Genetic assimilation

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 (Lamarckian) inheritance of acquired characters, although epigenetic inheritance could potentially influence the result. Genetic assimilation overcomes the barrier to selection imposed by genetic canalization of developmental pathways.

The classic example of genetic assimilation was a pair of experiments in 1942 and 1953 by Conrad H. Waddington, in which Drosophila fruit fly embryos were exposed to ether, producing a bithorax-like phenotype (a homeotic change). Flies which developed halteres (the modified hindwings of true flies, used for balance) with wing-like characteristics were chosen for breeding for 20 generations, by which point the phenotype could be seen without ether treatment. More recent evidence appears to confirm the existence of genetic assimilation in evolution.

Conrad H. Waddington's classic experiment (1942) induced an extreme environmental reaction in the developing embryos of Drosophila. In response to ether vapor, a proportion of embryos developed a radical phenotypic change, a second thorax. At this point in the experiment bithorax is not innate; it is induced by an unusual environment. Waddington then repeatedly selected Drosophila for the bithorax phenotype over some 20 generations. After this time, some Drosophila developed bithorax without the ether treatment.

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