Outbreeding depression occurs when hybridization of two species or mating of two populations of the same species separated by a great distance results in offspring that have lower fitness or reproductive success than the parent species. This process can occur in two ways, first locally adapted species can have their gene pool diluted by immigrants resulting in an intermediate or unfavorable phenotype (Lynch, 1991). An example of this is fish populations that receive regular immigrants from hatcheries. Hatchery fish are generally selected for large body size, gene flow between these immigrants and the native population can lead to a larger body size possibly unfavorable for the ecosystem (Fenster, 2000). The second mechanism for outbreeding depression is when locally co-adapted gene complexes get broken down by mating between individuals from differing genetic backgrounds. When the two sets of alleles adapted for positive effects in their respective environments are introduced to new genetic backgrounds the resulting phenotypes may not be favorable for any ecosystem. Outbreeding that results in negative effects results in outbreeding depression (Lynch, 1991).
Outbreeding doesn't always result in negative effects, it can restore heterozygosity to populations and act as a force to combat inbreeding depression. When outbreeding has positive effects it is known as outbreeding enhancement. This can be seen in many outbreeding situations as the first generation resulting from outbreeding is often more competitive for resources, but they have lower reproductive success. Following generations lose competitive advantage and can suffer an even greater loss in reproductive success (Edmands, 1999). Low levels of natural outbreeding preserve heterozygosity within populations. However outbreeding at high levels or between genetically distinct populations of the same or similar species can dilute and separate locally adapted alleles, resulting in loss of fitness and reproductive success negatively affecting parent populations (Lynch, 1991; Fester 2000).
1.) Lynch, M. (1991). The genetic interpretation of inbreeding depression and outbreeding depression. Evolution, 622-629.
2.) Edmands, S. (1999). Heterosis and outbreeding depression in interpopulation crosses spanning a wide range of divergence. Evolution, 1757-1768.
3.) Fenster, C. B., & Galloway, L. F. (2000). Inbreeding and outbreeding depression in natural populations of Chamaecrista fasciculata (Fabaceae).Conservation Biology, 14(5), 1406-1412.