G. G. Simpson was the first to explain the Baldwin Effect completely in terms of the theory of natural selection. A genetic version of a seemingly non-hereditary adaptation may arise when natural selection acts on the likelihood of having an adaptive trait not just on the trait itself. We present a quantitative model of the Simpson-Baldwin Effect. Organisms in the model have mutable ranges of phenotypic plasticity. The distribution of phenotypes in a population depends largely on the extent of environmental stochasticity. When the environment undergoes intermediate rates of fluctuation, the Simpson-Baldwin effect arises through the interaction of natural selection and mutation on norms of reaction. In a highly volatile environment, organisms benefit from plasticity, and consequently do not experience a Simpson-Baldwin channeling of phenotypic possibility.
