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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).
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