The Genetic Interpretation of Inbreeding Depression and Outbreeding Depression.

Inbreeding with close relatives and outbreeding with members of distant populations can both result in deleterious shifts in the means of fitness-related characters, most likely for very different reasons. Such processes often occur simultaneously and have important implications for the evolution of mating systems, dispersal strategies, and speciation. They are also relevant to the design of breeding strategies for captive populations of endangered species. A general expression is presented for the expected phenotype of an individual under the joint influence of inbreeding and crossbreeding. This expression is a simple function of the inbreeding coefficient, of source and hybridity indices of crossbreeding, and of specific forms of gene action. Application of the model may be of use in identifying the mechanistic bases for a number of evolutionary phenomena such as the shift from outbreeding enhancement to outbreeding depression that occurs with population divergence.