Chapter 11 - Phenotypic Variation in Larval Development and Evolution: Polymorphism, Polyphenism, and Developmental Reaction Norms

Abstract

This chapter provides an overview of the evolutionary causes and consequences of phenotypic variation. The focus is on phenotypic variation in larval forms, especially invertebrates. A phenotypic character may be determined entirely by an individual's genotype. In this case, phenotypic variation in a population arises because of genetic differences among individuals and is called genetic polymorphism. At the other extreme, such as the Pheidologeton ants, an individual may have the capacity to develop along several discretely different trajectories, and its eventual phenotype depends upon external stimuli received during development (polyphenism). At intermediate levels, an individual's developmental trajectory may depend on both genotype and environmental conditions and is described by a developmental reaction norm. The term polyphenism is used to distinguish environmentally induced phenotypic variation from genetic polymorphisms: “the occurrence of several phenotypes in a population, the differences between which are not the result of genetic differences.” The term polyphenism embraces a large number of phenomena, including morphological, physiological, and behavioral variation. Reaction norms explicitly describe the effects of both genotypic and environmental effects on an individual's phenotype. Reaction norms are maps of the developmental response (that is, the phenotype) of genotypes as a function of the environment.