Waddington's Experiment

Waddington's experiments on genetic assimilation, conducted in the 1950s on Drosophila melanogaster, are the foundational empirical demonstration that environmentally induced phenotypes can become genetically fixed through selection. Waddington subjected fly embryos to heat shock during a critical developmental window, which induced a cross-veinless wing phenotype in a subset of individuals. He then selected for this trait over 21 generations. By the end of the experiment, a substantial fraction of the population expressed the cross-veinless phenotype without any heat shock — the trait had been genetically assimilated. The experiments showed that the genome contains latent developmental capacity that is normally masked by canalization, and that selection can stabilize this capacity into a constitutive trait. The work was controversial in its time because it appeared to support Lamarckian inheritance, but Waddington correctly interpreted it within a Darwinian framework: the environment reveals variation, selection fixes it. Modern replications have confirmed the basic phenomenon across multiple taxa and stressors. The experiments remain the canonical reference for discussions of genetic assimilation, phenotypic switching, and the role of stress in revealing evolvable variation.