The genetics of phenotypic plasticity. VII. Evolution in a spatially‐structured environment

Previous models of the evolution of phenotypic plasticity have, for the most part, not considered the effects of genetic architecture and spatial structure. I examine those factors with an individual‐based simulation model. With regard to genetic architecture, I considered how the presence of different types of loci would affect medium‐term evolutionary outcomes. The types of loci differed in how the environment determined phenotypic expression and included loci that were insensitive to the environment (non‐plastic loci), sensitive in a linear fashion, and sensitive in a quadratic fashion (both plastic loci). With regard to spatial structure, I investigated the affects of migration patterns. These simulations demonstrated that two general conditions are necessary for phenotypic plasticity to be selected. (1) The environment must have a strong influence on genotypic expression. (2) The between‐generation changes in the environment must be large and predictable, in the current instance because of migration in a spatially‐structured (clinal) environment. Responses to selection were not simple, however. Rarely were pure strategies — genetic specialization or phenotypic plasticity — selected for. Instead, the existence of multiple types of loci led to mixed genetic outcomes. The result of this mixed outcome were individuals with reaction norms that were less steep than the optimal reaction norm (when non‐plastic and linear‐plastic loci were present) or individuals with curved reaction norms when the optimal reaction norms was linear (when all three types of loci were present). A pure plasticity strategy had the highest global fitness because plastic individuals would match the optimal phenotype everywhere. The reason that the metapopulation did not achieve this global fitness optimum is that local selection is stronger than global selection. Each deme is driven to a local fitness peak based on the combined, locally additive effects of the non‐plastic and plastic loci. Plasticity is only selected globally, so plasticity becomes more highly favored with high migration rates. This effect was greatest in parts of the cline where the plasticity loci were not being expressed and, thus, not locally selected upon. That is, in these demes local selection was weak or absent allowing global fitness effects to predominate.