Chapter 6 - Constraints on Variation from Genotype through Phenotype to Fitness

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Constraints on variation take place on multiple levels. At the source of variation, the intrinsic rates, biases, and genetic requirements of mutational mechanisms limit the range of possible genetic modifications. Moving up, the map from genes to traits is not perfect. Because of the influence of the environment, phenotypic variation can arise without genotypic variation; because of buffering within and among genes, genotypic variation can exist without corresponding phenotypic variation. This latter phenomenon—genetic robustness—significantly constrains the production of phenotypic novelty. Integrating across all three levels, natural selection itself can limit variation. A mutation may simply be deleterious and therefore be eliminated rapidly by natural selection. Moreover, if a gene is involved in the production of multiple phenotypes, then a mutation may produce innocuous or beneficial variation in the trait of interest while damaging another trait. Natural selection may eliminate the mutation by virtue of the pleiotropic deleterious effects and thereby constrain the variation possible in the trait of interest. This chapter explores these three levels of constraints on variation and refers to them as mechanistic, epistatic, and viability constraints. It considers the large bodies of theoretical and experimental research on these topics and generates further intuition using a realistic genotype–phenotype model that has recently provided profound insights into the nature of phenotypic variation.