Genetic assimilation
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.
More recent evidence appears to confirm the existence of genetic assimilation in evolution; in yeast, when a stop codon is lost by mutation, the reading frame is preserved much more often than would be expected.
[8] 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.
[10] Other evolutionary biologists have agreed that assimilation occurs, but give a different, purely quantitative genetics explanation in terms of Darwin's natural or artificial selection.
[12] In 2017, L. Fanti and colleagues replicated Waddington's experiments, but included DNA sequencing, revealing that the wing phenotypes were due to mutational events, small deletions and the insertions of transposable elements that were mobilised by the heat exposure.
[4] The evolutionary biologists Theodosius Dobzhansky and Ernst Mayr both thought that Waddington was using genetic assimilation to support so-called Lamarckian inheritance.
They denied that the inheritance of acquired characteristics had taken place, and asserted that Waddington had simply observed the natural selection of genetic variants that already existed in the study population.
[14] Waddington himself interpreted his results in a Neo-Darwinian way, particularly emphasizing that they "could bring little comfort to those who wish to believe that environmental influences tend to produce heritable changes in the direction of adaptation.
In particular, according to Wilkins, Waddington felt that the Neo-Darwinians badly neglected the phenomenon of extensive gene interactions and that the 'randomness' of mutational effects, posited in the theory, was false.
[18] Waddington did not deny the threshold-based conventional genetic interpretation of his experiments, but regarded it "as a told to the children version of what I wished to say" and considered the debate to be about "mode of expression, rather than of substance".
In the 1960s, Waddington and his colleague the animal geneticist 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, enabling them to evolve rapidly.
