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.[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:
Examples[edit]
The first mechanism has the greatest effects on fitness for polyploids, an intermediate effect on translocations, and a modest effect on centric fusions and inversions.[1] Generally this mechanism will be more prevalent in the first generation (F1) after the initial outcrossing when most individuals are made up of the intermediate phenotype.
Examples of the second mechanism include stickleback fish, which developed benthic and limnetic forms when separated. When crosses occurred between the two forms, there were low spawning rates. However, when the same forms mated with each other and no crossing occurred between lakes, the spawning rates were normal. This pattern has also been studied in Drosophila and leaf beetles, where the F1 progeny and later progeny resulted in intermediate fitness between the two parents. This circumstance is more likely to happen and occurs more quickly with selection than genetic drift.[1]
For the third mechanism, examples include poison dart frogs, anole lizards, and cichlid fish. Selection over genetic drift seems to be the dominant mechanism for outbreeding depression.[1]
In plants[edit]
For plants, outbreeding depression represents a partial crossing barrier.[3] Outbreeding depression is not understood well in angiosperms. After observing Ipomopsis aggregata over time by crossing plants that were between 10–100 m apart, a pattern was noticed that plants that were farther away spatially had a higher likelihood of outbreeding depression.[3] Some general takeaways from this were that spatial patterns of selection on plant genotypes will vary in scale and pattern, and outbreeding depression reflects the genetic constitution of "hybrid" progeny and the environments in which the parents and progeny grow.[3] This means that although outbreeding depression cannot be predicted in angiosperms yet, the environment has a role in it.