Outbreeding depression
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In evolutionary biology, Outbreeding depression refers to cases when progeny from crosses between individuals from different populations have lower fitness than progeny from crosses between individuals from the same population. This phenomenon can occur in two ways. First, selection in one population might produce a large body size, whereas in another population small body size might be more advantageous. Gene flow between these populations may lead to individuals with intermediate body sizes, which may not be adaptive in either population. A second way outbreeding depression can occur is by the breakdown of biochemical or physiological compatibilities between genes in the different populations. Within local, isolated populations, alleles are selected for their positive, overall effects on the local genetic background. Due to nonadditive gene action, the same genes may have rather different average effects in different genetic backgrounds--hence, the potential evolution of locally coadapted gene complexes. Offspring between parents from two different populations may have phenotypes that are not good for any environment. It is important to keep in mind that these two mechanisms of outbreeding depression can be operating at the same time. However, determining which mechanism is more important in a particular population is very difficult.
This article incorporates text from a National Park Service website([link]) that is a public domain work of the United States Government.
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[INBREEDING DEPRESSION AND OUTBREEDING DEPRESSION] by Michael Lynch
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