Hybrid speciation is a form ofspeciation wherehybridization between two differentspecies leads to a new species, reproductively isolated from the parent species. Previously, reproductive isolation between two species and their parents was thought to be particularly difficult to achieve, and thus hybrid species were thought to be very rare. With DNA analysis becoming more accessible in the 1990s, hybrid speciation has been shown to be a somewhat common phenomenon, particularly in plants.[1][2] Inbotanical nomenclature, a hybrid species is also called anothospecies.[3] Hybrid species are by their naturepolyphyletic.[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. Ifreproductive isolation is subsequently achieved, a separate species may arise. Reproductive isolation may be genetic,ecological,[5] behavioral, 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 into the parent population, a situation called anintrogression. Introgression is a source of genetic variation, and can in itself facilitate speciation. There is evidence that introgression is a ubiquitous phenomenon in plants and animals,[6][7] even in humans,[8] where genetic material fromNeanderthals andDenisovans is responsible for variousimmune genes in non-African populations.[9][10]
For a hybrid form to persist, it must be able to exploit the available resourcesbetter than either parent species, which, in most cases, it will have to compete with.For example: whilegrizzly bears andpolar bears may be able to mate and produce offspring, agrizzly–polar bear hybrid is apparently less- suited in either of the parents' ecological niches than the original parent species themselves. 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.[11]
Likewise,lions andtigers 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, andtigons, 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. theleopard. 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 theAsiatic lion is now restricted to theGir Forest National Park, where tigers are mostly absent).[13]
Some situations may favor hybrid population. One example is rapid turnover of available environment types, like the historical fluctuation of water level inLake Malawi, a situation that generally favors speciation.[14] A similar situation can be found where closely related species occupy achain 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][5] Genetics, too, can occasionally favor hybrids. In theAmboseli National Park in Kenya,yellow baboons andanubis 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.[16]
Genetics are more variable and malleable in plants than in animals, probably reflecting the higher activity level in animals.[citation needed] Hybrids' genetics will necessarily be less stable than those of species evolving through isolation, which explains why hybrid species appear more common in plants than in animals.[citation needed] Many agricultural crops are hybrids with double or even triple chromosome sets. Having multiple sets of chromosomes is calledpolyploidy. Polyploidy is usually fatal in animals where extra chromosome sets upsetfetal development, but is often found in plants.[17] A form of hybrid speciation that is relatively common in plants occurs when an infertile hybrid becomes fertile after doubling of thechromosome number.
Hybridization without change in chromosome number is calledhomoploid hybrid speciation.[1] This is the situation found in most animal hybrids. For a hybrid to be viable, the chromosomes of the two organisms will have to be very similar, i.e., the parent species must be closely related, or else the difference in chromosome arrangement will makemitosis problematic. With polyploid hybridization, this constraint is less acute.[citation needed]
Super-numerary chromosome numbers can be unstable, which can lead to instability in the genetics of the hybrid. The Europeanedible frog appears to be a species, but is actually a triploid semi-permanent hybrid betweenpool frogs andmarsh frogs.[18] In most populations, the edible frog population is dependent on the presence of at least one of the parent species to be maintained, as each individual need two gene sets from one parent species and one from the other. 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.[citation needed]
Hybrid speciation in animals is primarilyhomoploid. While thought not to be very common, a few animal species are the result of hybridization, mostlyinsects such astephritid fruitflies that inhabitLonicera plants[20] andHeliconius butterflies,[21][22] as well as somefish,[15] one marine mammal, theclymene dolphin,[23] a few birds.[24] and certainBufotes toads.[25]
One bird is anunnamed form ofDarwin's finch from the Galapagos island of Daphne Major, described in 2017 and likely founded in the early 1980s by a maleEspañola cactus finch from Española Island and a femalemedium ground finch from Daphne Major.[26] Another is thegreat skua, which has a surprising genetic similarity to the physically very differentpomarine skua; most ornithologists[who?] now assume it to be a hybrid between the pomarine skua and one of the southern skuas.[27] Thegolden-crowned manakin was formed 180,000 years ago by hybridization betweensnow-capped andopal-crowned manakins.[28]
A 2021 DNA study determined that theColumbian mammoth of North America was a hybrid species betweenwoolly mammoths and another lineage, discovered inKrestovka, descended fromsteppe mammoths. The two populations had diverged from the ancestral steppe mammoth earlier in the Pleistocene. Analysis of genetic material recovered from their remains showed that half of the ancestry of the Columbian mammoths originated from the Krestovka lineage and the other half from woolly mammoths, with the hybridization happening more than 420,000 years ago, during theMiddle Pleistocene. This is the first evidence of hybrid speciation obtained from prehistoric DNA.[29][30]
Rapidly diverging species can sometimes form multiple hybrid species, giving rise to aspecies complex, like several physically divergent but closely related genera ofcichlid fishes inLake Malawi.[14] The duck genusAnas (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.[31][32] While hybrid species generally appear rare in mammals,[15] the Americanred wolf appears to be a hybrid species of theCanis species complex, betweengray wolf andcoyote.[33] Hybridization may have led to the species-richHeliconiusbutterflies,[34] though this conclusion has been criticized.[35]
Hybrid speciation occurs when two divergent lineages (e.g., species) with independent evolutionary histories come into contact and interbreed. Hybridization can result in speciation when hybrid populations become isolated from the parental lineages, leading to divergence from the parent populations.
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.Polyploid speciation is commonly observed in plants because their nature allows them to support genome duplications. 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. Polyploids can arise through single step mutations or through triploid bridges. In single step mutations,allopolyploids are the result of unreduced gametes in crosses between divergent lineages. The F1 hybrids produced from these mutations are infertile due to failure of bivalent pairing of chromosomes and segregation into gametes which leads to the production of unreduced gametes by single division meiosis, which results in unreduced, diploid (2N) gametes. Triploid bridges occur in low frequencies in populations and are produced when unreduced gametes combine with haploid (1N) gametes to produce a triploid offspring that can function as a bridge to the formation of tetraploids.[36] In both paths, the polyploid hybrids are reproductively isolated from the parents due to the difference in ploidy. Polyploids manage to remain in populations because they generally experience lessinbreeding depression and have higher self-fertility.[36][37]
Homoploid (diploid) speciation is another result of hybridization, but the hybrids remain diploid. It is less common in plants than polyploid speciation because, without genome duplication, genetic isolation must develop through other mechanisms. Studies on diploid hybrid populations ofLouisiana irises show how these populations occur in Hybrid zones created by disturbances andecotones (Anderson 1949). Novel niches can allow for the persistence of hybrid lineages. For example, established sunflower (Helianthus) hybrid species showtransgressive phenotypes and display genomic divergence separating them from the parent species.[38]
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