Natural Hybridization

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Natural Hybridization What 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 vari
  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 insectspecies, especially butterflies. It also occurs in many plant species, especially in areas where there is anoverlapping of habitats.Explanations regarding natural hybridization are many and varied and some botanists and other learned peoplerefuse to acknowledge the existence of such happenings and refuse to name specimens known to be naturalhybrids.However investigations into natural hybridization have been going on for centuries. Carl Linnaeus (Also know asCarl von Linné. 1707-1778.) the man who laid the foundations for the modern system of nomenclature, thesystem used to name all natural things, living or dead, conducted many investigations into the existence of naturalhybrids.Carl Linnaeus named many hundreds of animal, insect and plant species, using the system which is still in usetoday. His system basically uses two names for every species, the first being the name of the genus and thesecond the specific or species name. The language used is still essentially Latin. The generic or name of thegenus should always start with a capital letter or upper-case letter, while the name of the species starts with asmall or lower-case letter. A good example is  Homo sapiens, the name which Carl Linnaeus assigned to us, the socalled modern human beings. He did however, place us in the order of   Primates which means he believed that weare 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 interestedin 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 inevolution. Specifically, his experiments were designed to expose an essential difference between hybrids andspecies.Gregor Mendel was born as Johann Mendel in 1822 to peasant parents in Heinzendorf, in the Czech Republic. Hetook 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 theUniversity 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 Societyof Brünn in 1865 and published in 1866, his famous paper stated the laws of dominance, segregation, andindependent 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 besurprising 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 Czechnationalism 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 wascautious, particularly in not reporting his hybrid experiments with white and grey mice (Iltis 1924), hissurroundings were conducive to scientific inquiry.Mendel had a long-standing interest in breeding and crossing. As a child, Mendel spent time in the orchard withhis father, who worked with fruit trees (Voipio 1990). His high school teacher also grew fruit trees, and thecurriculum included fruit growing and beekeeping (Moore 1954, cited by Voipio 1990). The Moravianagricultural community generated much interest in questions of sheep and plant breeding, and Abbott Napp'sinterest in plant hybridization had a noticeable influence in Brünn (Orel 1996). Much of Mendel's researchconcerned hybridism and its role in evolution. He transplanted unusual wild varieties of plants to his garden, andwhen they failed to converge with the known domestic forms he concluded that environmental influence, as inLamarckian 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 eighteenthcentury. In 1737 he held the special creationist view that all species had been created by God and could notdeviate from the limits of their proper kinds (Callender 1988). He later updated his theory to account for naturalhybrids. 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 thereare 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 wasErasmusDarwin, the grandfather of the even more famous Charles Darwin. Erasmus Darwin believed that   evolution hasoccurred in living things including humans, but he only had rather vague ideas about what might be responsiblefor this change. Natural hybridization in Butterfly species is well documented, especially between species of Heliconius andEueides, as shown in the following paper by James Mallet, Walter Neukirchen and Mauricio Linaresentitled: 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 todemonstrate that hybridization still occurs between closely related species. In Victorian times and early thiscentury, 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 viewof 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 beenrenewed interest in all aspects of biodiversity, including that within species, and it is possible to discern areturn 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 hasconcentrated, perhaps for obvious reasons, on zones where hybrids are easily obtained, such as thehybrid zone   between races of   Heliconius melpomene in Eastern Ecuador (see alsoHenry Walter Bates' (1863)pioneeringwork on natural hybridization in  Heliconius , andWilliam Beebe's (1955)first experimental crosses; these arenow considered to be hybridization between geographic races of the same species). Arguably, these studiescontribute little to understanding speciation (JM criticises himself here ... and hybrid zone studies are interestingfor 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 hybridizingspecies are recognizable hybrids, and often even fewer. What is not generally realized, however, is thatthefraction of all species that hybridize is high(Mallet 2005). A world-wide survey of birds has shown that around9% of species hybridize (Panov in Grant & Grant 1992), and in European butterflies including Hesperiidae, therate 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 formsis not considered as interspecific hybridization, unless hybrids are very rare in the zone of overlap. Some generaand higher groups have much higher rates, over 20% of species, for example in the American warblers, the birdsof 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 conclusiveevidence that at least one of the Heliconiina,  Heliconius heurippa , is a hybrid species, having characteristicsinherited 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 laboratoryhybrids 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 hybridviability 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 maymerely be due to the greater ease of detection of gynandromorphs in populations polymorphic for major colour  pattern differences. Larry Gilbert (pers. comm.) obtained an interestinggyndandromorph  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 beenexcluded from our database as far as possible; for example, we here show avery odd  Eueides caught in the wild,andan aberrant  Heliconius charithonia produced in an insectary. Other probable mutant specimens are shownhere.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 F 2 (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 extensivelythroughout W. Ecuador, Andean Colombia and Venezuela, and Central America. Here female hybrids have beenfound 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. Ina 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 areextremely closely related genetically, and the rarity of hybrids is due to very strong mate discrimination (Jigginset al. 2001). Some mtDNA studies put  H. cydno within the genealogy of   H. melpomene ; i.e.  H. cydno is littlemore 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 acomplete 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 areable to maintain genetic differences in spite of hybridization, interspecific gene flow between animal speciescould be quite common, and may even contribute to heritability and genetic variation within animal species, ashas been shown for the Darwin's finches by Grant & Grant (1992). There is very clear evidence in the specimenswe 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 insome 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 previouslyrealized. 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 newlyemerged 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.  References Brower, 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: EndlessForms: 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 byhybridization 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 rainforestareas of Northern Australia, and also the vast areas of coral reefs which form the “Great Barrier Reef” off thecoast 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 rocksin that area.
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