Genetic pollution

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Genetic pollution, genetic contamination or genetic swamping happens when original set of naturally evolved (wild) region specific genes / gene pool of wild animals and plants become hybridized with domesticated and feral varieties or with the genes of other nonnative wild species or subspecies from neighboring or far away regions.

Usually Genetic pollution happens as a result and a consequence of mankind's interaction with natural environment and a lack of foresight but in rare instances it has also been observed happening naturally more commonly in case of closely related subspecies of plants whose ranges overlap forming hybrid zones making it easier for insects to cross pollinate them.

In most cases nature has its own interspecies genetic barriers to guard against genetic pollution to keep species distinct. When rarely hybridization does occur naturally as in hybrid zones where the ranges of closely related wild species overlap, the hybrid crosses produced, even though they may display hybrid vigour (heterosis) in the first generation (F1 hybrid), are in the long run less fit then the two parent species which have evolved over hundreds of thousands of years specializing in exploiting their own particular niche in nature. It is extremely rare that the hybrids ever become fitter then the two wild parent species so that natural selection may then favor these individuals and it is even more extremely rarer that reproductive isolation is ever achieved to lead to the birth of a new species through the process known as Hybrid speciation.

Endangers wild species and quickens their extinction

Purebred naturally evolved region specific wild species can be threatened with extinction in a big way[1] through the process of Genetic Pollution i.e. uncontrolled hybridization, introgression and Genetic swamping which leads to homogenization or replacement of local genotypes as a result of either a numerical and/or fitness advantage of introduced plant or animal[2]. Nonnative species can bring about a form of extinction of native plants and animals by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species coming into contact with more abundant ones where the abundant ones can interbreed with them swamping the entire rarer gene pool creating hybrids thus driving the entire original purebred native stock to complete extinction. Attention has to be focused on the extent of this under appreciated problem that is not always apparent from morphological (outward appearance) observations alone. Some degree of gene flow may be a normal, evolutionarily constructive process, and all constellations of genes and genotypes cannot be preserved however, hybridization with or without introgression may, nevertheless, threaten a rare species' existence[3][4].

Widespread genetic pollution also leads to weakening of the naturally evolved (wild) region specific gene pool leading to weaker hybrid animals and plants which are not able to cope with natural environs over the long run and fast tracks them towards final extinction.

The gene pool of a species or a population is the complete set of unique alleles that would be found by inspecting the genetic material of every living member of that species or population. A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection. Meanwhile, low genetic diversity (see inbreeding and population bottlenecks) can cause reduced biological fitness and an increased chance of extinction amongst the reducing population of purebred individuals from a species.


Nowadays, with several domesticated and hybridized animals and plants living in towns and villages in and all around forests which still hold naturally evolved region specific original wild ancestor species and subspecies of their kind, genetic pollution has become a major threat to naturally evolved wild populations and their natural genetic makeup. Wild populations of animals and plants have evolved naturally over millions of years through a process of natural selection as apposed to human controlled selective breeding or artificial selection in animal husbandry and agriculture for desirable traits from the human point of view. These naturally evolved wild species (wildlife) and region specific subspecies are most adaptable to living in the particular regions of earth where they exist presently. They have scientifically the most endurable and viable genetic makeup to live where they live. Genetic pollution leads to hybridized animals and plants which, even though they may look exactly the same, are not as genetically strong as their naturally evolved region specific wild ancestors who can survive under natural wild conditions indefinitely without any human help or husbandry and have high adaptability to local climatic conditions and a high immunity to local natural diseases.

Throughout history with the movement and immigration of people from one part of the world to another, intentionally or unintentionally nonnative species (See: Exotic or Introduced species and Invasive species) have spread creating havoc on indigenous wild animals and plants and local ecosystems. Nonnative species were introduced intentionally into new regions because of aesthetic reasons or for hunting & sport and sometimes for emotional reasons as people missed them in their adopted homelands where they had settled. These nonnative species and subspecies have become another common source of genetic pollution as they readily crossbreed with closely related often much rarer indigenous forms.

In modern times even the national and international zoo associations have realized how important it is to maintain purebred naturally evolved region specific subspecies of wild animals which may one day have a chance to be reintroduced back and survive in the wild in their original home range. It has now become mandatory for zoos to maintain detailed breeding records of their wild animal stock, keep detailed studbooks and have registered breeding programs. Random hybridization of wild animals with subspecies from different regions was a common practice which plagued all of world's zoos until recently.

Usually genetic pollution happens as a result and a consequence of mankind's activities and interaction with natural environment and a lack of foresight but in rare instances it has also been observed happening naturally more commonly in case of closely related subspecies of plants whose ranges overlap forming hybrid zones making it easier for insects to cross pollinate them.

Significance to us

Amongst other threats it is also important to give due importance to and guard against genetic pollution so that we can conserve the naturally evolved region specific wild gene stock and genetic makeup of wild animals and wild plants for maintaining the health of natural ecosystems and the environment in general. Scientists now, also consider this naturally evolved biodiversity to be a valuable source of strong genes which in the future may be used on a continual basis to hybridize domesticated varieties, to make them even stronger and more resistant to climate and diseases, thus leading to an ongoing improvement in our food security and medicines etc. for us humans ourselves.


  • Native captive Asiatic Lions in Indian zoos till recently were genetically polluted with genes of Circus confiscated African Lions which had been randomly hybridized with them leading to widespread genetic pollution in the captive Asiatic Lion population. Once discovered this led to the complete shut down of the European (EEP) and the American endangered species registered breeding programs (SSP) for Asiatic Lions as the founder animals originally imported from India were ascertained to be genetically polluted with the genes of African lions. Since then India has corrected its mistake and now breeds only pure native Asiatic Lions which are critically endangered with extinction and has helped revive the European endangered species registered breeding program (EEP) for Asiatic Lions but the American SSP which completely shut down in early 80’s has yet to be revived and receive pure bred Asiatic Lions from India to form a new founder population for breeding in the zoos on the American continent[5][6][7][8][9][10].
  • Wild Jungle fowl, specifically Red Junglefowl in India and South Asia are the ancestor of all Poultry Chickens. These are thought to be facing a serious threat of extinction because of genetic pollution which is occurring at the edge of forests where domesticated free ranging chickens are commonly kept in bordering villages and towns, these meet and breed with their wild cousins indiscriminately genetically polluting their ancestors[11][12][13][14][15][16][17][18].
  • The Wild Asian Water Buffalo is facing a serious threat of extinction because of genetic pollution when it comes into contact with common abundant Domestic Asian Water Buffalo, which live in villages and towns all around forests. The domesticated animals daily come to graze within forests which have been designated as Wildlife Sanctuaries and National Parks for their very rare wild ancestors and interbreed with them genetically swamping the wild gene pool raising fears that no pure Wild Asian Water Buffalo may be alive today[19][20][21][22][23].
  • Many herds of surviving American Bison, even though they look exactly the same, were hybridized with genes of beef cattle by landowners in a short-sighted effort to improve their meat quality (see Cattalo), leading to widespread genetic pollution in the surviving bisons in America. The current American Bison population has been growing rapidly and is estimated at 350,000, but this is compared to an estimated 60–100 million in the mid-19th century. Most current herds, however are genetically polluted or partly crossbred with cattle hence are in fact what are called "beefalo"[24][25][26][27]; today there are only four genetically unmixed herds and only one that is also free of brucellosis: it roams Wind Cave National Park. A founder population from the Wind Cave herd was recently established in Montana by the World Wildlife Fund.
  • Tara, a hand-reared supposedly Bangal tigress acquired from Twycross Zoo in England in July 1976 was trained by Billy Arjan Singh and released to the wild in Dudhwa National Park, India with the permission of India’s then Prime Mininster Indira Gandhi in an attempt to prove the experts wrong that zoo bred hand reared Tigers can ever be released in the wild with success. In the 1990s, some tigers from Dhudhwa were observed which had the typical appearance of Siberian tigers: white complexion, pale fur, large head and wide stripes. With recent advances in science it was subsequently found that Siberian Tigers genes have polluted the otherwise pure Bengal Tiger gene pool of Dudhwa National Park. It was proved later that Twycross Zoo had been irresponsible and maintained no breeding records and had given India a hybrid Siberian-Bengal Tigress instead. Dudhwa tigers constitute about 1% of India's total wild population, but the possibility exists of this genetic pollution spreading to other tiger groups, at its worst, this could jeopardize the Bengal tiger as a distinct subspecies[28][29][30][31][32][33][34][35][36][37].
  • Release of feral Mallard Ducks worldwide is creating havoc on indigenous waterfowl, unlike wild Mallards these don't migrate and stay back in the local breeding season and interbreed with indigenous rare wild ducks devastating local populations of closely related species through genetic pollution by producing fertile offspring. Complete hybridization of various species of rare wild duck gene pools could result in the extinction of many indigenous waterfowl. Wild Mallard itself is the ancestor of most domestic ducks and their naturally evolved wild gene pool gets genetically polluted in turn by the domestic and feral populations[38][39][40][41][42].

Conventional hybridization for higher yield, Genetic Engineering and the resulting loss of Biodiversity, a threat to Food Security

In agriculture and animal husbandry, green revolution popularized the use of conventional hybridization to increase yield many folds by creating "High yielding varieties". Often the handful of breeds of plants and animals hybridized originated in developed countries and were further hybridized with local verities, in the rest of the developing world, to create high yield strains resistant to local climate and diseases. Local governments and industry since have been pushing hybridization with such zeal that several of the wild and indigenous breeds evolved locally over thousands of years having high resistance to local extremes in climate and immunity to diseases etc. have already become extinct or are in grave danger of becoming so in the near future. Due to complete disuse because of un-profitability and uncontrolled intentional, compounded with unintentional crosspollination and crossbreeding (genetic pollution) formerly huge gene pools of various wild and indigenous breeds have collapsed causing widespread genetic erosion and genetic pollution resulting in great loss in genetic diversity and biodiversity as a whole[43].

A Genetically Modified Organism (GMO) is an organism whose genetic material has been altered using the genetic engineering techniques generally known as recombinant DNA technology. Genetic Engineering today has become another serious and alarming cause of genetic pollution because artificially created and genetically engineered plants and animals in laboratories, which could never have evolved in nature even with conventional hybridization, can live and breed on their own and what is even more alarming interbreed with naturally evolved wild varieties. Genetically Modified (GM) crops today have become a common source for genetic pollution, not only of wild varieties but also of other domesticated varieties derived from relatively natural hybridization[44][45][46][47][48].

It is being said that genetic erosion coupled with genetic pollution is destroying that needed unique genetic base thereby creating an unforeseen hidden crisis which will result in a severe threat to our food security for the future when diverse genetic material will cease to exist to be able to further improve or hybridize weakening food crops and livestock against more resistant diseases and climatic changes[49].

Natural barriers to genetic pollution

It is common knowledge that plants can't reproduce with animals, nor can fishes with reptiles or birds with insects and so on. Different species within the same animal or plant kingdoms also can not reproduce with each other (by definition) and are not attracted to each other in the wild for the purpose of reproduction. Species which fall under the same broad families like donkeys and horses or tigers and lions will mate with each other only in captivity and only if raised together from young ones but most interspecies hybrids are born without functioning reproductive systems and hence can not reproduce any further. Common example of the most famous interspecies hybrid created and utilized by man is the cross between donkeys and horses, the mule which is commonly utilized as a hardy draught animal in rough terrain but mules can not reproduce any further. The term "mule" (Latin mulus) was formerly applied to the offspring of any two creatures of different species - in modern usage, a "hybrid". Mules are almost always sterile (see fertile mules for rare cases).

Interspecific hybrids like the mule 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 very often are sterile, this hybrid sterility prevents the movement of genes from one species to the other, keeping both species distinct.[50] 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, the offspring thus have 63 chromosomes which cannot evenly divide. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes because the extra chromosome cannot make a homologous pair at meiosis, meiosis is disrupted, and viable sperm and eggs are not formed.

Most often other mechanisms are used by plants and animals to keep gametes isolated and species distinct. Species often have different mating or courtship patterns or behavours, 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. The Lonicera fly is the first known animal species that resulted from natural hybridization. Until the discovery of the Lonicera fly, this process was known to occur in nature only among plants.

See also


  1. Hybridization and Introgression; Extinctions; from "The evolutionary impact of invasive species; by H. A. Mooney and E. E. Cleland" Proc Natl Acad Sci U S A. 2001 May 8; 98(10): 5446–5451. doi: 10.1073/pnas.091093398. Proc Natl Acad Sci U S A, v.98(10); May 8, 2001, The National Academy of Sciences
  2. Glossary: definitions from the following publication: Aubry, C., R. Shoal and V. Erickson. 2005. Grass cultivars: their origins, development, and use on national forests and grasslands in the Pacific Northwest. USDA Forest Service. 44 pages, plus appendices.; Native Seed Network (NSN), Institute for Applied Ecology, 563 SW Jefferson Ave, Corvallis, OR 97333, USA
  3. EXTINCTION BY HYBRIDIZATION AND INTROGRESSION; by Judith M. Rhymer , Department of Wildlife Ecology, University of Maine, Orono, Maine 04469, USA; and Daniel Simberloff, Department of Biological Science, Florida State University, Tallahassee, Florida 32306, USA; Annual Review of Ecology and Systematics, November 1996, Vol. 27, Pages 83-109 (doi: 10.1146/annurev.ecolsys.27.1.83), [1]
  4. Genetic Pollution from Farm Forestry using eucalypt species and hybrids; A report for the RIRDC/L&WA/FWPRDC; Joint Venture Agroforestry Program; by Brad M. Potts, Robert C. Barbour, Andrew B. Hingston; September 2001; 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
  5. Mitochondrial DNA sequence divergence among big cats and their hybrids by Pattabhiraman Shankaranarayanan* and Lalji Singh*, *Centre for Cellular and Molecular Biology, Uppal Road, Hyderabad 500 007, India, Centre for DNA Fingerprinting and Diagnostics, CCMB Campus, Uppal Road, Hyderabad 500 007, India
  6. Central Zoo Authority of India (CZA), Government of India
  7. "Indians Look At Their Big Cats' Genes", Science, Random Samples, Volume 278, Number 5339, Issue of 31 October 1997, 278: 807 (DOI: 10.1126/science.278.5339.807b) (in Random Samples),The American Association for the Advancement of Science
  8. BEASTLY TALES - African-Asian lion problem was first spotted in the US. It’s the price you pay for playing God. After toying with lion-breeding programmes for years, zoo officials in India are staring at a man-made evolutionary disaster. G.S. Mudur reports Sunday, December 26, 2004 The Telegraph, Calcutta, India
  10. “This second concern was shown to be all to real when a report titled "Evidence for African Origins of the Founders of the Asiatic Lion SSP" by S.J. O’Brien et al. was published in Zoo Biology in 1987. The report’s authors used genetic tests to compare animals in the wild population in Gir with those in captivity. Those results confirmed that the majority of the captive population, at that time, was not pure Asiatic. As a result of the O’Brien report the SSP was effectively discontinued.”, The Asiatic lion captive breeding program. Asiatic Lion Information Centre Accessed on September 19, 2007
  11. CONCERNS FOR THE GENETIC INTEGRITY AND CONSERVATION STATUS OF THE RED JUNGLEFOWL by I. Lehr Brisbin, Jr. Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802 (with permission from SPPA Bulletin, 1997, 2(3):1-2) FeatherSite. Retrieved on September 19, 2007, Society for the Preservation of Poultry Antiquities, Red Junglefowl (Gallus gallus) page & links, FeatherSite by Barry Koffler
  12. Morphological and Behavioral Characteristics of Genetically Pure Indian Red Junglefowl, Gallus gallus murghi By Tomas P. Condon. Retrieved on September 19, 2007, THE JUNGLEFOWL PAGES: These pages were created to provide a reference guide to the four species of the genus Gallus, commonly known as junglefowl. It contains information and photographs of the each of the species. By Tomas P. Condon
  13. Hawkins, W.P. (n.d.). Carolinas/Virginia Pheasant & Waterfowl Society. Red Junglefowl – Pure Stain. Retrieved on September 19, 2007
  14. Gautier, Z. 2002. "Gallus gallus" (On-line), Animal Diversity Web. Accessed September 19, 2007
  15. Genetic invasion threatens red jungle fowl, Wildlife Trust of India, New Delhi, 9 January, 2006, Accessed September 19, 2007, [2]
  16. Red Junglefowl genetically swamped, 01-06-2000, According to some scientists, truly wild populations of the Red Junglefowl Gallus gallus are either extinct or in grave danger of extinction due to introgression of genes from domestic or feral chickens. Tragopan No. 12, p. 10, World Birdwatch 22 (2). Accessed September 19, 2007 from BirdLife International, Red Junglefowl - BirdLife Species Factsheet, BirdLife International (2007) Species factsheet: Gallus gallus. Downloaded Accessed on 20/9/2007
  17. [Peterson, A.T. and I.L. Brisbin, Jr.: 1999. Genetic endangerment of wild red junglefowl (Gallus gallus) ? Bird Conservation International (Vol. 9) pp. 387-394.]
  18. [Brisbin, I. L., Jr. (1969). Behavioral differentiation of wildness in two strains of Red Junglefowl (abstract). Amer. Zool. 9:1072.]
  19. Wild buffalo faces extinction, Chattisgarh, 15th February 2006, Wildlife Trust of India, [3]
  20. The IUCN Red list of threatened species classifies "Wild Asian Water Buffalo" (B. arnee) as "Endangered"
  21. Animal Info - Wild Asian (Water) Buffalo (B. arnee) - Status: Endangered; By: Paul Massicot
  22. Project to conserve wild Asian buffaloes, by Bindu Shajan Perappadan, The Hindu National Newspaper, Feb 26, 2006, [4]
  23. Asian Wild Buffalo (Bubalus bubalis Linnaeus, 1758) by P.K.Mathur, P.K. Malik and P.D. Muley. From the “Wildlife Institute of India” website.
  24. Strands of undesirable DNA roam with Buffalo, By Jim Robbins, 9th January 2007, The New York Times
  25. Polzhiehn, R.O., C. Strobeck, J. Sheraton, and R. Beech (1995). Bovine mtDNA Discovered in North American Bison Populations. Conservation Biology 9:6; 1638-43.
  26. Halbert, N.D., Ward, T.J., Schnabel, R.D., Taylor, J.F and Derr, J.N. (2005) Conservation genomics: disequilibrium mapping of domestic cattle chromosomal segments in North American bison populations. Molecular Ecology (2005) 14, 2343–2362
  27. Halbert, Natalie Dierschke (2003) The utilization of genetic markers to resolve modern management issues in historic bison populations: implications for species conservation Ph. D. Dissertation, Texas A&M University, December 2003
  28. Indian tiger isn't 100 per cent “swadeshi (Made in India)”; by PALLAVA BAGLA; Indian Express Newspaper; November 19, 1998
  29. Tainted Royalty, WILDLIFE: ROYAL BENGAL TIGER, A controversy arises over the purity of the Indian tiger after DNA samples show Siberian tiger genes. By Subhadra Menon. INDIA TODAY, November 17, 1997
  30. The Tale of Tara, 4: Tara's Heritage from Tiger Territory website
  31. Genetic pollution in wild Bengal tigers, Tiger Territory website
  32. Interview with Billy Arjan Singh: Dudhwa's Tiger man, October 2000, Sanctuary Asia Magazine,
  33. Mitochondrial DNA sequence divergence among big cats and their hybrids by Pattabhiraman Shankaranarayanan* and Lalji Singh*, *Centre for Cellular and Molecular Biology, Uppal Road, Hyderabad 500 007, India, Centre for DNA Fingerprinting and Diagnostics, CCMB Campus, Uppal Road, Hyderabad 500 007, India
  34. Central Zoo Authority of India (CZA), Government of India
  35. "Indians Look At Their Big Cats' Genes", Science, Random Samples, Volume 278, Number 5339, Issue of 31 October 1997, 278: 807 (DOI: 10.1126/science.278.5339.807b) (in Random Samples),The American Association for the Advancement of Science
  36. BOOKS By & About Billy Arjan Singh
  37. Book - Tara : The Cocktail Tigress/Ram Lakhan Singh. Edited by Rahul Karmakar. Allahabad, Print World, 2000, xxxviii, 108 p., ills., $22. ISBN 81-7738-000-1. A book criticizing Billy Arjan Singh's release of hand reared hybrid Tigress Tara in the wild at Dudhwa National Park in India
  38. Mottled Ducks : The Problem – Hybridization; Florida Fish and Wildlife Conservation Commission,
  40. Invasive Alien Bird Species Pose A Threat, Kruger National Park, Siyabona Africa Travel (Pty) Ltd - South Africa Safari Travel Specialist
  41. Anas platyrhynchos, Domestic Duck; DigiMorph Staff - The University of Texas at Austin
  42. Mallard; Encyclopædia Britannica
  43. “Genetic Pollution: The Great Genetic Scandal”; Devinder Sharma can be contacted at: 7 Triveni Apartments, A-6 Paschim Vihar, New Delhi-110 063, India. Email: CENTRE FOR ALTERNATIVE AGRICULTURAL MEDIA (CAAM)., [5]
  44. THE YEAR IN IDEAS: A TO Z.; Genetic Pollution; By MICHAEL POLLAN, The New York Times, December 9, 2001
  45. Dangerous Liaisons? When Cultivated Plants Mate with Their Wild Relatives; by Norman C. Ellstrand; The Johns Hopkins University Press, 2003; 268 pp. hardcover , $ 65; ISBN 0-8018-7405-X. Book Reviewed in: Hybrids abounding; Nature Biotechnology 22, 29 - 30 (2004) doi:10.1038/nbt0104-29; Reviewed by: Steven H Strauss & Stephen P DiFazio; 1 Steve Strauss is in the Department of Forest Science, Oregon State University, Corvallis, Oregon 97331-5752, USA.; 2 Steve DiFazio is at Oak Ridge National Laboratory, Bldg. 1059, PO Box 2008, Oak Ridge, Tennessee 37831-6422 USA.
  46. “Genetic pollution: Uncontrolled spread of genetic information (frequently referring to transgenes) into the genomes of organisms in which such genes are not present in nature.” Zaid, A. et al. 1999. Glossary of biotechnology and genetic engineering. FAO Research and Technology Paper No. 7. ISBN 92-5-104369-8
  47. “Genetic pollution: Uncontrolled escape of genetic information (frequently refering to products of genetic engineering) into the genomes of organisms in the environment where those genes never existed before.” Searchable Biotechnology Dictionary. University of Minnesota. , [6]
  48. “Genetic pollution: Living organisms can also be defined as pollutants, when a non-indigenous species (plant or animal) enters a habitat and modifies the existing equilibrium among the organisms of the affected ecosystem (sea, lake, river). Non-indigenous, including transgenic species (GMOs), may bring about a particular version of pollution in the vegetal kingdom: so-called genetic pollution. This term refers to the uncontrolled diffusion of genes (or transgenes) into genomes of plants of the same type or even unrelated species where such genes are not present in nature. For example, a grass modified to resist herbicides could pollinate conventional grass many miles away, creating weeds immune to the most widely used weed-killer, with obvious consequences for crops. Genetic pollution is at the basis of the debate on the use of GMOs in agriculture.” The many facets of pollution; Bologna University web site for Science Communication. The Webweavers: Last modified Tue, 20 Jul 2005
  49. “Genetic Pollution: The Great Genetic Scandal”; Devinder Sharma can be contacted at: 7 Triveni Apartments, A-6 Paschim Vihar, New Delhi-110 063, India. Email: CENTRE FOR ALTERNATIVE AGRICULTURAL MEDIA (CAAM)., [7]
  50. ISBN 0-393-95021-2 Page 800