In order to study the evolution of these background features, models have been examined in which they are under the control of a modifier gene. It holds for several cases but fails for others. Other classes of models exist where reduction is not the only outcome, and other complex behavior can occur. This thesis developes a new approach to the problem. The central idea is that besides selection, another basic process, transformation, is occurring in deterministic population genetics models.
Transformation is any process that results in offspring being of different types from their parents.
This idea is formalized as transition probabilities from parent types to offpring types. Phenomena such as genetic load in populations at equilibrium, gene frequency cycling, and decreasing mean fitness, are shown to result from the presence of transformation. Modifier gene models concern the evolution of the transformations themselves. By using a general formulation of transformations, modifiers are treated in a unified way, including modifiers of gene conversion, unequal crossing over, sexual reproduction, and cultural transmission. A principle is offered to explain when it does not operate.