Natural HybridizationWhat is it and where does it occur?

By A.S. (John) Mewburn.

Natural hybridization is simply part of the process of evolution. It occurs sporadically amongst animal species, particularly in fish species such as the Cichlids of South Africa and South America and frequently in insect species, especially butterflies. It also occurs in many plant species, especially in areas where there is an overlapping of habitats.

Explanations regarding natural hybridization are many and varied and some botanists and other learned people refuse to acknowledge the existence of such happenings and refuse to name specimens known to be natural hybrids.

However investigations into natural hybridization have been going on for centuries. Carl Linnaeus (Also know as Carl von Linné. 1707-1778.) the man who laid the foundations for the modern system of nomenclature, the system used to name all natural things, living or dead, conducted many investigations into the existence of natural hybrids.

Carl Linnaeus named many hundreds of animal, insect and plant species, using the system which is still in use today. His system basically uses two names for every species, the first being the name of the genus and the second the specific or species name. The language used is still essentially Latin. The generic or name of the genus should always start with a capital letter or upper-case letter, while the name of the species starts with a small or lower-case letter. A good example is Homo sapiens, the name which Carl Linnaeus assigned to us, the so called modern human beings. He did however, place us in the order of Primates which means he believed that we are closer to apes and monkeys than we are to any other form of animals.

During his investigations into the various species of animals, plants, fish and insects that were living on our planet during his lifetime, he found evidence of many occurrences of natural hybridization, and in fact was of the belief that many of the new species that had been named, were in fact not new species at all, but natural hybrids. There is however some doubt about whether he actually acknowledged that natural hybrids were in fact, part of the process of evolution.

It was however Gregor Mendel's 1866 paper on plant hybridization that formed the basis for the modern study of genetics, it was also used in the 1940s in support of Darwin's theory of evolution. Mendel himself was interested in the question of evolution, but ironically his experiments were done in support of the theory of special creation. He worked in the tradition of Kolreuter and Gartner, studying Linnaeus's theory that hybrids played a role in evolution. Specifically, his experiments were designed to expose an essential difference between hybrids and species.

Gregor Mendel was born as Johann Mendel in 1822 to peasant parents in Heinzendorf, in the Czech Republic. He took the name Gregor in 1843 upon joining the Augustinian monastery at Brunn, the capital of the province of Moravia. There he became a high school supply teacher, and in 1851 he was sent to study natural science at the University of Vienna. He was ordained as a priest in 1847 and was ultimately elected to the position of Abbott. He is remembered for his research on inheritance in Pisum hybrids. First presented to the Natural History Society of Brünn in 1865 and published in 1866, his famous paper stated the laws of dominance, segregation, and independent assortment which are still used in the study of genetics.

Despite his occupation as a priest, Mendel was scientific in his approach to the question of evolution. It would be surprising for a "zealous defender of the faith" in 1866 to consider seriously ideas of evolution and in particular Darwinism (Bishop 1996), but Mendel's environment was uncommonly liberal (Voipio 1990). F. C. Napp, Mendel's predecessor as Abbott, was a member of many scientific societies and shared Mendel's interest in

breeding (Orel 1996). Other members of the monastery included F. M. Klacel, who shared Mendel's interest in evolution. Klacel had been prevented from teaching by the time Mendel arrived as a result of his Czech nationalism and Hegelian philosophy (Orel 1996). Mendel himself had a scientific education at the University of Vienna, and wrote about geology and organic evolution on his 1850 teaching examination. Although Mendel was cautious, particularly in not reporting his hybrid experiments with white and grey mice (Iltis 1924), his surroundings were conducive to scientific inquiry.

Mendel had a long-standing interest in breeding and crossing. As a child, Mendel spent time in the orchard with his father, who worked with fruit trees (Voipio 1990). His high school teacher also grew fruit trees, and the curriculum included fruit growing and beekeeping (Moore 1954, cited by Voipio 1990). The Moravian agricultural community generated much interest in questions of sheep and plant breeding, and Abbott Napp's interest in plant hybridization had a noticeable influence in Brünn (Orel 1996). Much of Mendel's research concerned hybridism and its role in evolution. He transplanted unusual wild varieties of plants to his garden, and when they failed to converge with the known domestic forms he concluded that environmental influence, as in Lamarckian evolution, could not account for the modification of species (Iltis 1924).

Mendel's idea that some species might begin as hybrids was introduced by Carolus Linnaeus in the eighteenth century. In 1737 he held the special creationist view that all species had been created by God and could not deviate from "the limits of their proper kinds" (Callender 1988). He later updated his theory to account for natural hybrids. Although he did not perform any careful experiments, he was confident that they existed (Olby 1966). First he classified them as "at least permanent varieties," and by 1759 he found it "impossible to doubt that there are new species produced by hybrid generation" (Callender 1988). He proposed that God had initially created one plant in each Order, which then crossed to form Genera and Species (Callender 1988.)

For further information on Mendel’s research and experiments:

Voipio, P. (1990). "When and how did Mendel become convinced of the idea of general, successive evolution?" Hereditas 113: 179-181.

Yet another famous naturalist who conducted investigations into natural hybrids and evolution was Erasmus Darwin, the grandfather of the even more famous Charles Darwin. Erasmus Darwin believed that evolution has occurred in living things including humans, but he only had rather vague ideas about what might be responsible for this change. 

Natural hybridization in Butterfly species is well documented, especially between species of Heliconius and Eueides, as shown in the following paper by James Mallet, Walter Neukirchen and Mauricio Linares entitled:

Hybrids between species of Heliconius and Eueides butterflies: a database.One of the best ways of showing evolutionary continuity between species and geographic races is to demonstrate that hybridization still occurs between closely related species. In Victorian times and early this century, naturalists were very interested, like "stamp-collectors", in freaks of nature, including rare hybrids between species. Between the 1930s and about 1980, there was decreased interest about the peculiarities of nature, and increased emphasis on the "fundamental" biological "realities" of animal species. (Hybridization between plant species is so abundant and easily shown, of course, this rather myopic view of "pure", "good" species has never really caught on among botanists). Widely used field guides from this period often omit pictures even of common hybrids of birds and butterflies that can be seen in the wild, while treating much rarer or even extinct species in the same book. Recently, however, there has been renewed interest in all aspects of biodiversity, including that within species, and it is possible to discern a return of interest in variants, hybrids, and exceptions ("bad species") as well as the good species. In some beautiful recent books, often thorough world treatments of particular groups of butterflies and birds, hybridization between species is again becoming well-documented.

There has also been an impressive amount of recent work on hybrid zones, but most of this work has concentrated, perhaps for obvious reasons, on zones where hybrids are easily obtained, such as the hybrid zone between races of Heliconius melpomene in Eastern Ecuador (see also Henry Walter Bates' (1863) pioneering

work on natural hybridization in Heliconius, and William Beebe's (1955) first experimental crosses; these are now considered to be hybridization between geographic races of the same species). Arguably, these studies contribute little to understanding speciation (JM criticises himself here ... and hybrid zone studies are interesting for other reasons!), because the forms that interact have clearly not speciated.

Hybrids between species are much rarer: usually less than one in a thousand individuals in a pair of hybridizing species are recognizable hybrids, and often even fewer. What is not generally realized, however, is that the fraction of all species that hybridize is high (Mallet 2005). A world-wide survey of birds has shown that around 9% of species hybridize (Panov in Grant & Grant 1992), and in European butterflies including Hesperiidae, the rate is about 12% of species (Guillaumin & Descimon 1976) - here species are classified conservatively using the polytypic species concept, not the so-called "phylogenetic concept", so hybridization between geographic forms is not considered as interspecific hybridization, unless hybrids are very rare in the zone of overlap. Some genera and higher groups have much higher rates, over 20% of species, for example in the American warblers, the birds of paradise, and Darwin's finches among the birds (Grant & Grant 1992). See also Mallet (2005) for a review of natural hybridization in animals which surveys a number of groups, including birds, mammals, as well as insects, and compares them to hybridization rates in plants.

A somewhat related topic is the topic of hybrid speciation. The speciation of taxa due to hybridization requires, of course, the existence of ongoing natural hybridization documented here. Recent publications provide conclusive evidence that at least one of the Heliconiina, Heliconius heurippa, is a hybrid species, having characteristics inherited from the local races of both Heliconius cydno and H. melpomene (Salazar et al. 2005, Mavárez et al. 2006).

We here provide an updated database of wild-caught interspecific Heliconius hybrids. In this butterfly genus, about 26% of species are known to hybridize (Mallet et al. 1998, Mallet 2005).

For many of these species, laboratory hybrids have now been produced.  We have excluded any laboratory hybrids from the database because we were interested here mainly in the potential for natural hybrids.  However, the artificially produced hybrids are a useful confirmation of the hybrid status of the wild-caught individuals. Several recent studies have dealt with laboratory hybridization, the inheritance of colour pattern, and hybrid viability and sterility between Heliconius species (Jiggins & McMillan 1997, McMillan et al. 1997, Naisbit et. al. 2002, 2003).  Gynandromorphs, presumably a result of chimaeric development of separately fertilized zygotes, are relatively common in hybridization experiments between geographic races of Heliconius species.  This may merely be due to the greater ease of detection of gynandromorphs in populations polymorphic for major colour pattern differences.  Larry Gilbert (pers. comm.) obtained an interesting gyndandromorph Heliconius cydno x H. melpomene hybrid, perhaps the only one of its kind.

Several specimens are unique and may be simple mutational variants, as opposed to hybrids.  These have been excluded from our database as far as possible; for example, we here show a very odd Eueides caught in the wild, and an aberrant Heliconius charithonia produced in an insectary. Other probable mutant specimens are shown here.

Between most pairs of species, hybrids are very rare in nature. The only exceptions are H. himera and H. erato, which hybridize wherever their ranges abut in contact zones. In this pair of species, there is no inviability or sterility among the hybrids, backcrosses, or F2 (McMillan et al. 1997). The species remain distinct because of mate choice (which is about 5% "leaky"), and strong ecological selection against hybrids.

In another good example, Heliconius melpomene and H. cydno hybridize regularly (though at low frequency, maybe 1/1000 individuals are hybrids) throughout their joint range, and their distributions overlap extensively throughout W. Ecuador, Andean Colombia and Venezuela, and Central America. Here female hybrids have been found in the laboratory to be sterile (Naisbit et. al. 2002, 2003), but wild hybrids are often backcross phenotypes, showing that males backcross in the wild as well as in captivity. Once hybrids have been produced in a local population, backcross phenotypes may survive at high frequency for several generations. In a collection of 103 H. cydno and H. melpomene made by Jesús Mavárez in the botanic garden of San Cristobal, Táchira, Venezuela,

seven were putative backcross hybrids, even though such hybrids are rare elsewhere. These two species are extremely closely related genetically, and the rarity of hybrids is due to very strong mate discrimination (Jiggins et al. 2001). Some mtDNA studies put H. cydno within the genealogy of H. melpomene; i.e. H. cydno is little more than a clade of Heliconius melpomene that has speciated,suggesting H. melpomene a paraphyletic remnant (Brower 1996).

An important conclusion that can be drawn from this kind of data is that speciation doesn't suddenly lead to a complete absence of gene flow. There may be several millions of years after speciation during which genes may be exchanged between newly-evolved, recognizably and ecologically distinct species. Given that new species are able to maintain genetic differences in spite of hybridization, interspecific gene flow between animal species could be quite common, and may even contribute to heritability and genetic variation within animal species, as has been shown for the Darwin's finches by Grant & Grant (1992). There is very clear evidence in the specimens we illustrate that wild hybrids between H. melpomene and H. cydno, and between H. erato and H. himera, backcross regularly. We are currently undertaking studies to investigate whether significant gene flow occurs in some parts of the genome between Heliconius melpomene and H. cydno, while leaving other parts of the genome, affecting ecological and colour pattern differences, intact.

Another conclusion is that reinforcement (adaptive evolution of mate choice) may be more likely than previously realized. Reinforcement is often seen as unlikely because the evolution of mating isolation has to race against the breakdown of the genetic differences due to hybridization - the latter will usually win. But, given that newly emerged species can stably maintain their genetic differences in the face of gene flow, further mate choice should be able to evolve adaptively to prevent the production of genetically inferior hybrids.

ReferencesBrower, A.V.Z. 1996. Parallel race formation and the evolution of mimicry in Heliconius butterflies: a phylogenetic hypothesis from mitochondrial DNA sequences. Evolution 50: 195-221.

Grant P.R. & Grant B.R. 1992. Hybridization of bird species. Science 256: 193-197.Guillaumin, M. & Descimon, H. 1976, in: Les Problèmes de l'Espèce dans le Règne Animal. Vol. 1. Eds: Bocquet, C., Génermont, J., & Lamotte, M., Société zoologique de France, Paris, 129-201.Jiggins, C.D. & McMillan, W.O. 1997. The genetic basis of an adaptive radiation: warning colour in two Heliconius species. Proc. Roy. Soc. Lond. B  264: 1167-1175.Jiggins, C.D., Naisbit, R.E., Coe, R.L. & Mallet, J. 2001.  Reproductive isolation caused by colour pattern mimicry.  Nature 411: 302-305.Mallet, J. 2005. Hybridization as an invasion of the genome. Trends in Ecology and Evolution 20: 229-237.Mallet, J., McMillan, W.O. & Jiggins, C.D. 1998. Mimicry and warning color at the boundary between races and species. In: Endless Forms: Species and Speciation. Eds: Howard, DJ & Berlocher, SH, Oxford Univ. Press, New York, 390-403.Mavárez, J., Salazar, C., Bermingham, E., Salcedo, C., Jiggins, C.D. & Linares, M. 2006. Speciation by hybridization in Heliconius butterflies. Nature 441: 868-871.McMillan, W.O., Jiggins, C.D., & Mallet, J. 1997. What initiates speciation in passion-vine butterflies? Proc. Natl. Acad. Sci. USA 94: 8628-8633.

During my years of study in Australia, I was involved in a considerable amount of field work in the rainforest areas of Northern Australia, and also the vast areas of coral reefs which form the “Great Barrier Reef” off the coast of Queensland, Australia.

I was fortunate enough to find and observe many natural hybrids between species of the genus Orchidaceae. These finds culminated in my discovery of an extremely rare double natural hybrid.

Natural hybrids between Dendrobium speciosum var curvicaule F. M. Bail. and Dendrobium ruppianum A.D. Hawkes. are reasonably common in North Queensland and can be found both in the oak tree forests and on rocks in that area.

These plants are known as Dendrobium X ruppiosum, the large X designates it as a natural hybrid, and the rupp being the first part of “ruppianum” while the iosum is the second part of “speciosum.”

The specimen I found on the Atherton Tableland however was quite different. As it was quite a large plant when I found it, the features of the plant were quite different to the X ruppiosums that I had found before. Investigations carried out by a leading botanist confirmed my suspicions that it was indeed a double natural hybrid. The parentage was most probably Dendrobium X ruppiosum x Dendrobium speciosum var. curvicaule F. M. Bail.

The specimen of Dendrobium X ruppiosum x Dendrobium speciosum var. curvicaulle found by the author near Atherton North Queensland Australia.

This was only the second “Double natural hybrid” to have ever been found in Australia at that time. The first was found near the tip of Cape York Peninsula and was a hybrid between DendrobiumX superbiens Reichb. anf Dendrobium bigibbum var bigibbum Ldl. This plant was originally named Dendrobium bigibbum var. venosum in 1891 by F. M. Bail. but later this was revised and it was re-named Dendrobium bigibbum forma venosum F. M. Bail. in 1902.Many natural hybrids do not even get names like the aforementioned Den. X ruppiosum, they are just called Dendrobium (species name) x Dendrobium (species name).One orchid that was given a name however was the natural hybrid between Dendrobium kingianum Bidw. and Dendrobium gracilicaule F. Muell. It was originally described as Dendrobium suffusum by Leo Cady in 1964. The type plant was found growing on North Brother Mountain at Laurieton New South Wales in September 1961. The name was later changed to Dendrobium X suffusum to designate it as a natural hybrid.

More than 20 natural hybrids between species of Orchidaceae have been found in Australia but I am sure there are many more to be found as hundreds have been found throughout the world. Here is a list of the Dendrobium natural hybrids found in Australia:

Dendrobium discolor Lindley X Dendrobium semifuscum Lavarack and CribbDendrobium discolor Lindley X Dendrobium antennatum Lindley.Dendrobium discolor Lindley X Dendrobium nindii W. HillDendrobium discolor Lindley X Dendrobium bigibbum Lindley=Dendrobium X superbiens H.G. ReichbDendrobium discolor Lindley X Dendrobium bigibbum var. bigibbum subvar. compactum C. T. White. = Dendrobium X vinicolor St. Cloud.Dendrobium canaliculatum R. Br. X Dendrobium semifuscum Lavarack and Cribb

Dendrobium linguaforme Var. nugentii Bailey X Dendrobium rasemossum Nicholls.Dendrobium linguaforme Var. nugentii Bailey X Dendrobium teretifolium var. fasciolatum Rupp.= Dendrobium X grimesii C. T. White and Summerh.Dendrobium pugioniforme A. Cunn. X Dendrobium Tenuissimum Rupp.Dendrobium speciosum var. speciosum J. E. Sm. X Dendrobium gracilicaule F. Muell = D. X gracilimum Rupp.Dnedrobium speciosum var. speciosum J. E. Sm. X Dendrobium kingianum Bidw. = X delicatum F. M. Bailey.Dnedrobium gracilicaule F. Muell. X Dendrobium kingianum Bidw. = Dendrobium X suffusum Cady.

The following is a list of some of the Phalaenopsis natural hybrids that have been found.Phalaenopsis × amphitrita (P. sanderiana × P.stuartiana; Philippines).Phalaenopsis × gersenii (P. sumatrana × P. violacea; Borneo, Sumatra). Phalaenopsis × intermedia (P. aphrodite × P.equestris; Star of Leyte; Philippines) (First recognized Phalaenopsis hybrid) Phalaenopsis × leucorrhoda (P. aphrodite × P.schilleriana; Philippines). Phalaenopsis × singuliflora (P. bellina × P.sumatrana; Borneo). Phalaenopsis × veitchiana (P. equestris × P.schilleriana; Philippines).

Some information about a more recently discovered natural hybrid:

New orchid natural hybrid blooms in UK04-8-2006

Two rare orchids have crossed and produced a natural hybrid which recently flowered for the first time.Tests have proved that the two parents, both endangered species are the monkey orchid (Orchis simian) and the lady orchid (Orchis purpurea.)The flower of the new orchid resembles that of another rare species the military orchid (Orchis militaris,) this has given rise to thoughts that the military orchid may not be a species at all, but a natural hybrid between Orchis simian and Orchis purpurea that occurred long ago. This would account for the limited distribution of Orchis militaris and its rarity.I believe that natural hybridization is occurring everywhere around us and is happening everyday. How do you recognize a natural hybrid? A good knowledge of both of the parents involved in the hybridization is necessary to be able recognize if it is a genuine natural hybrid, or merely a form of mutation. Gaining this knowledge is a long-term project and often involves long periods of study in the field. This can be seen by the following experience that I had after many years of diving on the Great Barrier Reef.I was also fortunate to find an extremely rare natural hybrid between two species of butterfly fish, namely Chaetodon myeri and Chaetodon ornatissimus, on Euston Reef, about 45 kilometers off the coast from Cairns North Queensland.Chaetodon myeri is a very rare fish in Australian waters, but is quite common on the reefs of other islands in the Pacific Ocean. Chaetodon ornatissimus is quite common on the Great Barrier Reef, but only in the northern parts and on the outer reefs.I observed a number of specimens of natural hybrids from the mating of two very beautiful species of butterfly fish. The offspring distinctly showed the features of both of the parents.

Chaetodon meyeri Chaetodon ornatissimus Hybrid between Chaetodon meyeri Bloch and Schneider 1801 Cuvier 1831 and Chaetodon ornatissimus First sighted on Euston reef by the author.

Another natural hybrid between two Chaetodon species is this cross between Chaetodon reticulatus Cuvier 1831 and Chaetodon meyeri Bloch and Schneider 1801 (See photo above.)

Chaetodon reticulates Cuvier 1831. All Chaetodon photos reproduced with permission from http://www.edge-of-reef.com

Since those sightings there have been many specimens collected of other natural hybrids between species of both butterfly fishes and saltwater angel fish particularly among the genus Centropyge. These are among some of the definite records of natural hybrids between Centropyge species.Centropyge eibli x flavissimusCentropyge eibli x vrolikiiCentropyge flavissimus x vrolikiiCentropyge loriculus x potteriCentropyge multifasciatus x venustus

Natural hybrid between Centropyge flavissimus x vrolikii.

In fresh water fish, the records are innumerable especially in the genus Cichlidae. Species in this genus are found in both South America and South Africa. The species found in South Africa live in lakes and are usually small (Two to three inches) while the South American species are found in rivers and are much larger.

The South African species number around 600 but it is possible that some of the named species are in fact natural hybrids from a mating that occurred long ago. Many of these species tend to live in small colonies in lakes such as Lake Malawi. Natural hybrids occur frequently in these areas because the occasional adventurous specimen may swim into neighboring territory, if he/she is accepted, then mating may occur, giving rise to a new batch of natural hybrids, which may then move and inhabit a different area of the lake.

In South America there are also many records of natural hybrids, especially in the genus Cichla, which is under review at the time of writing.

The number of natural hybrids amongst the species of grasses is innumerable, and is continuing everyday, somewhere in the world. As is natural hybridization in trees and many other types of plants.

Regardless of whether a God made the original inhabitants on our planet or we originated from some very primitive form of life, one thing is certain, life in one form or another has been on our planet for a long, long time and during that time the process of evolution has played a big part in bringing the life forms on our planet to where they are today.

Natural hybridization is a part of that ongoing process. Most natural hybrids are generally stronger than their parents and can therefore survive under harder conditions than their parents can adapt to. The part they play in the evolutionary process is to produce stronger, more resilient life forms, so they can cope with the changing conditions on our planet.

Is natural hybridization something we should be concerned about? Of course not! It has been ongoing for millions of years, and will continue, unless we destroy our planet in the meantime. After all, the human race (Homo sapiens) as we know it today, is in fact only a natural hybrid of other life forms that have existed for thousands (Possibly millions) of years.

Written by Andrew S. MewburnLecturer: President University.Jababeka Education Park. Indonesia.