Genetic Assimilation - Related Concepts

Related Concepts

Genetic assimilation generally describes the production of phenotypes with altered or decreased responsiveness to environmental conditions; the phenotype produced under a stressful condition becomes the phenotype for every condition. Genetic accommodation can be used to refer more broadly to changes in gene frequency that result from environmentally induced phenotypes. When used by contrast with genetic assimilation, the term can be applied more specifically to refer to the outcome that may be obtained when selection under stressful conditions is used to obtain a phenotype with increased responsiveness to environmental conditions. For example, H.F. Nijhout found that a black mutant line of Manduca sexta caterpillars sometimes became green under heat-shock conditions; selection of green caterpillars for thirteen generations yielded a polyphenic line that reliably became green under heat-shock, but remained black at cool temperatures. Either genetic assimilation or (other) genetic accommodation can be produced by similar selection procedures, and it may not be possible to predict in advance which phenomenon will occur. The underlying biological basis of these phenomena can be quite similar — temperature sensitive mutations and mutations affecting the activity of a gene without temperature sensitivity can each be produced by a small change in the sequence of a protein.

Genetic compensation describes the situation that occurs when an environmental condition changes the phenotype, but the new phenotype is not favored by selection. The outcome is a genetic change that shifts the expressed phenotype back to its original state despite the altered environment. For example, in salmon, anadromous sockeye populations migrate into the ocean to develop, where they ingest high levels of carotenoids which they use to produce an intense red coloration. "Residuals", salmon which do not enter the ocean, do not receive this nutrition and are a green color. However, they are thought to be the progenitors of nonanadromous kokanee salmon, which despite remaining in freshwater lakes develop an intense red coloration. Similar situations can be described for the pigmentation of tanagers and guppies. Genetic compensation may play a role in speciation by creating genetic incompatibilities between phenotypically similar populations within a species.

Genetic assimilation experiments have been comparatively rare in modern studies, because most geneticists are more interested in relating the activity of a gene to that of other genes. Those relationships are pursued by the study of genetic interaction, which is similar in concept. In a genetic interaction study, the experimenter begins with a strain that has a weak phenotype due to a known mutant allele, and screens flies for second mutations that create a stronger phenotype. Genetic interaction studies are typically used to identify mutant alleles with relatively severe effects, at least in the genetic background of the known mutant allele, which can be readily localized by genetic mapping and further characterized. The objective of these studies is to work out which genes have related functions --- often genes paired in this manner are later shown to code proteins that physically interact within the cell or catalyze sequential steps of a chemical reaction.