 (Dezember 1, 2008): 899-908.
2. Jonathan M W Slack, Conrad Hal Waddington: the last Renaissance biologist?, Nature Reviews. Genetics 3, Nr. 11 (November 2002): 889-895.
3. Christian Braendle and Thomas Flatt, A role for genetic accommodation in evolution?, BioEssays 28, no. 9 (September 1, 2006): 868-873.


Tags: Evolution Genetics Theory

 
   

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