Hybrid speciation

[4] A hybrid may occasionally be better fitted to the local environment than the parental lineage, and as such, natural selection may favor these individuals.

There is evidence that introgression is a ubiquitous phenomenon in plants and animals,[6][7] even in humans,[8] where genetic material from Neanderthals and Denisovans is responsible for much of the immune genes in non-African populations.

So: although the hybrid is fertile (i.e. capable of reproduction and thus theoretically could propagate), this poor adaptation would be unlikely to support the establishment of a permanent population.

[12] No tiger-lion hybrids are known from the wild, and the ranges of the two species no longer overlap (tigers are not found in Africa, and while there was formerly overlap in the distribution of the two species in Asia, both have been extirpated from much of their respective historic ranges, and the Asiatic lion is now restricted to the Gir Forest National Park, where tigers are mostly absent).

One example is rapid turnover of available environment types, like the historical fluctuation of water level in Lake Malawi, a situation that generally favors speciation.

Polyploidy is usually fatal in animals where extra chromosome sets upset fetal development, but is often found in plants.

Also, the male sex determination gene in the hybrids is only found in the genome of the pool frog, further undermining stability.

[19] Such instability can also lead to rapid reduction of chromosome numbers, creating reproductive barriers and thus allowing speciation.

While thought not to be very common, a few animal species are the result of hybridization, mostly insects such as tephritid fruitflies that inhabit Lonicera plants[20] and Heliconius butterflies,[21][22] as well as some fish,[15] one marine mammal, the clymene dolphin,[23] a few birds.

[14] The duck genus Anas (mallards and teals) has a very recent divergence history, many of the species are inter-fertile, and quite a few of them are thought to be hybrids.

[35] Hybrid speciation occurs when two divergent lineages (e.g., species) with independent evolutionary histories come into contact and interbreed.

In cases where the first-generation hybrids are viable but infertile, fertility can be restored by whole genome duplication (polyploidy), resulting in reproductive isolation and polyploid speciation.

Polyploids are considered a new species because the occurrence of a whole genome duplication imposes post-zygotic barriers, which enable reproductive isolation between parent populations and hybrid offspring.

It is less common in plants than polyploid speciation because, without genome duplication, genetic isolation must develop through other mechanisms.