Question: What are the conditions required for natural selection to produce phenotypes specially adapted to the various habitats available in nature?
Model: Assume there are two habitat types and one or two phenotypes of the same or different species. The phenotypes do not recognize any spatial differences among patches of the same habitat type. Possible evolutionary winners can do better in one habitat only by relinquishing some ability in the other.
Results: If only one phenotype is present, it will be an intermediate (unless one of the two habitat types is so rare and unproductive that its effects can be ignored by natural selection). Even if two phenotypes are introduced, natural selection should generally restore monomorphism if habitat selection is not ever favored (e.g. if search costs are high). But if search costs and environmental variation are zero, dimorphism can be expected. And if they are small, then although monomorphism is stable, its basin of attraction is small, and invasion by a second form (such as a sibling species) can provide the discontinuous jump needed to put the system in the other basin of attraction. Once there, dimorphic extremism coevolves. Each successful morph is as specialized as possible on one of the habitats. Competition between the morphs is eliminated. Environmental variation may constrict the basin, but once a point is captured by it, the system approaches dimorphic extremism anyway. In general, whatever promotes the behavior of habitat selection also promotes the evolution of extreme morphologies and physiologies.