Fig. 2. The concept of genetic assimilation, seen in the modern light of reaction norms and phenotypic plasticity. The population is initially occupying one environment (A), although there is an unexpressed capacity for plasticity, should the environment change. If the environment does change (B), the pre-existing reaction norm allows the population to persist, producing a novel phenotype with no initial genetic change. Finally, if natural selection keeps operating only in the new environment (C), the novel phenotype may become genetically fixed (assimilated), and the original reaction norm may lose plasticity, for example because of drift or costs associated with maintaining plasticity when it is not favored by natural selection (because the old environment is no longer experienced).