Hybrid speciation

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Two species mate resulting in a fit hybrid that is unable to mate with members of its parent species.

Hybrid speciation is a form of

polyphyletic.[4]

Ecology

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,[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 an

Denisovans is responsible for much of the immune genes in non-African populations.[9][10]

Ecological constraints

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. For example: while grizzly bears and polar bears may be able to mate and produce offspring, a grizzly–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,

Gir 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 in

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.[16]

Genetics of hybridization

Genetics are more variable and malleable in plants than in animals, probably reflecting the higher activity level in animals.[

polyploidy. Polyploidy is usually fatal in animals where extra chromosome sets upset fetal 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 the chromosome
number.

Hybridization without change in chromosome number is called

homoploid 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 make mitosis 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 European edible frog appears to be a species, but is actually a triploid semi-permanent hybrid between pool frogs and marsh 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

Closely related Heliconius species

Homoploid hybrid speciation

Hybrid speciation in animals is primarily

Lonicera plants[20] and Heliconius butterflies,[21][22] as well as some fish,[15] one marine mammal, the clymene dolphin,[23] a few birds.[24] and certain Bufotes toads.[25]

One bird is an

pomarine skua; most ornithologists[who?] 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]

A 2021 DNA study determined that the

Middle Pleistocene. This is the first evidence of hybrid speciation obtained from prehistoric DNA.[29][30]

Multiple hybrids during rapid divergence

Rapidly diverging species can sometimes form multiple hybrid species, giving rise to a

gray wolf and coyote.[33] Hybridization may have led to the species-rich Heliconius butterflies,[34] though this conclusion has been criticized.[35]

Hybrid speciation in plants

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.

Polyploid hybrid speciation

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.

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 less inbreeding depression and have higher self-fertility.[36][37]

Homoploid hybrid speciation

Homoploid (diploid) speciation is another result of hybridization, but unlike polyploid speciation, it is observed less commonly because the hybrids are not characterized by a genome duplication and isolation must develop through other mechanisms. In homoploid speciation, the hybrids remain diploid. Studies on diploid hybrid populations of Louisiana irises show how these populations occur in Hybrid zones created by disturbances and ecotones (Anderson 1949). The existence of these novel niches allows for the persistence of hybrid lineages. For example, established sunflower (Helianthus) hybrid species represent transgressive phenotypes and display genomic divergence separating them from the parent species.[38]

See also

References

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  12. ^ a b c d Mott, M. (2005, August 5). Retrieved February 13, 2013, from Liger Facts. Big Cat Rescue
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