Outbreeding depression

In biology, outbreeding depression occurs when individuals from different populations cross and create offspring that have lower fitness than progeny from crosses between individuals from the same population. The concept is opposed to inbreeding depression. Outbreeding depression manifests most significantly in two ways:

• Intermediate genotypes may be disfavored. For example, 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. It might be that, in a certain environment, having either a large or a small body is advantageous, whereas an intermediate-sized body is comparatively disadvantageous.

• There may be a breakdown of biochemical or physiological compatibilities between genes in the different breeding populations. Within local, isolated breeding 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. On the other hand, according to the overdominance hypothesis in genetics, it is believed that certain combinations of alleles (which can only be obtained by outbreeding) are especially advantageous when paired in a heterozygous individual, which is one explanation for the existence of hybrid vigor.

A third, but neutral, effect of outbreeding is the loss of allopatric speciation of a particular group, that lends its distinctness and contributes to the diversity of said types in either group by the exclusive retention of select traits.

Strength of depression

The different mechanisms of outbreeding depression can operate at the same time. However, determining which mechanism is more important in a particular population is very difficult.

Considering the second mechanism of outbreeding depression cited above, individuals from Population A will tend to have alleles that are selected to combine well with alleles common in Population A. However, alleles found in Population A will not have been selected for the quality of crossing well with alleles common in Population B. Therefore outbreeding can undermine vitality by reducing positive epistasis and/or increasing negative epistasis. An extreme case of this type of outbreeding depression is the sterility and other fitness-reducing effects often seen in interspecific hybrids (such as mules), which involves not only different alleles of the same gene (as in distinct populations of a single species) but even different orthologous genes.

Reduced inbreeding depression in first generation hybrids can, in some circumstances, be strong enough to more than make up for outbreeding depression. Because of this, farmers make first-generation hybrids, F1 hybrids. The parents of the F1 hybrids are purebred strains, to improve the uniformity of the offspring. The F1 generation is not used for further breeding because of unpredictable phenotypes in their offspring and because outbreeding depression will remain or worsen.

As a general rule of thumb, hybrid vigor (another way of saying a reduction of inbreeding depression) is strongest in first generation hybrids and gets weaker over time. In contrast, outbreeding depression can be relatively weak in the first generation. Unless there is strong selective pressure, outbreeding depression will increase in power through the further generations as co-adapted gene complexes are broken apart without the forging of new co-adapted gene complexes to take their place.

However, there is usually strong enough selective pressure acting on each generation. In large enough populations, outbreeding depression is not normally a long-term occurrence. Natural selection will usually lead to rapid recovery and often eventually to higher fitness. Unless the F1 hybrid generation is sterile or of very low fitness, selection will act on the higher diversity to adapt to the environment. The hybrid population is likely to go through a temporary decline in fitness for a few generations and then rebound

See also

• Outbreeding
• Heterozygote advantage
• Dominance versus overdominance
• Haldane's Rule
• Miscegenation
• Heterosis
• Inbreeding depression

References

 This article incorporates public domain material from the National Park Service document "Inbreeding depression and outbreeding depression" by Michael Lynch.