The Evolution of Threshold Traits in Animals

Abstract

Within a population there are frequently several discrete morphs. While in some cases, particularly color polymorphisms, this variation can be explained by simple Mendelian modes of inheritance, in many cases the evidence suggests a polygenic pattern of inheritance. The threshold model of quantitative genetics, in which discrete morphs are determined by some underlying continuously distributed trait and a threshold(s) of expression, is applied appropriately in these cases. The discrete morphs exhibited in cyclomorphosis, pedomorphosis, pedogenesis, "protective" dimorphisms, trophic dimorphisms, wing dimorphism, and mating strategies can all be analyzed by using this model. Analyses of a wide range of different types of threshold traits show that there is typically a large additive genetic component, but that there is also strong environmental induction. A review of studies shows that no morph has a universally higher fitness, but that there is a tradeoff, with the relative fitnesses of two morphs being contingent upon environmental conditions. For example, exuberant structures that serve to protect organisms from predators reduce other components of fitness, such as development time and fecundity. Environmental induction is an adaptive norm of reaction, in that cues of current or future conditions are used to increase the probability that the morph produced is that which has the highest fitness under the expected conditions. Most models for the evolution of threshold traits have focused on the phenotype and have not addressed the crucial question of what maintains genetic variation, and hence permits continued evolutionary change. Phenotypic models show that noninducible polymorphic variation cannot be maintained by spatial variation alone, but can be favored in an environment that is temporally variable. Multiple phenotypes may evolve in a spatially variable environment if there are cues that allow the organism to assess the type of patch in which it is developing; thus spatial variation is expected to lead to the evolution of inducible phenotypes. Considerable genetic variation can be maintained by mutation, even in the face of strong directional selection. Frequency-dependent selection, shown to play an important role in the maintenance of phenotypic variation, may also be significant in the maintenance of genetic variation.