Genetic Assimilation

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

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.

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.