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:''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."〔http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-G/genetic_assimilation.html〕 This usage is unrelated to the usage below.'' Genetic assimilation is a process by which a phenotype originally produced in response to an environmental condition, such as exposure to a teratogen, later becomes genetically encoded via artificial selection or natural selection. Despite superficial appearances, this does not require the inheritance of acquired characters, although epigenetic inheritance could potentially influence the result. Genetic assimilation is merely a method of overcoming the barrier to selection imposed by genetic canalization of developmental pathways. If there is no canalization of a developmental pathway, genetic variation of pathway components results in a continuous spectrum of phenotypes, often distributed in a bell curve. In these cases artificial selection can be done in a straightforward way, by choosing offspring from one end of the curve and using them to breed the next generation. However, when a pathway is strongly canalized, all of the individuals, except perhaps a few at the furthest extreme of the bell curve, physically look the same regardless of their genotype under normal environmental circumstances. However, a given genetic make-up does not predetermine the same outcome under all possible circumstances; instead, it determines a norm of reaction that varies with the environment (phenotypic plasticity). There may be a way to stress an organism so that canalization breaks down, and many aberrant individuals can be selected for further breeding; these are said to phenocopy the desired genetic trait. With several generations of artificial selection in this manner, perhaps aided by mutagenesis, the genetic variation can be reduced to that of the furthest extreme of the original population, until canalization is overwhelmed even under normal environmental conditions. At this point the environmentally induced abnormality has been duplicated genetically. The classic example of genetic assimilation was a 1953 experiment by C. H. Waddington, in which ''Drosophila'' embryos were exposed to ether, producing a bithorax-like phenotype (a homeotic change). Flies which developed halteres with wing-like characteristics were chosen for breeding for 20 generations, by which point the phenotype could be seen without ether treatment. == Genetic assimilation in natural selection == It has not been proven that genetic assimilation occurs in natural evolution, but it is difficult to rule it out from having at least a minor role, and research continues into the question. Mathematical modeling suggests that under certain circumstances, natural selection will favor the evolution of canalization that is designed to fail under extreme conditions. If the result of such a failure is favored by natural selection, genetic assimilation will occur. In the 1960s C. H. Waddington and J. M. Rendel argued for the importance of genetic assimilation in natural adaptation as a means of providing new and potentially beneficial variation to populations under stress. Their contemporary Williams argued that genetic assimilation proceeds at the cost of a loss of developmental plasticity, and should be a minor mechanism. If it occurs frequently, genetic assimilation could contribute to punctuated equilibrium in evolution, as organisms repeatedly evolve systems of canalization, then break out of them under adverse circumstances. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「genetic assimilation」の詳細全文を読む スポンサード リンク
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