page_title: Genetic assimilation
section_title: Waddington's experiments
hierarchical_section_title: Genetic assimilation / History / Waddington's experiments
caption_attribution_description: English: Normal and mutant cross-veinless Drosophila
context_page_description: Genetic assimilation is a process described by Conrad H. Waddington 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. Waddington stated that genetic assimilation overcomes the barrier to selection imposed by what he called canalization of developmental pathways; he supposed that the organism's genetics evolved to ensure that development proceeded in a certain way regardless of normal environmental variations.
  The classic example of genetic assimilation was a pair of experiments in 1942 and 1953 by Waddington. He exposed Drosophila fruit fly embryos to ether, producing an extreme change in their phenotype: they developed a double thorax, resembling the effect of the bithorax gene. This is called a homeotic change.
context_section_description: 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.
  Waddington carried out a similar experiment in 1953, this time inducing the cross-veinless phenocopy in Drosophila with a heat shock, with 40% of the flies showing the phenotype prior to selection. Again he selected for the phenotype over several generations, applying heat shock each time, and eventually the phenotype appeared even without heat shock.
caption: Normal and cross-veinless Drosophila wings
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