THE EVOLUTION OF THRESHOLD TRAITS IN ANIMALS

Within a population there are frequently several discrete morphs. Whil e in some cases, particularly color polymorphisms, this variation can be explained by simple Mendelian modes of inheritance, in many Gases t he evidence suggests a polygenic pattern of inheritance. The threshold model of quantitative genetics, in which discrete morphs are determin ed by some underlying continuously distributed trail and a threshold(s ) of expression, is applied appropriately in these cares. The discrete morphs exhibited in cyclomorphosis, pedomorphosis, pedogenesis, ''pro tective'' dimorphisms, trophic dimorphisms, wing dimorphism, and matin g strategies can all be analysed by using this model. Analyses of a wi de range of different types of threshold traits show that there is typ ically a large additive genetic component, but that there is also stro ng environmental induction. A review of studies shows that no morph ha s a universally higher fitness, but that there is a tradeoff with the relative fitnesses of two morphs being contingent upon environmental c onditions. For example, exuberant structures that serve to protect org anisms from predators reduce other components of fitness, such as deve lopment time and fecundity. Environmental induction is an adaptive non of reaction, in that cues of current or future conditions are used to increase the Probability that the morph produced is that which has th e highest fitness under the expected conditions. Most models for the e volution of threshold traits have focused on the phenotype and have no t addressed the crucial question of what maintains genetic variation, and hence permits continued evolutionary change. Phenotypic models sho w that noninducible polymorphic variation cannot be maintained by spat ial variation alone, but can be favored in an environment that is temp orally variable. Multiple phenotypes may evolve in a spatially variabl e environment if there are cues that allow the organism to assess the type of patch in which it is developing. thus spatial variation is exp ected to lead to the evolution of inducible phenotypes. Considerable g enetic variation can be maintained by mutation, even in the face of st rong directional selection. Frequency-dependent selection, shown to pl ay an important role in the maintenance of phenotypic variation, may a lso be significant in the maintenance of genetic variation.