Hybrid speciation

Hybrid speciation is a form of speciation where hybridization between two different species leads to a new species, reproductively isolated from the parent species. From the 1940s, reproductive isolation between hybrids and their parents was thought to be particularly difficult to achieve and thus hybrid species were thought to be extremely rare. With DNA analysis becoming more accessible in the 1990s, hybrid speciation has been shown to be a fairly common phenomenon, particularly in plants.

In botanical nomenclature, a hybrid species is also called a nothospecies. Hybrid species are by their nature polyphyletic.

A hybrid may occasionally be better fitted to the local environment than the parental lineage and as such natural selection may favor these individuals. If reproductive isolation is subsequently achieved, a separate species may arise. Reproductive isolation may be genetic, ecological, behavioural, spatial, or a combination of these.

If reproductive isolation fails to establish, the hybrid population may merge with either or both parent species. This will lead to an influx of foreign genes in the parent population, a situation called an introgression. Introgression is a source of genetic variation, and can in itself facilitate speciation. There is evidence that introgression is a ubiquitous phenomenon in plants, animals, and even humans, where genetic material from Neanderthals and Denisovans is responsible for much of the immune genes in non-African populations.

For a hybrid form to persist, it must be able to exploit the available resources better than either parent species, which, in most cases, it will have to compete with. While grizzly bears and polar bears may have offspring, a grizzly–polar bear hybrid will likely be less suited in either of the ecological roles than the parents themselves. Although the hybrid is fertile, this poor adaptation would prevent the establishment of a permanent population.

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