A theoretical muddle of the conditional strategy: a comment on Neff and Svensson

Abstract

Within apopulation therearefrequentlyseveraldiscretemorphs. While in somecases, particularly color polymorphisms, variation this can be explained by simple Mendelian modes of inheritance, in many cases the evidence suggests a polygenic pattern of inheritance. The threshold model ofj quantitative genetics, in which discrete morphs are determined by some underlying continuously distributed and a threshold(s) trait of expression, is applied appropriately in these cases. The discretemorphsexhibitedincyclomorphosis, pedogenesis, pedomorphosis, 'protective"dimorphisms, trophic dimorphisms, dimorphism, wing and mating strategies can all be analysed by using this model. Analyses of a wide range of different of threshold show that types traits 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 higherfitness, but that there is a tradeoff, with the relativefitnesses of two morphs being contingent environmental upon conditions. For example, exuberant structures serve that to protect organisms from predators reduce other components of fitness, such as development andfecundity. time Environmental induction is an adaptive norm of reaction, in that cues of current orfuture conditions are used to increase probability the the that morph produced is that which has the highest fitness under expected the conditions. Most models for the evolution of threshold havefocused traits on the phenotype and have not addressed crucial the 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 evolve may in a spatially variable environment are cues if there that allow the organism to assess the type of patch in which it is developing; spatial variation thus is expected to lead to the evolution of inducible phenotypes. Considerablegenetic variation can be maintained mutation, by even in the face of strong directional selection. Frequency-dependent shown selection, to play an important in the role maintenance of phenotypic variation, also be significant maintenance may in the of genetic variatio