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Hybrid speciation is a form of speciation wherein hybridization between two different closely related species leads to a novel 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 considered to be extremely rare. With DNA analysis becoming more accessible in the 1990s, hybrid species have been showed to be a fairly common phenomenon, particularly in plants.[1][2]
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A hybrid may have a distinct trait (phenotype). This phenotype may in very rare cases be better fitted to the local environment than the parental lineage and as such natural selection may favor these individuals. If reproductive isolation subsequently is achieved, it will lead to a separate species. The reproductive isolation may be genetic, ecological, behavioural or spatial, or a combination.
For the hybrid individual to form have superior fitness, it will generally have to be able to exploit the available resources better than either parent species, which it in most cases will have to compete with. While grizzly bears and polar bears may have offspring, a grizzly–polar bear hybrid will be less suited in either of the ecological roles than the parents themselves. While the hybrid is fertile, this will stop the establishment of a permanent population.[3] Likewise, lions and tigers historically overlapped in much of their range and can produce hybrids, the fertile hybrids (the tiglons) are smaller than either parent species and might in the wild come into competition with smaller carnivores, e.g. the leopard. No wild tiger-lion hybrids are known from the wild.
Some situations may favour 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 favour speciation.[4] A similar situation can be found where closely relates species occupy a chain of islands. This will allow any present hybrid population to move into a new unoccupied habitats, avoiding direct competition with parent species and giving a hybrid population time and space to establish.[5]
Many agricultural crops are hybrids with double or even triple chromosome sets. Having multiple sets of chromosomes is called polyploidy (or polyploidity). Polyploidy is usually fatal in animals where extra chomosome sets upset fetal development, but is often found in plants.[6] 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 the difference in chromosome arrangement will make mitosis problematic. With polyploid hybridization, this constraint is less acute.
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 triploid semi-permanent hybrids between pool frogs and marsh frogs.[7] In most populations, the edible frog population is dependent on the presence of at least one of the parents species to be maintained as each individial 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.[8] Such instability can also lead to rapid reduction of chromosome numbers, creating reproductive barriers and thus allowing speciation.
Hybrid speciation in animals is primarily homoploid. While not very common, a few animal species have been recognized as being the result of hybridization, mostly insects and fish.[5] The Lonicera fly is an example of a novel animal species that resulted from natural hybridization. The great skua has a surprising genetic similarity to the physically dissimilar pomarine skua, and most ornithologists now assume the great skua is a hybrid species between the pomarine skua and one of the northern skua species.[9]
Rapidly diverging species can sometimes form multiple hybrid species, giving rise to a species complex, like several physically divergent by closely related genera of chiclid fishes in Lake Malawi.[4] While hybrid species generally appear rare in mammals[5], the American red wolf too appear to be a hybrid species of the Canis species complex, between gray wolf and coyote.[10] This is also hypothesized to have lead to the species rich Heliconius butterflies,[11] though the conclusion has been criticized.[12]
With plants being more tolerant of polyploidity, hybrid species are a lot more common in plants than in animals. Estimates indicate as much as 2-4% of all flowering plants and 7% of all fern species are the results of polyploid hybridization.[13] Many of the crop species are hybrids,[13] and hybridization is an important factor in speciation in some plant groups.[14] Hybrids of the flower genus Saxifraga is commonly used in gardening, and a tetraploid natural hybrid, Saxifraga osloenis is estimated to have formed at the end of the last ice age.[15][16]
Homoploid speciation has given rise to several species of sunflower.[17] [18]
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