In biology a hybrid, also known as cross breed, is the result of mixing, through sexual reproduction, two animals or plants of different breeds, varieties, species or genera. Using genetic terminology, it may be defined as follows.
- Hybrid generally refers to any offspring resulting from the breeding of two genetically distinct individuals, which usually will result in a high degree of heterozygosity, though hybrid and heterozygous are not, strictly speaking, synonymous.
- a genetic hybrid carries two different alleles of the same gene
- a structural hybrid results from the fusion of gametes that have differing structure in at least one chromosome, as a result of structural abnormalities
- a numerical hybrid results from the fusion of gametes having different haploid numbers of chromosomes
- a permanent hybrid is a situation where only the heterozygous genotype occurs, because all homozygous combinations are lethal.
From a taxonomic perspective, hybrid refers to:
- Offspring resulting from the interbreeding between two animal species or plant species. See also hybrid speciation.
- Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are sometimes known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids. Extremely rare interfamilial hybrids have been known to occur (such as the guineafowl hybrids). No interordinal (between different orders) animal hybrids are known.
- The third type of hybrid consists of crosses between populations, breeds or cultivars within a single species. This meaning is often used in plant and animal breeding, where hybrids are commonly produced and selected, because they have desirable characteristics not found or inconsistently present in the parent individuals or populations.
The term hybrid is derived from Latin hybrida, meaning the "offspring of a tame sow and a wild boar", "child of a freeman and slave", etc. The term came into popular use in English in the 19th century, though examples of its use have been found from the early 17th century.
There is a popular convention of naming hybrids by forming portmanteau words. The template for this is the naming of tiger-lion hybrids as liger and tigon in the 1920s. This was playfully (but unsystematically) extended to a number of other hybrids, or hypothetical hybrids, such as beefalo (1960s), humanzee (1980s), cama (1998).
Types of hybrids
- Single cross hybrids — result from the cross between two true breeding organisms and produces an F1 generation called an F1 hybrid (F1 is short for Filial 1, meaning "first offspring"). The cross between two different homozygous lines produces an F1 hybrid that is heterozygous; having two alleles, one contributed by each parent and typically one is dominant and the other recessive. Typically, the F1 generation is also phenotypically homogeneous, producing offspring that are all similar to each other.
- Double cross hybrids — result from the cross between two different F1 hybrids.
- Three-way cross hybrids — result from the cross between one parent that is an F1 hybrid and the other is from an inbred line.
- Triple cross hybrids — result from the crossing of two different three-way cross hybrids.
- Population hybrids — result from the crossing of plants or animals in a population with another population. These include crosses between organisms such as interspecific hybrids or crosses between different breeds.
- Stable hybrid – a horticultural term which typically refers to an annual plant that, if grown and bred in a small monoculture free of external pollen (e.g., an air-filtered greenhouse) will produce offspring that are "true to type" with respect to phenotype; i.e., a true breeding organism.
- Hybrid species – results from hybrid populations evolving reproductive barriers against their parent species through hybrid speciation.
Interspecific hybrids are bred by mating two species, normally from within the same genus. The offspring display traits and characteristics of both parents. The offspring of an interspecific cross are very often sterile; thus, hybrid sterility prevents the movement of genes from one species to the other, keeping both species distinct. Sterility is often attributed to the different number of chromosomes the two species have, for example donkeys have 62 chromosomes, while horses have 64 chromosomes, and mules and hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes, because differences in chromosome structure prevent appropriate pairing and segregation during meiosis, meiosis is disrupted, and viable sperm and eggs are not formed. However, fertility in female mules has been reported with a donkey as the father.
Most often other processes occurring in plants and animals keep gametic isolation and species distinction. Species often have different mating or courtship patterns or behaviors, the breeding seasons may be distinct and even if mating does occur antigenic reactions to the sperm of other species prevent fertilization or embryo development. Hybridisation is much more common among organisms that spawn indiscriminately, like soft corals and among plants.
While it is possible to predict the genetic composition of a backcross on average, it is not possible to accurately predict the composition of a particular backcrossed individual, due to random segregation of chromosomes. In a species with two pairs of chromosomes, a twice backcrossed individual would be predicted to contain 12.5% of one species' genome (say, species A). However, it may, in fact, still be a 50% hybrid if the chromosomes from species A were lucky in two successive segregations, and meiotic crossovers happened near the telomeres. The chance of this is fairly high: (where the "two times two" comes about from two rounds of meiosis with two chromosomes); however, this probability declines markedly with chromosome number and so the actual composition of a hybrid will be increasingly closer to the predicted composition.
While not very common, a few animal species have been recognized as being the result of hybridization. The Lonicera fly is an example of a novel animal species that resulted from natural hybridization. The American red wolf appears to be a hybrid species between gray wolf and coyote, although its taxonomic status has been a subject of controversy. The European edible frog appears to be a species, but is actually a semi-permanent hybrid between pool frogs and marsh frogs. The edible frog population is dependent on the presence of at least one of the parents species to be maintained. Evidence found in cave paintings indicates that the European bison is a descendant of the "Higg's bison", a natural hybrid between aurochs and steppe bison.
Hybrid species of plants are much more common than animals. Many of the crop species are hybrids, and hybridization appears to be an important factor in speciation in some plant groups.
Examples of hybrid animals and animal populations derived from hybrids
- Equid hybrids
- Mule, a cross of female horse and a male donkey.
- Hinny, a cross between a female donkey and a male horse. Mule and hinny are examples of reciprocal hybrids.
- hybrid ass, a cross between a donkey and an onager or Asian wild ass.
- Bovid hybrids
- Dzo, zo or yakow; a cross between a domestic cow/bull and a yak.
- Beefalo, a cross of an American bison and a domestic cow. This is a fertile breed; this along with genetic evidence has caused them to be recently reclassified into the same genus, Bos.
- Żubroń, a hybrid between wisent (European bison) and domestic cow.
- "Higgs bison", a naturally occurring hybrid between aurochs and steppe bison that appeared during the Pleistocene; likely gave rise to the wisent.
- Sheep-goat hybrid is the cross between a sheep and a goat, which belong to different genera.
- Ursid hybrids, such as the grizzly-polar bear hybrid, occur between black bears, brown bears, and polar bears.
- Felid hybrids
- Savannah cat are a fertile breed developed originally from a cross between the serval (Leptailurus serval) and a domestic cat (Felis catus).
- A hybrid between a Bengal tiger and a Siberian tiger is an example of an intra-specific hybrid. It also includes the Indochinese tiger, Sumatran tiger too.
- Pumapards are the hybrid crosses between a puma and a leopard.
- Ligers and tigons (crosses between a lion and a tiger - the difference in name due to what species the mother and father were - ligers have a lion father and a tiger mother) and other Panthera hybrids such as the lijagulep. Various other wild cat crosses are known involving the lynx, bobcat, leopard, serval, etc.
- Bengals are a fertile breed developed originally from a cross between the Asian leopard cat [Prionailurus bengalensis] and the domestic cat [Felis catus].
- Fertile canid hybrids occur between coyotes, wolves, dingoes, jackals and domestic dogs.
- Hybrids between black and white rhinoceroses have been recognized.
- Hybrid camel, a cross between a bactrian camel and a dromedary camel
- Cama, a cross between a camel and a llama, also an intergeneric hybrid.
- Wholphin, a fertile but very rare cross between a false killer whale and a bottlenose dolphin.
- At Chester Zoo in the United Kingdom, a cross between an African elephant (male) and an Asian elephant (female). The male calf was named Motty. He died of intestinal infection after twelve days.
- Bornean and Sumatran orangutan hybrids have occurred in captivity.
- Hybrids between spotted owls and barred owls
- Cagebird breeders sometimes breed hybrids between species of finch, such as goldfinch × canary. These birds are known as mules.
- The perlin is a peregrine falcon – merlin hybrid.
- Gamebird hybrids, hybrids between gamebirds and domestic fowl, including chickens, guineafowl and peafowl, interfamilial hybrids.
- Numerous macaw hybrids and lovebird hybrids are also known in aviculture.
- Red kite × black kite: five bred unintentionally at a falconry center in England. (It is reported that the black kite (the male) refused female black kites but mated with two female red kites.)
- The mulard duck, hybrid of the domestic pekin duck and domesticated muscovy ducks.
- In Australia, New Zealand and other areas where the Pacific black duck occurs, it is hybridised by the much more aggressive introduced mallard. This is a concern to wildlife authorities throughout the affected area, as it is seen as Genetic pollution of the black duck gene pool.
- Hybridisation in gulls is a reasonably frequent occurrence in the wild.
- Hybrid iguana, a single‐cross hybrid resulting from natural interbreeding between male marine iguanas and female land iguanas since the late 2000s.
- Crestoua, a cross between a Rhacodactylus Ciliatus (crested gecko) and a Rhacodactylus Chahoua.
- Colubrid snakes of the tribe Lampropeltini have been shown to produce fertile hybrid offspring.
- Hybridization between the endemic Cuban crocodile (Crocodilus rhombifer) and the widely distributed American crocodile (Crocodilus acutus) is causing conservation problems for the former species as a threat to its genetic integrity.
- Saltwater crocodiles (Crocodylus porosus) have mated with Siamese crocodiles (Crocodylus siamensis) in captivity producing offspring which in many cases have grown over 20 feet (6.1 metres) in length. It is likely that wild hybridization occurred historically in parts of southeast Asia.
- Many species of boas and pythons are known to produce hybrids, such as carball (a cross between a ball python and a carpet python) or a bloodball (a cross between a blood python and a ball python) however, most of these only occur in captivity. Contrary to popular belief, boa–python hybrids are not possible due to their differing reproductive functions. Boas only produce hybrids with other species of boas, and pythons only produce hybrids with other species of pythons.
- Japanese giant salamanders and Chinese giant salamanders have created hybrids that threaten the survival of Japanese giant salamanders due to the competition for similar resources in Japan.
- Blood parrot cichlid, which is probably created by crossing a red head cichlid and a Midas cichlid or red devil cichlid
- A group of about 50 hybrids between Australian blacktip shark and the larger common blacktip shark was found by Australia's East Coast in 2012. This is the only known case of hybridization in sharks.
- Silver bream and common bream commonly produce sterile hybrids.
- Tiger muskie is a sterile hybrid between northern pike and muskellunge.
- Killer bees were created during an attempt to breed a strain of bees that would produce more honey and be better adapted to tropical conditions. This was done by crossing a European honey bee and an African bee.
- The Colias eurytheme and C. philodice butterflies have retained enough genetic compatibility to produce viable hybrid offspring.
Many hybrids are created by humans, but natural hybrids occur as well. Plant species hybridize more readily than animal species, and the resulting hybrids are more often fertile hybrids and may reproduce, though there still exist sterile hybrids and selective hybrid elimination where the offspring are less able to survive and are thus eliminated before they can reproduce. A number of plant species are the result of hybridization and polyploidy with many plant species easily cross pollinating and producing viable seeds, the distinction between each species is often maintained by geographical isolation or differences in the flowering period. Since plants hybridize frequently without much work, they are often created by humans in order to produce improved plants. These improvements can include the production of more or improved seeds, fruits or other plant parts for consumption, or to make a plant more winter or heat hardy or improve its growth and/or appearance for use in horticulture. Much work is now being done with hybrids to produce more disease resistant plants for both agricultural and horticultural crops. In many groups of plants hybridization has been used to produce larger and more showy flowers and new flower colors. Hybridization may be restricted to the desired parent species through the use of pollination bags.
Many plant genera and species have their origins in polyploidy. Autopolyploidy results from the sudden multiplication in the number of chromosomes in typical normal populations caused by unsuccessful separation of the chromosomes during meiosis. Tetraploids (plants with four sets of chromosomes rather than two) are common in a number of different groups of plants and over time these plants can differentiate into distinct species from the normal diploid line. In Oenothera lamarchiana the diploid species has 14 chromosomes, this species has spontaneously given rise to plants with 28 chromosomes that have been given the name Oenothera gigas. When hybrids are formed between the tetraploids and the diploid population, the resulting offspring tend to be sterile triploids, thus effectively stopping the intermixing of genes between the two groups of plants (unless the diploids, in rare cases, produce unreduced gametes).
Another form of polyploidy called allopolyploidy occurs when two different species mate and produce polyploid hybrids. Usually the typical chromosome number is doubled, and the four sets of chromosomes can pair up during meiosis, thus the polyploids can produce offspring. Usually, these offspring can mate and reproduce with each other but cannot back-cross with the parent species. Allopolyploids may be able to adapt to new habitats that neither of their parent species inhabited.
|Species||Common name||Family||Hybridization||Confirmed or putative hybridization?||Putative parental/introgressive species||Polyploid or homoploid?||Polyploid chromosome count||References||Notes|
|Abelmoschus esculentus (L.) Moench||Okra||Malvaceae||Allopolyploid origin||Putative||Uncertain||Polyploid (tetraploid)||usually 2n=4x=130||Joshi and Hardas, 1956; Schafleitner et al., 2013||Variable ploidy|
|Actinidia deliciosa (A. Chev.) C.F.Liang & A.R.Ferguson||Kiwifruit||Actinidiaceae||Allopolyploid origin||Putative||Actinidia chinensis Planch. and Unknown||Polyploid (hexaploid)||2n=6x=174||Atkinson et al., 1997|
|Agave fourcroydes Lem.||Henequen||Asparagaceae||Allopolyploid origin||Confirmed||Uncertain||Polyploid (usu. pentaploid, triploid)||2n=5x(3x)=150(90)||Robert et al., 2008; Hughes et al., 2007||Variable ploidy, polyploid event not recent|
|Agave sisalana Perrine||Sisal||Asparagaceae||Allopolyploid origin||Confirmed||Uncertain||Polyploid (usu. pentaploid, hexaploid)||2n=5x(6x)=150(180)||Robert et al., 2008||Variable ploidy, polyploid event not recent|
|Allium ampeloprasum L.||Great headed garlic||Amaryllidaceae||Intraspecific hybrid origin||Putative||Allium ampeloprasum L.||Homoploid||–||Guenaoui et al., 2013|
|Allium cepa L.||Common onion||Amaryllidaceae||Interspecific hybrid origin||Putative||Uncertain: Allium vavilovii Popov & Vved., A. galanthum Kar. & Kir. or A. fistulosum L.||Homoploid||–||Gurushidze et al., 2007|
|Allium cornutum Clementi||Triploid onion||Amaryllidaceae||Triparental allopolyploid origin||Confirmed||Allium cepa L., A. roylei Stearn, unknown||Polyploid (triploid)||2n=3x=24||Fredotovic et al., 2014|
|Ananas comosus (L.) Merr is||Pineapple||Bromeliaceae||Interspecific introgression||Putative||Ananas ananassoides (Baker) L.B. Smith||Homoploid||–||Duval et al., 2003|
|Annona x atemoya||Atemoya||Annonaceae||Interspecific hybrid origin||Confirmed||Annona cherimola Mill. and A. squamosa L.||?||–||Perfectti et al., 2004; Jalikop, 2010|
|Arachis hypogaea L.||Peanut||Fabaceae||Allopolyploid origin||Confirmed||Arachis duranensis Krapov. & W.C. Greg. and A. ipaënsis Krapov. & W.C. Greg.||Polypoid (tetraploid)||2n=4x=40||Kochert et al., 1996; Bertioli et al., 2011|
|Armoracia rusticana P.Gaertn., B.Mey. & Scherb.||Horseradish||Brassicaceae||Interspecific hybrid origin||Putative||Uncertain||?||–||Courter and Rhodes, 1969|
|Artocarpus altilis (Parkinson ex F.A.Zorn) Fosberg||Breadfruit||Moraceae||Interspecific introgression||Putative||Artocarpus mariannensis Trécul||?||–||Zerega et al., 2005; Jones et al., 2013|
|Avena sativa L.||Oat||Poaceae||Allopolyploid origin||Confirmed||Uncertain||Polyploid (hexaploid)||2n=6x=42||Linares et al., 1998; Oliver et al., 2013|
|Brassica carinata A.Braun||Ethiopian mustard||Brassicaceae||Allopolyploid origin||Confirmed||Brassica oleracea L. and B. nigra (L.) K.Koch||Polyploid (tetraploid)||2n=4x=19||Arias et al., 2014|
|Brassica juncea (L.) Czern.||Indian mustard||Brassicaceae||Allopolyploid origin||Confirmed||Brassica nigra (L.) K.Koch and B. rapa L.||Polyploid (tetraploid)||2n=4x=18||Arias et al., 2014|
|Brassica napus L.||Rapeseed, rutabega||Brassicaceae||Allopolyploid origin||Confirmed||Brassica rapa L. and B. oleracea L.||Polyploid (tetraploid)||2n=4x=19||Arias et al., 2014|
|Cajanus cajan (L.) Millsp.||Pigeon pea||Fabaceae||Intraspecific introgression, interspecific introgression||Putative||Wild Cajanus cajan and other species||Homoploid||–||Kassa et al., 2012|
|Cannabis sativa L.||Hemp||Cannabaceae||Intraspecific introgression||Putative||Cannabis sativa L. 'Indica' and 'Sativa' types||Homoploid||–||de Meijer and van Soest, 1992|
|Carica pentagona Heilborn||Babaco||Caricaceae||Interspecific hybrid origin||Confirmed||Uncertain (Carica stipulata V.M.Badillo, Vasconcellea pubescens A.DC., Vasconcellea weberbaueri (Harms) V.M. Badillo)||?||–||Van Droogenbroeck et al., 2002; Van Droogenbroeck et al., 2006|
|Carya illinoinensis (Wangenh.) K.Koch||Pecan||Juglandaceae||Interspecific hybrid origin||Putative||Uncertain||?||–||Grauke et al., 2011|
|Castanea dentata (Marshall) Borkh||Chestnut||Fagaceae||Interspecific introgression||Confirmed||Castanea pumila (L.) Mill.||Homoploid||–||Li and Dane, 2013||Also ongoing efforts to introgress blight resistance from Castanea mollissima Blume (see Jacobs et al., 2013)|
|Castanea sativa Mill.||Chestnut||Fagaceae||Intraspecific introgression||Confirmed||Castanea sativa Eurosiberian and Mediterranean populations||Homoploid||–||Villani et al., 1999; Mattioni et al., 2013|
|Chenopodium quinoa Willd.||Quinoa||Chenopodiaceae||Allopolyploid origin||Putative||Uncertain||Polyploid (tetraploid)||–||Heiser, 1974; Ward, 2000; Maughan et al., 2004|
|Cicer arietinum L.||Chickpea (pea-shaped)||Fabaceae||Intraspecific hybrid origin||Putative||Cicer arietinum L. Desi and Kabuli Germplasm||?||–||Upadhyaya et al., 2008; Keneni et al., 2011|
|Cichorium intybus L.||Radicchio||Asteraceae||Interspecific introgression||Confirmed||Wild Cichorium intybus L.||Homoploid||–||Kiaer et al., 2009|
|Citrus aurantiifolia (Christm.) Swingle||Key lime||Rutaceae||Interspecific hybrid origin||Confirmed||Citrus medica L. and C. subg. Papeda||?||–||Ollitrault and Navarro, 2012; Penjor et al., 2014; Nicolosi et al., 2000; Moore, 2001|
|Citrus aurantium L.||Sour oranges||Rutaceae||Interspecific hybrid origin||Confirmed||Citrus maxima (Burm.) and C. reticulata Blanco||?||–||Wu et al., 2014; Moore, 2001|
|Citrus clementina hort.||Clementine||Rutaceae||Interspecific hybrid origin||Confirmed||Citrus sinensis (L.) Osbeck and C. reticulata Blanco||?||–||Wu et al., 2014|
|Citrus limon (L.) Osbeck||Lemon, lime||Rutaceae||Interspecific hybrid origin||Confirmed||Citrus medica L., C. aurantiifolia (Christm.) Swingle, and uncertain||?||–||Nicolosi et al., 2000; Moore, 2001|
|Citrus paradisi Macfad.||Grapefruit||Rutaceae||Interspecific hybrid origin||Confirmed||Citrus sinensis (L.) Osbeck and C. maxima (Burm.)||?||–||Wu et al., 2014; Moore, 2001|
|Citrus reticulata Blanco||Mandarin||Rutaceae||Interspecific introgression||Confirmed||Citrus maxima (Burm.)||?||–||Wu et al., 2014|
|Citrus sinensis (L.) Osbeck||Sweet orange (blood, common)||Rutaceae||Interspecific hybrid origin||Confirmed||Uncertain||?||–||Wu et al., 2014; Moore, 2001|
|Cocos nucifera L.||Coconut||Arecaceae||Intraspecific introgression||Confirmed||Cocos nucifera L. Indo-Atlantic and Pacific lineages||Homoploid||–||Gunn et al., 2011|
|Coffea arabica L.||Coffee||Rubiaceae||Allopolyploid origin||Confirmed||Coffea eugenioides S.Moore and C. canephora Pierre ex A.Froehner||Polyploid (tetraploid)||2n=4x=44||Lashermes et al., 1999|
|Corylus avellana L.||Hazelnut||Betulaceae||Intraspecific introgression||Confirmed||Wild Corylus avellana L. in Southern Europe||Homoploid||–||Campa et al., 2011; Boccacci et al., 2013|
|Cucurbita pepo L.||Winter squash, pumpkin||Cucurbitaceae||Intraspecific introgression||Putative||Cucurbita pepo var. texana (Scheele) D.S.Decker||Homploid||–||Kirkpatrick and Wilson, 1988|
|Daucus carota subsp. sativus (Hoffm.) Arcang.||Carrot||Apiaceae||Intraspecific introgression||Confirmed||Daucus carota L. subsp. carota||Homoploid||–||Iorizzo et al., 2013; Simon, 2000|
|Dioscorea L. spp.||Yam||Dioscoreaceae||Interspecific hybrid origin, introgression||Putative||Uncertain||Variable||–||Terauchi et al., 1992; Dansi et al., 1999; Bhattacharjee et al., 2011; Mignouna et al., 2002||Multiple species of putative hybrid (perhaps allopolyploid) origin including Dioscorea cayennensis subsp. rotundata (Poir.) J.Miège. and D. cayennensis Lam.|
|Diospyros kaki L.f.||Persimmon||Ebenaceae||Allopolyploid origin||Putative||Uncertain||Polyploid (hexaploid)||Yonemori et al., 2008|
|Ficus carica L.||Fig||Moraceae||Interspecific introgression||Putative||Uncertain||?||Aradhya et al., 2010|
|Fragaria ananassa (Duchesne ex Weston) Duchesne ex Rozier||Strawberries||Rosaceae||Interspecific hybrid origin||Confirmed||Fragaria virginiana Mill. (octoploid), F. chiloensis (L.) Mill. (octoploid)||Homoploid relative to parentals (octoploid)||2n=8x=56 Evans, 1977; Hirakawa et al., 2014||Uncertain which species formed the octoploid progenitors|
|Garcinia mangostana L.||Mangosteen||Clusiaceae||Allopolyploid origin||Putative||Garcinia celebica L. and G. malaccensis Hook.f. ex T.Anderson||Polyploid (tetraploid)||? Richards, 1990||Recent work shows this may not be of hybrid origin (Nazre, 2014)|
|Gossypium hirsutum L.||Upland cotton||Malvaceae||hybrid origin||Confirmed||Uncertain, referred to as 'A' and 'D'||Polyploid (formed <1MYA)||2n =4x=52 Wendel and Cronn 2003||Polyploidization likely led to agronomically significant traits (Applequist et al., 2001)|
|Hibiscus sabdariffa L.||Roselle||Malvaceae||Allopolyploid origin||Putative||Uncertain||Polyploid (tetraploid)||2n=4x=72 Menzel and Wilson, 1966; Satya et al., 2013|
|Hordeum vulgare L.||Barley||Poaceae||Introgression||Confirmed||Hordeum spontaneum K.Koch||Homoploid||Badr et al., 2000; Dai et al., 2012|
|Humulus lupulus L.||Hops||Cannabaceae||Intraspecific introgression||Confirmed||Humulus lupulus L. North American and European Germplasm||Homoploid||Reeves and Richards, 2011; Stajner et al., 2008; Seefelder et al., 2000|
|Ipomoea batatas (L.) Lam.||Sweet potato||Convolvulaceae||Intraspecific introgression; Interspecific introgression?||Putative||Ipomoea batatas (L.) Lam. Central American and South American Germplasm||Homoploid relative to parentals||Roullier et al., 2013|
|Juglans regia L.||Walnut||Juglandaceae||Interspecific hybridization||Confirmed||Juglans sigillata Dode||Homoploid||Gunn et al., 2010|
|Lactuca sativa L.||Lettuce||Asteraceae||Intraspecific hybrid origin||Putative||Lactuca serriola L. and other L. spp.||Homoploid||de Vries, 1997|
|Lagenaria siceraria (Molina) Standl.||Bottle Gourd||Cucurbitaceae||Intraspecific introgression||Confirmed||Lagenaria siceraria (Molina) Standl. African/American and Asian Germplasm||Homoploid||Clarke et al., 2006|
|Lens culinaris Medik. ssp. culinaris||Lentil||Fabaceae||Intraspecific introgression||Putative||Wild lentil, Lens culinaris subsp. orientalis (Boiss.) Ponert||Homoploid||Erskine et al., 2011|
|Macadamia integrifolia Maiden & Betche||Macadamia||Proteaceae||Interspecific hybrid origin, interspecific introgression||Confirmed||Macadamia tetraphylla L.A.S.Johnson, and other M. spp.||Homoploid||Hardner et al., 2009; Steiger et al., 2003; Aradhya et al., 1998|
|Malus domestica Borkh.||Apple||Rosaceae||Interspecific hybrid origin||Confirmed||Malus sieversii (Ledeb.) M.Roem., M. sylvestris (L.) Mill., and possibly others||Homoploid||Cornille et al., 2012|
|Mentha piperita L.||Peppermint||Lamiaceae||Allopolyploid origin||Confirmed||Mentha aquatica L. and M. spicata L.||Polyploid (12-ploid)||2n=12x=66 or 72 Harley and Brighton, 1977; Gobert et al., 2002|
|Musa paradisiaca L.||Banana||Musaceae||Allopolyploid origin||Confirmed||Musa acuminata Colla, M. balbisiana Colla||Polyploid (usually triploid)||2n=3x=33 Simmonds and Shepherd, 1955; Heslop-Harrison and Schwarzacher, 2007; De Langhe et al., 2010|
|Nicotiana tabacum L.||Tobacco||Solanaceae||Allopolyploid origin||Confirmed||Uncertain (Nicotiana sylvestris Speg. & S. Comes and N. tomentosiformis Goodsp.)||Polyploid (tetraploid)||2n=4x=48 Kenton et al., 1993; Murad et al., 2002|
|Olea europaea L.||Olive||Oleaceae||Intraspecific introgression||Putative||Wild Olea europaea L., Eastern and Western Germplasm||Homoploid||Kaniewski et al., 2012; Besnard et al., 2013; Breton et al., 2006; Rubio de Casas et al., 2006; Besnard et al., 2007; Besnard et al., 2000|
|Opuntia L. spp.||Opuntia||Cactaceae||Interspecific hybrid origin, Allopolyploid origin||Putative||Including Opuntia ficus-indica (L.) Mill.||Polyploid, homoploid||Hughes et al., 2007; Griffith, 2004|
|Oryza sativa L.||Rice||Poaceae||Intraspecific introgression, interspecific introgression||Putative||Oryza sativa L. 'Japonica' and 'Indica' Germplasm, Oryza rufipogon Griff.||Homoploid||Caicedo et al., 2007; Gao and Innan, 2008|
|Oxalis tuberosa Molina||Oca||Oxalidaceae||Allopolyploid origin||Putative||Uncertain||Polyploid (octaploid)||2n=8x=64 Emswhiller and Doyle, 2002; Emshwiller 2002; Emswhiller et al., 2009|
|Pennisetum glaucum (L.) R.Br.||Pearl millet||Poaceae||Intraspecific introgression||Confirmed||Wild Pennisetum glaucum (L.) R.Br.||Homoploid||Oumar et al., 2008|
|Persea americana Mill.||Avocado (Hass and other cultivars)||Lauraceae||Intraspecific introgression||Confirmed||Persea americana Mill. 'Guatamalensis', 'Drymifolia', and' Americana'||Homoploid||Chen et al., 2008; Davis et al., 1998; Ashworth and Clegg, 2003; Douhan et al., 2011|
|Phoenix dactylifera L.||Date palm||Arecaceae||Interspecific hybrid origin||Putative||Uncertain||Homoploid||El Hadrami et al., 2011; Bennaceur et al., 1991|
|Piper methysticum G.Forst.||Kava||Piperaceae||Allopolyploid origin||Putative||Piper wichmannii C. DC. and P. gibbiflorum C.DC.||Polyploid (decaploid)||2n=10x=130 Singh, 2004; Lebot et al., 1991|
|Pistacia vera L.||Pistachio||Anacardiaceae||Interspecific introgression||Putative||Pistacia atlantica Desf., P. chinensis subsp. integerrima (J. L. Stewart ex Brandis) Rech. f.||Homoploid||Kafkas et al., 2001|
|Pisum abyssinicum A.Braun||Pea||Fabaceae||Interspecific hybrid origin||Confirmed||Uncertain (Pisum fulvum Sibth. & Sm. and other P. spp.)||Homoploid||Vershinin et al., 2003|
|Pisum sativum L.||Pea||Fabaceae||Interspecific hybrid origin||Confirmed||Uncertain (Pisum sativum subsp. elatius (M.Bieb.) Asch. & Graebn. and other P. spp.)||Homoploid||Vershinin et al., 2003|
|Plinia cauliflora (Mart.) Kausel||Jaboticaba||Myrtaceae||Intraspecific hybridization, interspecific hybridization||Putative||Plinia 'Jaboticaba' and 'Cauliflora' Germplasm; P. peruviana (Poir.) Govaerts||Homoploid||Balerdi et al., 2006|
|Prunus cerasus L.||Cherry||Rosaceae||Allopolyploid origin||Confirmed||Prunus avium (L.) L. and P. fruticosa Pall.||Polyploid (tetraploid)||2n=4x=32 Tavaud et al., 2004; Olden and Nybom, 1968|
|Prunus domestica L.||Plum||Rosaceae||Allopolyploid origin||Confirmed||Uncertain (P. cerasifera Ehrh. and P. spinosa L.)||Polyploid (hexaploid)||2n=6x=48 Zohary, 1992; Hartmann and Neumuller, 2009||Japanese plum is also of hybrid origin (see Hartmann and Neumuller 2009). Also hybridizes with other cultivated Prunus spp.|
|Prunus dulcis (Mill.) D.A.Webb||Almond||Rosaceae||Interspecific introgression||Confirmed||Prunus orientalis (Mill.) Koehne and other P. spp.||Homoploid||Delplancke et al., 2012; Delplancke et al., 2013|
|Pyrus L. species||Pear||Rosaceae||Interspecific hybrid origin||Confirmed||Many species||Homoploid||Silva et al., 2014||Also introgression with semidomesticated populations (see Iketani et al. 2009)|
|Raphanus raphanistrum subsp. sativus (L.) Domin||Radish||Brassicaceae||Intraspecific introgression||Confirmed||Raphanus raphanistrum L. subsp. raphanistrum||Homoploid||Ridley et al., 2008|
|Rheum L. cultivated species||Rhubarb||Polygonaceae||Interspecific hybrid origin||Putative||Unclear||Homoploid relative to parentals (tetraploid)||Foust and Marshall, 1991; Kuhl and Deboer, 2008||Hybrids include: Rheum rhaponticum L., R. rhabarbarum L., R. palmatum L.|
|Rubus L. spp.||Red raspberry, blackberry, tayberry, boysenberry, etc.||Rosaceae||Allopolyploid origin, interspecific hybridization||Confirmed||Many||Polyploid||Alice et al., 2014; Alice and Campbell, 1999|
|Saccharum spp.||Sugarcane||Poaceae||Allopolyploid origin||Confirmed||Saccharum officinarum L. and S. spontaneum L.||Polyploid||Variable, 2n=10-13x=100-130 Grivet et al., 1995; D'Hont et al., 1996|
|Secale cereale L.||Rye||Poaceae||Interspecific hybrid origin||Confirmed||Uncertain (Secale montanum Guss., S. vavilovii Grossh.)||Homoploid||Bartos et al., 2008; Korzun et al., 2001; Hillman, 1978; Tang et al., 2011; Salamini et al., 2002|
|Sechium edule (Jacq.) Sw.||Chayote||Cucurbitaceae||Interspecific introgression||Putative||Sechium compositum (Donn. Sm.) C. Jeffrey||Homoploid||Newstrom, 1991|
|Setaria italica (L.) P.Beauv.||Foxtail millet||Poaceae||Interspecific introgression||Confirmed||Setaria viridis (L.) P.Beauv.||Homoploid||Till-Bottraud et al., 1992|
|Solanum L. spp. Section Petota||Potato||Solanaceae||Interspecific hybrid origin, allopolyploid origin, interspecific introgression||Confirmed||Including Solanum tuberosum L., S. ajanhuiri Juz. & Bukasov, S. curtilobum Juz. & Bukasov, S. juzepczukii Bukasov||Homoploid and Polyploid||Rodriguez et al., 2010|
|Solanum lycopersicum L.||Tomato||Solanaceae||Intraspecific introgression, interspecific introgression||Confirmed||Solanum lycopersicum var. cerasiforme (Dunal) D.M. Spooner, G.J. Anderson & R.K. Jansen and S. pimpinellifolium L.||Homoploid||Blanca et al., 2012; Causse et al., 2013; Rick, 1950|
|Solanum melongena L.||Eggplant||Solanaceae||Interspecific hybrid origin, interspecific introgression, intraspecific introgression||Confirmed||Solanum undatum Lam. and others; wild S. melongena L. (=S. insanum L.)||Homoploid||Knapp et al., 2013; Meyer et al., 2012||Hybrid origin is not confirmed, but introgression is well documented|
|Solanum muricatum Aiton||Pepino dulce||Solanaceae||Interspecific hybrid origin, interspecific introgression||Likely||Solanum species in Series Caripensia||Homoploid||Blanca et al., 2007||Polyphyletic origin and extensive, ongoing introgression with wild species|
|Solanum tuberosum||Potato||Solanaceae||Intraspecific Hybrid||Confirmed||Diploid potato lines crossed with S. chacoense containing Sli gene||Homoploid||Lindhout et al., 2011|
|Spondias purpurea L.||Jocote||Anacardiaceae||Interspecific introgression||Putative||Spondias mombin L.||Homoploid||Miller and Schaal, 2005|
|Theobroma cacao L.||Cacao (Trinitario-type)||Malvaceae||Intraspecific hybrid origin||Confirmed||Theobroma cacao L. 'Forastero' and 'Criollo' Germplasm||Homoploid||Yang et al., 2013|
|Triticum aestivum L.||Bread wheat, spelt||Poaceae||Allopolyploid origin||Confirmed||Triticum turgidum L. (tetraploid) with Aegilops tauschii Coss.||Polyploid (hexaploid)||2n=6x=42 Matsuoka, 2011; Dvorak, 2012|
|Triticum turgidum L.||Emmer wheat, durum wheat||Poaceae||Allopolyploid origin||Confirmed||Triticum urartu Thumanjan ex Gandilyan and Aegilops speltoides Tausch||Polyploid (tetraploid)||2n=4x=28 Dvorak et al., 2012; Matsuoka, 2011; Yamane and Kawahara, 2005|
|Vaccinium corymbosum L.||Highbush blueberry||Ericaceae||Interspecific hybrid origin, interspecific introgression||Putative||Vaccinium tenellum Aiton, V. darrowii Camp, (V. virgatum Aiton, V. angustifolium Aiton)||Uncertain||Vander Kloet, 1980; Bruederle et al., 1994; Lyrene et al., 2003; Boches et al., 2006||Possible hybrid origin during the Plesitocene|
|Vanilla tahitensis J.W. Moore||Tahitian vanilla||Orchidaceae||Allopolyploid origin||Confirmed||Vanilla planifolia Jacks. ex Andrews and V. odorata C.Presl||Polyploid||Variable, 2n=2x(4x)=32(64) Lubinsky et al., 2008|
|Vitis rotundifolia Michx.||Grape||Vitaceae||Interspecific introgression||Confirmed||ManyVitis spp.||Homoploid||2n=2x=38 Reisch et al., 2012; This et al., 2006|
|Zea mays L.||Maize||Poaeceae||Intraspecific introgression||Confirmed||Wild Zea mays L. (teosinte, =subsp. parviglumis Iltis & Doebley)||Homoploid||Van Heerwaarden et al., 2011; Hufford et al., 2013|
Sterility in a non-polyploid hybrid is often a result of chromosome number; if parents are of differing chromosome pair number, the offspring will have an odd number of chromosomes, leaving them unable to produce chromosomally balanced gametes. While this is undesirable in a crop such as wheat, where growing a crop which produces no seeds would be pointless, it is an attractive attribute in some fruits. Triploid bananas and watermelons are intentionally bred because they produce no seeds (and are parthenocarpic).
Hybrids are sometimes stronger than either parent variety, a phenomenon most common with plant hybrids, which when present is known as hybrid vigor (heterosis) or heterozygote advantage. A transgressive phenotype is a phenotype displaying more extreme characteristics than either of the parent lines. Plant breeders make use of a number of techniques to produce hybrids, including line breeding and the formation of complex hybrids. An economically important example is hybrid maize (corn), which provides a considerable seed yield advantage over open pollinated varieties. Hybrid seed dominates the commercial maize seed market in the United States, Canada and many other major maize producing countries.
Examples of plant hybrids
The multiplication symbol × (not italicised) indicates a hybrid in the Latin binomial nomenclature. Placed before the binomial it indicates a hybrid between species from different genera (intergeneric hybrid):-
- × Fatshedera lizei, a hybrid between Hedera helix and Fatsia japonica
- × Heucherella, a hybrid genus between Heuchera and Tiarella
- × Philageria veitchii is a hybrid between Lapageria rosea and Philesia magellanica; it is more similar in appearance to the former
- Leyland cypress, [× Cupressocyparis leylandii] hybrid between Monterey cypress and Nootka cypress
- Triticale, [× Triticosecale] a wheat–rye hybrid
- × Urceocharis, a hybrid between Eucharis and Urceolina
Interspecific plant hybrids include:
- Dianthus × allwoodii (Dianthus caryophyllus × Dianthus plumarius)
- Limequat Citrus × floridana, key lime Citrus aurantiifolia and kumquat Citrus japonica hybrid
- Loganberry Rubus × loganobaccus, a hybrid between raspberry Rubus idaeus and blackberry Rubus ursinus
- London plane (Platanus orientalis × Platanus occidentalis), thus forming Platanus × acerifolia
- Magnolia × alba (Magnolia champaca × Magnolia montana)
- Peppermint, a hybrid between spearmint and water mint
- Quercus × warei (Quercus robur × Quercus bicolor) 'Nadler' (marketed in the United States under the trade name Kindred Spirit hybrid oak)
- Tangelo, a hybrid of a Mandarin orange and a pomelo which may have been developed in Asia about 3,500 years ago
- Wheat; most modern and ancient wheat breeds are themselves hybrids. Bread wheat is a hexaploid hybrid of three wild grasses; durum (pasta) wheat is a tetraploid hybrid of two wild grasses
- Grapefruit, hybrid between a pomelo and the Jamaican sweet orange
Some natural hybrids:
- Iris albicans, a sterile hybrid which spreads by rhizome division
- Evening primrose, a flower which was the subject of famous experiments by Hugo de Vries on polyploidy and diploidy
Hybrids in nature
Hybridization between two closely related species is actually a common occurrence in nature but is also being greatly influenced by anthropogenic changes as well. Hybridization is a naturally occurring genetic process where individuals from two genetically distinct populations mate. As stated above, it can occur both intraspecifically, between different distinct populations within the same species, and interspecifically, between two different species. Hybrids can be either sterile/not viable or viable/fertile. This affects the kind of effect that this hybrid will have on its and other populations that it interacts with. Many hybrid zones are known where the ranges of two species meet, and hybrids are continually produced in great numbers. These hybrid zones are useful as biological model systems for studying the mechanisms of speciation (Hybrid speciation). Recently DNA analysis of a bear shot by a hunter in the North West Territories confirmed the existence of naturally-occurring and fertile grizzly–polar bear hybrids.
Changes to the environment caused by humans, such as fragmentation and Introduced species, are becoming more widespread. This increases the challenges in managing certain populations that are experiencing introgression, and is a focus of conservation genetics.
Introduced species and habitat fragmentation
Humans have introduced species worldwide to environments for a long time, both intentionally such as establishing a population to be used as a biological control, and unintentionally such as accidental escapes of individuals out of agriculture. This causes drastic global effects on various populations, including through hybridization.
When habitats become broken apart, one of two things can occur, genetically speaking. The first is that populations that were once connected can be cut off from one another, preventing their genes from interacting. Occasionally, this will result in a population of one species breeding with a population of another species as a means of surviving such as the case with the red wolves. Their population numbers being so small, they needed another means of survival. Habitat fragmentation also led to the influx of generalist species into areas where they would not have been, leading to competition and in some cases interbreeding/incorporation of a population into another. In this way, habitat fragmentation is essentially an indirect method of introducing species to an area.
The hybridization continuum
There is a kind of continuum with three semi-distinct categories dealing with anthropogenic hybridization: hybridization without Introgression, hybridization with widespread introgression, and essentially a Hybrid swarm. Depending on where a population falls along this continuum, the management plans for that population will change. Hybridization is currently an area of great discussion within Wildlife management and habitat management. Global climate change is creating other changes such as difference in population distributions which are indirect causes for an increase in anthropogenic hybridization.
Hybridization can be a less discussed way toward extinction than within detection of where a population lies along the hybrid continuum. The dispute of hybridization is how to manage the resulting hybrids. When a population experiences hybridization with substantial introgression, there still exists parent types of each set of individuals. When a complete hybrid swarm is created, all the individuals are hybrids.
Management of hybrids
Conservationists disagree on when is the proper time to give up on a population that is becoming a hybrid swarm or to try and save the still existing pure individuals. Once it becomes a complete mixture, we should look to conserve those hybrids to avoid their loss. Most leave it as a case-by-case basis, depending on detecting of hybrids within the group. It is nearly impossible to regulate hybridization via policy because hybridization can occur beneficially when it occurs "naturally" and there is the matter of protecting those previously mentioned hybrid swarms because if they are the only remaining evidence of prior species, they need to be conserved as well.
Expression of parental traits in hybrids
When two distinct types of organisms breed with each other, the resulting hybrids typically have intermediate traits (e.g., one parent has red flowers, the other has white, and the hybrid, pink flowers). Commonly, hybrids also combine traits seen only separately in one parent or the other (e.g., a bird hybrid might combine the yellow head of one parent with the orange belly of the other).
In a hybrid, any trait that falls outside the range of parental variation is termed heterotic. Heterotic hybrids do have new traits, that is, they are not intermediate. Positive heterosis produces more robust hybrids, they might be stronger or bigger; while the term negative heterosis refers to weaker or smaller hybrids. Heterosis is common in both animal and plant hybrids. For example, hybrids between a lion and a tigress ("ligers") are much larger than either of the two progenitors, while a tigon (lioness × tiger) is smaller. Also the hybrids between the common pheasant (Phasianus colchicus) and domestic fowl (Gallus gallus) are larger than either of their parents, as are those produced between the common pheasant and hen golden pheasant (Chrysolophus pictus). Spurs are absent in hybrids of the former type, although present in both parents.
Genetic mixing and extinction
Regionally developed ecotypes can be threatened with extinction when new alleles or genes are introduced that alter that ecotype. This is sometimes called genetic mixing. Hybridization and introgression of new genetic material can lead to the replacement of local genotypes if the hybrids are more fit and have breeding advantages over the indigenous ecotype or species. These hybridization events can result from the introduction of non native genotypes by humans or through habitat modification, bringing previously isolated species into contact. Genetic mixing can be especially detrimental for rare species in isolated habitats, ultimately affecting the population to such a degree that none of the originally genetically distinct population remains.
Effect on biodiversity and food security
In agriculture and animal husbandry, the Green Revolution's use of conventional hybridization increased yields by breeding "high-yielding varieties". The replacement of locally indigenous breeds, compounded with unintentional cross-pollination and crossbreeding (genetic mixing), has reduced the gene pools of various wild and indigenous breeds resulting in the loss of genetic diversity. Since the indigenous breeds are often well-adapted to local extremes in climate and have immunity to local pathogens this can be a significant genetic erosion of the gene pool for future breeding. Therefore, commercial plant geneticists strive to breed "widely adapted" cultivars to counteract this tendency.
A number of conditions exist that limit the success of hybridization, the most obvious is great genetic diversity between most species. But in animals and plants that are more closely related hybridization barriers can include morphological differences, differing times of fertility, mating behaviors and cues, physiological rejection of sperm cells or the developing embryo.
In plants, barriers to hybridization include blooming period differences, different pollinator vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and structural differences of the chromosomes.
Mythical, legendary and religious hybrids
Ancient folktales often contain mythological creatures, sometimes these are described as hybrids (e.g., hippogriff as the offspring of a griffin and a horse, and the Minotaur which is the offspring of Pasiphaë and a white bull). More often they are kind of chimera, i.e., a composite of the physical attributes of two or more kinds of animals, mythical beasts, and often humans, with no suggestion that they are the result of interbreeding, e.g., harpies, mermaids, and centaurs.
In the Bible, the Old Testament contains several passages which talk about a first generation of hybrid giants who were known as the Nephilim. The Book of Genesis (6:4) states that "the sons of God went to the daughters of humans and had children by them". As a result, the offspring was born as hybrid giants who became mighty heroes of old and legendary famous figures of ancient times. In addition, the Book of Numbers (13:33) says that the descendants of Anak came from the Nephilim, whose bodies looked exactly like men, but with an enormous height. According to the apocryphal Book of Enoch the Nephilim were wicked sons of fallen angels who had lusted with attractive women.
- Artificial selection
- Bird hybrids
- Canid hybrid
- Chimera (genetics)
- Chloroplast capture (botany)
- Felid hybrids
- F1 hybrids
- Genetic admixture
- Genetic erosion
- Grex (horticulture)
- Heterosis (hybrid vigor)
- Hybrid lovebird
- Hybrid (mythology)
- Hybrid name (botany)
- Hybrid speciation
- Hybrid swarm
- Hybrid zone
- Interspecific pregnancy
- Intraspecific breeding
- Macropod hybrids
- Selective breeding
- Sheep-goat hybrids
- Species barrier
- "Hybrid". Merriam Webster. Retrieved 2014-06-12.
- Rieger, R.; Michaelis A.; Green, M. M. (1991). Glossary of Genetics (5th ed.). Springer-Verlag. ISBN 0-387-52054-6 page 256
- Keeton, William T. 1980. Biological science. New York: Norton. ISBN 0-393-95021-2 page A9.
- Ghigi A. 1936. "Galline di faraone e tacchini" Milano (Ulrico Hoepli)
- Oxford English Dictionary Online, Oxford University Press 2007.
- "When the sire is a lion the result is termed a Liger, whilst the converse is a Tigon." Edward George Boulenger, World natural history, B. T. Batsford ltd., 1937, p. 40.
- Wricke, Gunter, and Eberhard Weber. 1986. Quantitative genetics and selection in plant breeding. Berlin: W. de Gruyter. Page 257.
- J. O. Rawlings, C. Clark Cockerham Analysis of Double Cross Hybrid Populations. J. O. Rawlings, C. Clark Cockerham Biometrics, Vol. 18, No. 2 (Jun., 1962), pp. 229-244 doi:10.2307/2527461
- Roy, Darbeshwar. 2000. Plant breeding analysis and exploitation of variation. Pangbourne, UK: Alpha Science International. Page 446.
- Toogood, A.(ed.) (1999). Plant Propagation (1st American ed.). American Horticultural Society. ISBN 0-7894-5520-X page 21
- Arnold, M.L. (1996). Natural Hybridization and Evolution. New York: Oxford University Press. p. 232. ISBN 978-0-19-509975-1.
- Keeton, William T. 1980. Biological science. New York: Norton. ISBN 0-393-95021-2 Page 800
- Rong, R; Chandley, A. C.; Song, J; McBeath, S; Tan, P. P.; Bai, Q; Speed, R. M. (1988). "A fertile mule and hinny in China". Cytogenetics and cell genetics. 47 (3): 134–9. doi:10.1159/000132531. PMID 3378453.
- Esch, Mary. "Study: Eastern wolves are hybrids with coyotes". Associated Press.
- Rutledge, Linda Y.; Wilson, Paul J.; Klütsch, Cornelya F.C.; Patterson, Brent R.; White, Bradley N. (2012). "Conservation genomics in perspective: A holistic approach to understanding Canis evolution in North America" (PDF). Biological Conservation. 155: 186–192. doi:10.1016/j.biocon.2012.05.017. Retrieved 2013-07-01.
- Chambers, Steven M.; Fain, Steven R.; Fazio, Bud; Amaral, Michael (2012). "An account of the taxonomy of North American wolves from morphological and genetic analyses". North American Fauna. 77: 1–67. doi:10.3996/nafa.77.0001. Retrieved 2013-07-02.
- Dumbacher, J., Review of Proposed Rule Regarding Status of the Wolf Under the Endangered Species Act, NCEAS (January 2014)
- 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.
- Soubrier, Julien; Gower, Graham; Chen, Kefei; Richards, Stephen M.; Llamas, Bastien; Mitchell, Kieren J.; Ho, Simon Y. W.; Kosintsev, Pavel; Lee, Michael S. Y. (2016-10-18). "Early cave art and ancient DNA record the origin of European bison". Nature Communications. 7. doi:10.1038/ncomms13158. ISSN 2041-1723.
- Daley, Jason. "Cave Paintings Help Unravel the Mystery of the 'Higgs Bison'". Smithsonian. Retrieved 2016-10-20.
- R.W. Bulliet The Camel and the Wheel (Cambridge Mass. '75) 164-75
- Amphibians.org May 2014 Godzilla vs. Godzilla—How the Chinese Giant Salamander is taking a toll on its Japanese Comic Counterpart
- Voloder, Dubravka (3 January 1012). "Print Email Facebook Twitter More World-first hybrid sharks found off Australia". ABC News. Retrieved 5 January 2012.
- Grula, John W.; Taylor, Orley R. (1980). "The Effect of X-Chromosome Inheritance on Mate-Selection Behavior in the Sulfur Butterflies, Colias eurytheme and C. Philodice". Evolution. 34 (4): 688–95. doi:10.2307/2408022.
- "Lilium Hybrids". Pacific Bulb Society. Retrieved 2015-03-22.
- Warschefsky, E.; Penmetsa, R. V.; Cook, D. R.; von Wettberg, E. J. B. (8 October 2014). "Back to the wilds: Tapping evolutionary adaptations for resilient crops through systematic hybridization with crop wild relatives". American Journal of Botany. 101 (10): 1791–1800. doi:10.3732/ajb.1400116. PMID 25326621.
- University of Colorado Principles of Genetics (MCDB 2150) Lecture 33: Chromosomal changes: Monosomy, Trisomy, Polyploidy, Structural Changes Archived 14 October 2012 at the Wayback Machine.
- "Evaluating the utility of Arabidopsis thaliana as a model for understanding heterosis in hybrid crops", Euphytica, Springer Netherlands, 156 (1-2): 157–171, July 2007, doi:10.1007/s10681-007-9362-1, ISSN 0014-2336
- Rieseberg, Loren H.; Margaret A. Archer; Robert K. Wayne (July 1999). "Transgressive segregation, adaptation and speciation". Heredity. 83 (4): 363–372. doi:10.1038/sj.hdy.6886170. PMID 10583537.
- Smith C. Wayne. Corn: Origin, History, Technology, and Production. Wiley Series in Crop Science, 2004, p. 332.
- Allendorf, Fred W.; R.F. Leary; P. Spruell; J.K. Wenburg (November 2001). "The problems with hybrids: setting conservation guidelines". TRENDS in Ecology & Evolution. 16 (11): 613–622. doi:10.1016/S0169-5347(01)02290-X.
- Allendorf, Fred (2007). Conservation and the Genetics of Populations. Malden, MA: Blackwell Publishing. p. 534.
- Allendorf, Fred (2007). Conservation and the Genetics of Populations. Malden, MA: Blackwell Publishing. pp. 421–448.
- "Hybrid bear shot dead in Canada". BBC News. 13 May 2006.
- Ehrlich, Paul; John Holdren (26 March 1971). "Impact of population Growth". Science. 171 (3977): 1212–1216. doi:10.1126/science.171.3977.1212.
- Vitousek, Peter; Carla M. D'Antonio; Lloyd L. Loope; Marcel Rejmánek; Randy Westbrooks (1997). "Introduced Species: A Significant Component of Human-cause Global Change". New Zealand Journal of Ecology. 21 (1): 1–16.
- McCarthy, Eugene M. 2006. Handbook of Avian Hybrids of the World. Oxford: Oxford University Press. Pp. 16-17.
- McCarthy, Eugene M. 2006. Handbook of Avian Hybrids of the World. Oxford: Oxford University Press. P. 17.
- Darwin, C. 1868. Variation of Animals and Plants under Domestication, vol. II, p. 125
- Spicer, J. W. G. 1854. Note on hybrid gallinaceous birds. The Zoologist, 12: 4294-4296 (see p. 4295).
- Mooney, H. A.; Cleland, E. E. (2001). "The evolutionary impact of invasive species". Proc Natl Acad Sci U S A. 98 (10): 5446–5451. doi:10.1073/pnas.091093398. PMC 33232. PMID 11344292.
- Rhymer, JM; Simberloff, D (1996). "Extinction by Hybridization and Introgression". Annual Review of Ecology and Systematics. 27: 83–109. doi:10.1146/annurev.ecolsys.27.1.83.
- Brad M. Potts, Robert C. Barbour, Andrew B. Hingston (2001) Genetic Pollution from Farm Forestry using eucalypt species and hybrids; A report for the RIRDC/L&WA/FWPRDC; Joint Venture Agroforestry Program; RIRDC Publication No 01/114; RIRDC Project No CPF - 3A; ISBN 0-642-58336-6; ISSN 1440-6845; Australian Government, Rural Industrial Research and Development Corporation
- Devinder Sharma "Genetic Pollution: The Great Genetic Scandal" Archived 18 May 2009 at the Wayback Machine.; Bulletin 28. hosted by www.farmedia.org
- Troyer, A. Forrest. Breeding Widely Adapted Cultivars: Examples from Maize. Encyclopedia of Plant and Crop Science, 27 February 2004.
- "Barriers to hybridization of Solanum bulbocastanumDun. and S. VerrucosumSchlechtd. and structural hybridity in their F1 plants", Euphytica, Springer Netherlands, 25 (1): 1–10, January 1976, doi:10.1007/BF00041523, ISSN 0014-2336
- James L. Kugel (2009), "Traditions of the Bible: A Guide to the Bible As It Was at the Start of the Common Era", Harvard University Press, p. 198
- James L. Kugel (1997), The Bible as it was, Harvard University Press, p. 110
- Gregory A. Boyd, God at War: The Bible & Spiritual Conflict, p. 177
|Wikimedia Commons has media related to Hybridization.|
- Artificial Hybridisation – Artificial Hybridisation in orchids
- Domestic Fowl Hybrids
- Hybrid Mammals
- Hybridisation in animals Evolution Revolution: Two Species Become One, Study Says (nationalgeographic.com)
- Hybrids of wildcats with domestic cats
- Scientists Create Butterfly Hybrid – Creation of new species through hybridization was thought to be common only in plants, and rare in animals.
- What is a human admixed embryo?
- Video of Mirror carp & Gold fish spawning at a fishing venue in France