Selection in a Fine-Grained Environment

Two models of selection in a fine-grained environment are developed. In the first model the viability of an individual in each part of its life depends on in which environment it is located. In the second model the fitness of an individual is a function of the amount of a product accumulated during its life. The amount of the product accumulated during each part of its life depends on in which environment it is. For both models it is shown that there must be a heterozygotic advantage in order for polymorphism to be maintained. However, under some circumstances a heterozygotic advantage can result from selection in opposite directions in two environments. The applicability of adaptive functions and fitness sets to selection in fine- and coarse-grained environments is also discussed.