ON THE EVOLUTION OF PHENOTYPIC PLASTICITY IN A SPATIALLY HETEROGENEOUS ENVIRONMENT

A genetic model for the dynamics of a quantitative trait is analyzed i n terms of gene frequencies, linkage disequilibria, and environmental effects on the trait. In a randomly mating population, at each generat ion progeny move to niches where they are subject to weak Gaussian sel ection on the trait, with different fitness levels in the different ni ches. Initially, the variability of the trait is due to additive loci with heterozygous homeostasis. The evolution of plasticity is then des cribed in terms of the invasion of the population by genetic modifiers that may epistatically affect the trait, its optimum in each niche, t he strengths of selection, and other parameters characteristic of the niches. We show that the evolution of trait means within niches depend s on the overall evolution in the whole system, and in general, optimu m phenotypic values are not attained. The reaction norm and genotype-e nvironment interaction may evolve even if the only effects of the modi fier are on individual rates of dispersal, or on fitness effects resul ting from the different environments in the different niches; this evo lution does not require that the modifier affect parameters that influ ence the values of the trait. It is conjectured that in the least freq uently reached niches with low fitness levels, the deviations from the trait optima should be larger than those in more commonly experienced and less stringent niches. Our analysis makes explicit the different contribution of between- and within-niche effects on the evolutionary dynamics of phenotypic plasticity in heterogeneous environments.