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3/10/2015 Lions, Tigons, and Pizzly Bears, Oh My! » Synapse | Blog Archive | Boston University

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Hercules is a liger, a liontiger hybrid.Photo Credit | Acarvin via WikimediaCommons

Lions, Tigons, and PizzlyBears, Oh My!Are humanmediated changes to the environmentdestroying species?

By Ashley Jones

His father was an African prince, his mother, afiercely independent Indian huntress. By all lawsgoverning society, geography, and biology, Herculesshould have never been born. Yet in 2002, thenewborn cub rose to fame as the largest hybrid felineat the Jungle Island Zoo in Miami, Florida. The liontiger mix inherited the best of both worlds, growinginto a 12footlong, 900pound carnivore with thecapability of running 50 miles per hour and consuming 100 pounds of meat in a

single sitting.1 His caretakers insisted that the birth was a mere “accident”, butcan such hybridization ever be truly natural?

What is Hybridization?Hybridization, the interbreeding of two related yet genetically distinct species, is

estimated to occur in at least 25% of plants and 10% of animals.2 A hybriddisplays traits intermediate between those of its father and those of its mother,indicating either the creation of new characteristics or the introgression of

genes from one taxon directly into another.3 Likewise, the species is identifiedby a portmanteau system of naming in which the first half consists of the

father’s species, and the second of the mother’s species.4 The hybrid lineagemay end with this newborn, the first member of the F1 (first hybrid) generation.



For successful generations of hybrids to occur, the two parent species mustshare at least some breeding traits, be geographically and reproductivelyavailable to each other, and produce viable and fertile offspring. Often,members of the F1 generation will mate with each other as opposed to either of

the parent species, thereby laying the foundation for the establishment of a

brand new species with selective traits.3

Natural Versus AnthropogenicHybridization in the wild, especially among plant species, is essentially an

indication of disturbance.5Anthropogenic, or human, have facilitated an

abnormal rise in hybrid mating.6 Such alterations to the environment couldpossibly result in decreased genetic diversity among species as unique allelesare is lost to a homogenized gene pool. Given enough time, the fit hybrids have

the potential to replace their weakly adapted ancestors.2

It’s Getting Hot in Here…One of the major anthropogenic factors affecting not only species hybridization,but the ecosystem as a whole, is an increase in global temperatures due tocarbon emissions. The influence of the greenhouse effect, coupled with levelsof carbon approaching 400ppm, has initiated the migration of species fromwarmer to cooler climates (and vice versa) as organisms attempt to maintain

their ideal temperature ranges.7 As organisms encounter similar species intheir new habitats, they may reproduce with them. A substantial number ofnatural hybridizations occurred during the postglacial Pleistocene era when

carbon levels exceeded those of today.8

The most recent example of temperature mediatedhybridization is between polar bears (Ursus maritimus), and



The pizzly bear is a polarbeargrizzly bear hybrid.Photo Credit | Messybeastvia Wikimedia Commons

grizzlies (Ursus arctos)in a hybrid tentatively referred to as a“pizzly” or “grolar” bear. In 2006, an Inuvialuit hunter fromVictoria Island shot what appeared to be polar bear.However, the animal possessed characteristics of agrizzly, with brown patches of fur, long claws, a concavefacial profile, and a humped back. DNA testing confirmedthe individual to be first documented case of a polargrizzly hybrid in the wild, a genetically possible butimprobable phenomenon given the differing habitat ranges

and breeding rituals of the two related species.7Humaninduced climate change, which has caused some animalsto shift their range northward, may have driven grizzlybears into polar bear territory and grizzly bears have beenrecorded in Canada’s western Arctic Nunavut province periodically throughoutthe last 50 years. An alternate theory proposes that polar bears have beendriven southward by the melting of the ice caps, bringing them into closercontact with grizzly bears. Three more hybrid individuals were reported after2006, and scientists expect more encounters between grizzlies and polar bearsin the future.

Habitat Destruction and FragmentationDeforestation and urbanization result in decreased habitat area, leading toincreased competition between individuals and smaller populations. Habitatdestruction may either isolate a singular region from a larger range, or cancombine the ranges of several species via the removal of a natural barrier such

as a mountain range or river way.3 Facing the deterioration of their territory,organisms extend their ranges to overlap with those of other individuals,leading to a higher density of individuals per unit space. Along the borderswhere one territory overlaps another, individuals are forced to share resourcesincluding food, shelter, and potential mates. Natural selection predicts that the



Yellow baboons (top) andanubis baboons (bottom) onlystarted interbreeding recently.Credit | Wikimedia Commons

most successful reproducers will be those most fit for the new environment,whether or not the parents are of the same species.

For example, yellow baboons (Papio cynocephalus) and Anubisbaboons (Papio anubis) in Kenya did not interbreed 30 yearsago; however, the proportion of hybrids has increasedover time with the construction of tourist

lodges.2 Historically, the yellow baboons occupied thesouth and east territories and the neighboring Anubislived in the north and west. Typically, baboons stay closetheir natal locations, separated from other populations byphysical obstacles like long stretches of waterless land.However, researchers observed an alternate immigrationpattern of Anubis baboons that were foraging from touristlodges. The immigration of Anubis coupled with their successful reproductionled to hybridization zones between the two baboon species. The hybrids arenot selected against as would be if they had a genetic fitness lower than theirpredecessors. Male hybrids undergo puberty earlier in life than the purebredmales and as a result, have higher reproductive and competition success.Researchers noted an increase in overall Anubis alleles in yellow baboonpopulations as well as in the number of offspring resulting from mating betweenhybrids. By 2000, almost 1/3 of the previously homogenous population had a

mixed genome.2 Clearly, hybridization poses a great threat to the geneticintegrity of indigenous populations, especially if interbreeding occurs with anonnative species.

Aliens among UsNonnative, or invasive, species are particularly threatening to genetic diversitybecause they carry foreign alleles and often lack predators and diseases thatwould naturally eliminate them from the population. An invasive species may be



more aggressive than its indigenous cousins, and thus more successful infinding mates. Genetic pollution from the invasive species may lead tohomogenization or even extinction of the local population as its natural alleles

are replaced.3 An invasive species, which tends to increase exponentially whencolonizing a new territory, also has the potential to outnumber the area’s nativeindividuals.

Introduction of an alien species may be accidental, as was the case of themarsh frog Pelophylax esculentus family, which is derived from a cross between P.

lessonae and P. ridibuntus.9Translocations of water frogs into new habitats have beenassociated with stocking of fish populations and garden ponds. The geneticintegrity of native frog population has been aggravated by the recurrentintroductions of alien species, and hybridization persists at a steady rate. Asthe number of hybridogenetic individuals increases, scientists have been ableto identify three subsets and potential new species of water frog, each adaptedto a different geographic location. The hybrids have been shown to succeed indifferent environments given its heterozygotic nature, suggesting that the hybrid

may eventually overtake one or both of its parent species.9 What does thismean for the future of this species, and so many others that have beengenetically altered as a result of anthropogenic hybridization?

Positives and NegativesNot all effects of hybridization are negative; in fact, many individuals can benefitfrom reproducing with other species. An intermediate amount of genetic drift in

http://www.bu.edu/synapse/files/2012/04/Frogs.jpg



a population corresponds to increased genetic diversity, allowing for newfitness characteristics including disease resistance and higher habitat

tolerance.9 Hybridization can help to prevent extinction of rare and endangeredspecies by preserving their unique homozygous alleles within theheterogeneous genome of the F1 generation. Yet the strong adaptive capability

of hybrids can also work against the parent species if environmental conditionsfavor hybrids and thus change the relative fitness of the entire population. Inthis case, the presence of a very fit F1 generation may drive one or both

parents to extinction.6

Still, there is no guarantee that a newborn hybrid will survive past birth, muchless live to reproduce and replace its parent species. Many individuals sufferfrom embryonic fatality or premature death due to fatal recessive or deleterious

mutations.9 Those that survive are often sterile, either genetically or physically,

and therefore unable to reproduce and continue their lineage.3 In addition tomutations, hybrids may also inherit lessthandesirable traits from the weaker ofthe parent species, resulting in a reduced capability to handle physical stress. Apoorly adapted hybrid may also display lower disease resistance, ineffective

foraging ability, or reduced predation survival compared to its parents.10 Eventhough some hybrids inherit the “good genes,” there is an equal potential for a

mixed genome to lead to the individual’s demise.11 Interfering with the processof natural selection certainly will not help anyone: humans, natural species, oranthropogenicallycreated hybrids.

A Mixed FutureWhat constitutes an organism as a hybrid? Biologists still lack an explicitoperational rule to define species boundaries on genetic tree



information.8 While some hybrids have been classified as new species, such asthe red wolf, others have been denied the rights to conservation based on lack

of genetic integrity.9 The decision whether to preserve a species or not islargely influenced by the method of hybridization, while natural hybrids are

eligible for protection, anthropogenic hybrids are not.9 Yet in the case ofendangered species, perhaps conserving hybrid offspring will be crucial inpreserving their distinct alleles for the future. In either case, management of

population biodiversity must be sustainable.10 Going forward we must askourselves what type of selectionnatural or anthropogenic – will ensure thecontinuation of a species.

References1Mott, M. (2005, August 5). Ligers make a “dynamite” leap into the limelight.National Geographic. Retrieved March 10, 2012, fromhttp://news.nationalgeographic.com/news.

2Tung, J., Charpentier, M., Garfield, D., Altmann, J., & Albert, C. (2008).Genetic evidence reveals temporal change in hybridization patterns in a wildbaboon population. Molecular Ecology, 17, 19982011.

3Largiader, C. (2007). Hybridization and introgression between native and alienspecies. Ecological Studies, 193, 275288

4Paugy, D & Leveque, C. (1999). Taxonomy and systematics. ASFA 1: BiologicalSciences and Living Resources. pp. 97119.

5Lamont, B., He, T., Enright, J., Krauss, S., & Miller, B. (2003). Anthropogenic



disturbance promotes hybridization between Banksia species by altering theirbiology. J. Evol. Biol., 16, 551557.

6Keller, B., Wolinska, J., Manca, M., & Spaak, P. (2008). Spatial,environmental, and anthropogenic effects on the taxon composition ofhybridizing Daphnia. Philosophical Transactions of the Royal Society, 363, 29432952.

7Roach, J. (2006, May 16). GrizzlyPolar Bear hybrid found — but what does itmean? National Geographic, 2006. Retrieved October 28, 2011, fromhttp://news.nationalgeographic.com/news/2006/05/polarbears_1.html.

8Randi, E. (2010). Wolves in the Great Lakes region: a phylogeographicpuzzle. Molecular Ecology,19, 43864388.

9Holsbeek, G., & Jooris, R. (2010). Potential impact of genome exclusion byalien species in the hybridogenetic water frogs (Pelphylax esculentus complex). BiolInvasions, 12, 113.

10Aguiar, J. B., Jara, P. G., Ferrero, M., SánchezBarbudo, I., Virgos, E.,Villafuerte, R., et al. (2008). Assessment of game restocking contributions toanthropogenic hybridization: the case of the Iberian redlegged partridge. AnimalConservation, 11, 535545.

11Lingle, S. (1993). The hybrid’s dilemma. Discover, 14, 14. Retrieved November 12011, from General Science Full Text database.

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