Hybrid speciation

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.[11]

Likewise, lions and tigers have historically overlapped in a portion of their range and can theoretically produce wild hybrids: ligers, which are a cross between a male lion and female tiger, and tigons, which are a cross between a male tiger and a female lion; however, tigers and lions have thus far only hybridized in captivity.[12] In both ligers and tigons, the females are fertile and the males are sterile.[12] One of these hybrids (the tigon) carries growth-inhibitor genes from both parents and thus is smaller than either parent species[12] and might in the wild come into competition with smaller carnivores, e.g. the leopard. The other hybrid, the liger, ends up larger than either of its parents: about a thousand pounds (450 kilograms) fully grown.[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 absent).[13]

Some situations may favor hybrid population. 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.[14] A similar situation can be found where closely related species occupy a chain of islands. This will allow any present hybrid population to move into new, unoccupied habitats, avoiding direct competition with parent species and giving a hybrid population time and space to establish.[15][16] Genetics, too, can occasionally favor hybrids. In the Amboseli National Park in Kenya, yellow baboons and anubis baboons regularly interbreed. The hybrid males reach maturity earlier than their pure-bred cousins, setting up a situation where the hybrid population may over time replace one or both of the parent species in the area.[17]

Hybrid speciation in animals is primarily homoploid. 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[21] and Heliconius butterflies,[22][23] as well as some fish,[15] one marine mammal, the clymene dolphin,[24] and a few birds.[25]

One bird is an unnamed form of Darwin's finch from the Galapagos island of Daphne Major, described in 2017 and likely founded in the early 1980s by a male Española cactus finch from Española Island and a female medium ground finch from Daphne Major.[26] Another is the great skua, which has a surprising genetic similarity to the physically very different pomarine skua; most ornithologists now assume it to be a hybrid between the pomarine skua and one of the southern skuas.[27] The golden-crowned manakin was formed 180,000 years ago by hybridization between snow-capped and opal-crowned manakins.[28]

Rapidly diverging species can sometimes form multiple hybrid species, giving rise to a species complex, like several physically divergent but closely related genera of cichlid fishes in Lake Malawi.[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.[29] While hybrid species generally appear rare in mammals,[15] the American red wolf appears to be a hybrid species of the Canis species complex, between gray wolf and coyote.[30] Hybridization may have led to the species-rich Heliconius butterflies,[31] though this conclusion has been criticized.[32]

Saxifraga osloensis, a natural tetraploid hybrid species

Since plants are more tolerant of polyploidy, hybrid species are more common than in animals. Estimates indicate as many as 2–4% of all flowering plants and 7% of all fern species are the results of polyploid hybridization.[33] Many crop species such as wheat are hybrids,[33] and hybridization is an important factor in speciation in some plant groups.[34] Garden flowers in the genus Saxifraga are often hybrids, and a tetraploid natural hybrid, Saxifraga osloenis, is thought to have formed at the end of the last ice age.[35][36]. Homoploid speciation also occurs in plants, and has for example given rise to several species of sunflower.[37][38]