Outbreeding depression

In biology, outbreeding depression happens when crosses between two genetically distant groups or populations result in a reduction of fitness.^[1] The concept is in contrast to inbreeding depression, although the two effects can occur simultaneously on different traits.^[2] Outbreeding depression is a risk that sometimes limits the potential for genetic rescue or augmentations.^[1] It is considered postzygotic response because outbreeding depression is noted usually in the performance of the progeny.^[3]

Outbreeding depression manifests in two ways:

• Generating intermediate genotypes that are less fit than either parental form. For example, selection in one population might favor a large body size, whereas in another population small body size might be more advantageous, while individuals with intermediate body sizes are comparatively disadvantaged in both populations. As another example, in the Tatra Mountains, the introduction of ibex from the Middle East resulted in hybrids which produced calves at the coldest time of the year.^[4]
• Breakdown of biochemical or physiological compatibility. Within isolated breeding populations, alleles are selected in the context of the local genetic background. Because the same alleles may have rather different effects in different genetic backgrounds, this can result in different locally coadapted gene complexes. Outcrossing between individuals with differently adapted gene complexes can result in disruption of this selective advantage, resulting in a loss of fitness.