hybrid speciation
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Hybrid speciation is a form of
speciation Speciation is the evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are ...

speciation
where hybridization between two different
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
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 Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such ...
, a hybrid species is also called a nothospecies. Hybrid species are by their nature
polyphyletic 300px, Cladogram of the primates, showing a monophyly (the simians, in yellow), a paraphyly">monophyly.html" ;"title="primates, showing a monophyly">primates, showing a monophyly (the simians, in yellow), a paraphyly (the prosimians, in cyan, inc ...

polyphyletic
.


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 right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
may favor these individuals. If
reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, ethology, behaviors and physiology, physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensur ...
is subsequently achieved, a separate species may arise. Reproductive isolation may be genetic,
ecological Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms In biology ...
, 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 Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed fo ...
. 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 (, also Neandertals, ''Homo neanderthalensis'' or ''Homo sapiens neanderthalensis'') are an extinct species In biology Biology is the natural science that studies life and living organisms, including their anatomy, phys ...
and
Denisovans The Denisovans or Denisova hominins ) are an extinct Extinction is the termination of a kind of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system ...
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. While
grizzly bear The grizzly bear (''Ursus arctos horribilis''), also known as the North American brown bear or simply grizzly, is a or of the inhabiting . In addition to the mainland grizzly (''Ursus arctos horribilis''), other morphological forms of brow ...
s and
polar bear The polar bear (''wikt:ursus#Latin, Ursus wikt:maritimus#Latin, maritimus'') is a Hypercarnivore, hypercarnivorous bear whose native range lies largely within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surroun ...

polar bear
s may have offspring, a
grizzly–polar bear hybrid A grizzly–polar bear hybrid (also named grolar bear, pizzly bear, grizzlar, or nanulak) is a rare ursid hybrid that has occurred both in captivity and in the wild. In 2006, the occurrence of this hybrid in nature was confirmed by testing the ...
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. Likewise,
lion The lion (''Panthera leo'') is a large Felidae, cat of the genus ''Panthera'' native to Africa and India. It has a muscular, deep-chested body, short, rounded head, round ears, and a hairy tuft at the end of its tail. It is sexually dimorphic; ...

lion
s and
tiger The tiger (''Panthera tigris'') is the largest extant taxon, living Felidae, cat species and a member of the genus ''Panthera''. It is most recognisable for its dark vertical stripes on orange fur with a white underside. An apex predator, it ...

tiger
s have historically overlapped in a portion of their range and can theoretically produce wild hybrids:
ligers The liger is a hybrid (biology), hybrid offspring of a male lion (''Panthera leo'') and a female tiger (''Panthera tigris''). The liger has parents in the same genus but of different species. The liger is distinct from the similar hybrid called ...

ligers
, which are a cross between a male lion and female tiger, and
tigon A tigon () or tiglon () is the hybrid offspring of a male tiger The tiger (''Panthera tigris'') is the largest extant taxon, living Felidae, cat species and a member of the genus ''Panthera''. It is most recognisable for its dark vertic ...
s, which are a cross between a male tiger and a female lion; however, tigers and lions have thus far only hybridized in captivity. 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 leopard (''Panthera pardus'') is one of the five extant in the ', a member of the cat , . It occurs in a wide range in , in some parts of and , , and on the to and . It is listed as on the because leopard populations are threatened ...

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 The Asiatic lion is a '' Panthera leo leo'' population In biology, a population is a number of all the organisms of the same group or species In biology, a species is the basic unit of biological classification, classification and a ...

Asiatic lion
is now restricted to the
Gir Forest National Park Gir National Park and Wildlife Sanctuary, also known as Sasan Gir, is a forest and wildlife sanctuary near Talala (Gir), Talala Gir in Gujarat, India. It is located north-east of Somnath, south-east of Junagadh and south-west of Amreli. It wa ...

Gir Forest National Park
, where tigers are 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 Tanzania (;This approximates the Kiswahili pronunciation. However, is also heard in English. ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a coun ...

Lake Malawi
, a situation that generally favors speciation. A similar situation can be found where closely related species occupy a . 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 'Amboseli National Park'', formerly Maasai Amboseli Game Reserve, is a national park in Kajiado South Constituency in Kajiado County, Kenya. The park is in size at the core of an ecosystem that spreads across the Kenya-Tanzania border. The loc ...

Amboseli National Park
in Kenya,
yellow baboon The yellow baboon (''Papio cynocephalus'') is a baboon in the family of Old World monkeys. The species epithet means "dog-head" in Greek, due to the dog-like shape of the muzzle and head. Yellow baboons have slim bodies with long arms and legs, ...
s and
anubis baboon The olive baboon (''Papio anubis''), also called the Anubis baboon, is a member of the family Cercopithecidae (Old World monkeys). The species is the most wide-ranging of all baboons, being found in 25 countries throughout Africa, extending from ...
s 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 (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
y. Polyploidy is usually fatal in animals where extra chromosome sets upset
fetal development Prenatal development () includes the development of the embryo and of the foetus during a viviparous animal's gestationGestation is the period of development during the carrying of an embryo An embryo is the early stage of development of a ...

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 long DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A mole ...

chromosome
number. Hybridization without change in chromosome number is called
homoploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Somatic cells, Tissue (biology), tissues, and I ...
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 In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proces ...

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 The edible frog (''Pelophylax'' kl. ''esculentus'') is a species of common European frog A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order An ...
appears to be a species, but is actually a triploid semi-permanent hybrid between pool frogs and
marsh frog The marsh frog (''Pelophylax ridibundus'') is a species of water frog native to Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any str ...

marsh frog
s.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 Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Somatic cells, Tissue (biology), tissues, and I ...
. While thought not to be very common, a few animal species are the result of hybridization, mostly
insect Insects (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Rom ...

insect
s such as tephritid fruitflies that inhabit ''
Lonicera Honeysuckles (''Lonicera'', ; synonymy, syn. ''Caprifolium'' Philip Miller, Mill.) are arching shrubs or Vine#Twining vines, twining vines in the family Caprifoliaceae, native to northern latitudes in North America and Eurasia. Approximately 180 ...

Lonicera
'' plants and ''
Heliconius ''Heliconius'' comprises a colorful and widespread genus of Nymphalidae, brush-footed butterflies commonly known as the longwings or heliconians. This genus is distributed throughout the tropical and subtropical regions of the New World, from So ...

Heliconius
'' butterflies, as well as some
fish Fish are , , -bearing animals that lack with . Included in this definition are the living , s, and and as well as various extinct related groups. Around 99% of living fish species are ray-finned fish, belonging to the class , with over 95 ...

fish
, one marine mammal, the
clymene dolphin The Clymene dolphin (''Stenella clymene''), in older texts known as the short-snouted spinner dolphin, is a dolphin endemic to the Atlantic Ocean. It is the only confirmed case of hybrid speciation Hybrid speciation is a form of speciation wher ...
, a few birds. and certain ''
Bufotes ''Bufotes'', the Eurasian green toads or Palearctic green toads, is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that unde ...
'' toads. 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. 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 manakin, snow-capped and opal-crowned manakins.


Multiple hybrids during rapid divergence

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 Lake Malawi, also known as Lake Nyasa in Tanzania Tanzania (;This approximates the Kiswahili pronunciation. However, is also heard in English. ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a coun ...

Lake Malawi
. 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'' species complex, between gray wolf and coyote. Hybridization may have led to the species-rich ''
Heliconius ''Heliconius'' comprises a colorful and widespread genus of Nymphalidae, brush-footed butterflies commonly known as the longwings or heliconians. This genus is distributed throughout the tropical and subtropical regions of the New World, from So ...

Heliconius
'' butterfly, 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 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. 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.


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 segregation, transgressive phenotypes and display genomic divergence separating them from the parent species.


See also

* Clymene dolphin * Eastern coyote * Coywolf * Genetic pollution * Hybrid name * New Mexico whiptail * Secondary contact * Ring species


References

{{Evolution Genetics Speciation Evolutionary biology terminology Interspecific hybrids