Genetic Assimilation

Genetic assimilation is a process discovered experimentally by C.H. Waddington in the 1950s: a phenotypic trait that initially appears only under environmental stress can, after selection across multiple generations, become expressed in the absence of that stress — as if it had been assimilated into the normal developmental program. Waddington induced cross-veins in Drosophila wings by heat shock, selected for the trait, and after several generations produced flies that expressed it without any heat shock at all. No new mutation had occurred; rather, the selection had uncovered and stabilized genetic variation that was already present but normally hidden by canalization.

The concept is important because it provides a mechanism for Lamarckian-looking evolution within a fully Darwinian framework: environment shapes phenotype (via stress-induced developmental change), selection acts on phenotype, and genetics follows. The environment does not directly change the genome — it instead overloads the buffering system, revealing variation that selection can then fix. This is the direct connection between Homeostasis at the developmental level and evolution at the population level: the tighter the canalization, the larger the stress needed to trigger assimilation, and the more dramatic the release of hidden variation when it occurs.