A concept introduced by Waddington in 1953, and having many similarities with the Baldwin effect, in order to account for how a Lamarckian form of inheritance might be mimicked during biological evolution. It expresses the idea that a novel phenotype (also called a phenocopy) induced by particular environmental circumstances becomes genetically fixed in a population if it is adaptive, so that it no longer requires those circumstances for it to be expressed. This outcome is achieved in the following sequence of events: 1. a change the environment (i.e., directional selection) produces an extension of the range of phenotypes, 2. one of the phenotypes produced will be novel and adaptive in the new environment, thus giving it a genetically selective advantage in that environment, 3. after a number of generations, it will be reproduced by some hereditary mechanism independently of external influences. The concept was something of an anathema to supporters of the Modern synthesis, even though it abided by most of its general principles, at least with regard to natural selection. The main reasons were that it smacked of Lamarckism, attempts at identifying its genetic locus were unsuccessful, and there was no guarantee that the initial acquired character would have been adaptive in the circumstances that created it. Nevertheless, genetic assimilation has some interesting features that tally with the current endeavor to establish an evolutionary developmental biology (e.g., variability in developmental pathways, which is known to increase under relaxed selection, could provide the necessary variation for natural selection to act upon).
See Adaptation, Baldwin effect, Diachronic biology, Epigenetics, Evolutionary developmental biology, Lamarckism, Modern synthesis, Theory of natural selection