CONCEPT

Genetic Assimilation

Waddington's experimental demonstration that a trait first evoked in fruit flies by environmental stress could, under selection, become hereditary—the cleanest biological model for how a flexible, learned response gets baked into fixed structure, and the most precise analogy for what happens when training dispositions pass from the plastic to the structural regime in a neural network.

In 1942, Conrad Waddington subjected fruit fly embryos to brief heat shocks and found that a fraction developed an abnormal wing trait the heat had evoked. He then selected those flies, bred them, shocked their offspring, selected again, and repeated for many generations. After enough rounds, flies began developing the trait without any heat shock at all. A characteristic that originally required an environmental trigger became hereditary, produced by the genome alone. He called it genetic assimilation: the conversion of an acquired, environmentally-evoked response into a built-in, genetically-determined one. The mechanism was not Lamarckian—the heat did not rewrite the genes. It was Darwinian: the capacity to respond with the trait already lurked in standing genetic variation, and selection repeatedly favoring that response gradually lowered the developmental threshold until the trait appeared without the trigger. The parallel to machine learning is

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