Hybrid speciation is a form of

speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

where hybridization between two different

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

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. In

botanical nomenclature Botanical nomenclature is the formal, scientific naming of plants. It is related to, but distinct from taxonomy (biology), taxonomy. Plant taxonomy is concerned with grouping and classifying plants; Botany, botanical nomenclature then provides na ...

, a hybrid species is also called a nothospecies. Hybrid species are by their nature

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

Ecology

A hybrid may occasionally be better fitted to the local environment than the parental lineage, and as such,

natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...

may favor these individuals. If reproductive isolation is subsequently achieved, a separate species may arise. Reproductive isolation may be genetic, ecological, 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

introgression Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Introg ...

. 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, even in humans, where genetic material from

Neanderthals Neanderthals ( ; ''Homo neanderthalensis'' or sometimes ''H. sapiens neanderthalensis'') are an extinction, extinct group of archaic humans who inhabited Europe and Western and Central Asia during the Middle Pleistocene, Middle to Late Plei ...

and

Denisovans The Denisovans or Denisova hominins ( ) are an extinct species or subspecies of archaic human that ranged across Asia during the Lower Paleolithic, Lower and Middle Paleolithic, and lived, based on current evidence, from 285 thousand to 25 thou ...

is responsible for much of the immune genes in non-African populations.

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 bear The grizzly bear (''Ursus arctos horribilis''), also known as the North American brown bear or simply grizzly, is a population or subspecies of the brown bear inhabiting North America. In addition to the mainland grizzly (''Ursus arctos horr ...

s and

polar bear The polar bear (''Ursus maritimus'') is a large bear native to the Arctic and nearby areas. It is closely related to the brown bear, and the two species can Hybrid (biology), interbreed. The polar bear is the largest extant species of bear ...

s 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. Likewise,

lion The lion (''Panthera leo'') is a large Felidae, cat of the genus ''Panthera'', native to Sub-Saharan Africa and India. It has a muscular, broad-chested body (biology), body; a short, rounded head; round ears; and a dark, hairy tuft at the ...

s and

tiger The tiger (''Panthera tigris'') is a large Felidae, cat and a member of the genus ''Panthera'' native to Asia. It has a powerful, muscular body with a large head and paws, a long tail and orange fur with black, mostly vertical stripes. It is ...

s 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

tigon The tigon is a hybrid offspring of a male tiger (''Panthera tigris'') and a female lion, or lioness (''Panthera leo''). In both ligers and tigons, the females are fertile and the males are sterile.Mott, M. (2005, August 5). Retrieved February 13, 2013, fro
Liger Facts. Big Cat Rescue
/ref> One of these hybrids (the tigon) carries growth-inhibitor genes from both parents and thus is smaller than either parent species 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. 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). 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 Lake Malawi, also known as Lake Nyasa in Tanzania and Lago Niassa in Mozambique, () is an African Great Lakes, African Great Lake and the southernmost lake in the East African Rift system, located between Malawi, Mozambique and Tanzania. It is ...

, a situation that generally favors speciation. 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. 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.

Genetics of hybridization

Genetics are more variable and malleable in plants than in animals, probably reflecting the higher activity level in animals. 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. Many agricultural crops are hybrids with double or even triple chromosome sets. Having multiple sets of chromosomes is called

polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired sets of (Homologous chromosome, homologous) chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning ...

y. Polyploidy is usually fatal in animals where extra chromosome sets upset fetal development, but is often found in plants. A form of hybrid speciation that is relatively common in plants occurs when an infertile hybrid becomes fertile after doubling of the

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

number. Hybridization without change in chromosome number is called homoploid hybrid speciation. 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 Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

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 a triploid semi-permanent hybrid between pool frogs and marsh frogs.Frost, Grant, Faivovich, Bain, Haas, Haddad, de Sá, Channing, Wilkinson, Donnellan, Raxworthy, Campbell, Blotto, Moler, Drewes, Nussbaum, Lynch, Green, and Wheeler 2006. The amphibian tree of life. Bulletin of the American Museum of Natural History. Number 297. New York. Issued March 15, 2006. 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. Such instability can also lead to rapid reduction of chromosome numbers, creating reproductive barriers and thus allowing speciation.

Hybrid speciation in animals

Homoploid hybrid speciation

Hybrid speciation in animals is primarily homoploid. While thought not to be very common, a few animal species are the result of hybridization, mostly

insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

s such as tephritid fruitflies that inhabit '' Lonicera'' plants and '' Heliconius'' butterflies, as well as some

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

, one marine mammal, the clymene dolphin, a few birds. and certain '' Bufotes'' toads. One bird is an unnamed form of

Darwin's finch Darwin's finches (also known as the Galápagos finches) are a group of about 18 species of passerine birds. They are well known for being a classic example of adaptive radiation and for their remarkable diversity in beak form and function. The ...

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. 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. The golden-crowned manakin was formed 180,000 years ago by hybridization between snow-capped and opal-crowned manakins. A 2021 DNA study determined that the Columbian mammoth of North America was a hybrid species between woolly mammoths and another lineage, discovered in Krestovka, descended from steppe 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 the

Middle Pleistocene The Chibanian, more widely known as the Middle Pleistocene (its previous informal name), is an Age (geology), age in the international geologic timescale or a Stage (stratigraphy), stage in chronostratigraphy, being a division of the Pleistocen ...

. This is the first evidence of hybrid speciation obtained from prehistoric DNA.

Multiple hybrids during rapid divergence

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

species complex In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...

, like several physically divergent but closely related genera of cichlid fishes in

. 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.A mid-sized species: Bernor, R.L.; Kordos, L. & Rook, L. (eds)
Recent Advances on Multidisciplinary Research at Rudabánya, Late Miocene (MN9), Hungary: A compendium
. ''Paleontographica Italiana'' 89: 3–36. While hybrid species generally appear rare in mammals, the American red wolf appears to be a hybrid species of the ''

Canis ''Canis'' is a genus of the Caninae which includes multiple extant taxon, extant species, such as Wolf, wolves, dogs, coyotes, and golden jackals. Species of this genus are distinguished by their moderate to large size, their massive, well-develo ...

'' species complex, between

gray wolf The wolf (''Canis lupus''; : wolves), also known as the grey wolf or gray wolf, is a canine native to Eurasia and North America. More than thirty subspecies of ''Canis lupus'' have been recognized, including the dog and dingo, though gr ...

and coyote. Hybridization may have led to the species-rich '' Heliconius'' butterflies, though this conclusion has been criticized.

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.

Polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired sets of (Homologous chromosome, homologous) chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning ...

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. 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 Inbreeding depression is the reduced biological fitness caused by loss of genetic diversity as a consequence of inbreeding, the breeding of individuals closely related genetically. This loss of genetic diversity results from small population siz ...

and have higher self-fertility.

Homoploid hybrid speciation

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 of '' Louisiana irises'' show how these populations occur in Hybrid zones created by disturbances and ecotones (Anderson 1949). Novel niches can allow for the persistence of hybrid lineages. For example, established sunflower (''

Helianthus ''Helianthus'' () is a genus comprising around 70 species of annual and perennial flowering plants in the daisy family Asteraceae commonly known as sunflowers. Except for three South American species, the species of ''Helianthus'' are native to ...

'') hybrid species show transgressive phenotypes and display genomic divergence separating them from the parent species.

References

{{Evolution Genetics Speciation Evolutionary biology terminology Interspecific hybrids

Ecology

Ecological constraints

Genetics of hybridization

Hybrid speciation in animals

Homoploid hybrid speciation

Multiple hybrids during rapid divergence

Hybrid speciation in plants

Polyploid hybrid speciation

Homoploid hybrid speciation

See also

References