Lions, Tigons, and Pizzly Bears, Oh My! - BU Blogs

3/10/2015
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Lions, Tigons, and Pizzly
Bears, Oh My!
Are human­mediated changes to the environment
destroying species?
By Ashley Jones
His father was an African prince, his mother, a
fiercely independent Indian huntress. By all laws
governing society, geography, and biology, Hercules
should have never been born. Yet in 2002, the
newborn cub rose to fame as the largest hybrid feline
at the Jungle Island Zoo in Miami, Florida. The lion­
Hercules is a liger, a lion­tiger hybrid.
Photo Credit | Acarvin via Wikimedia
tiger mix inherited the best of both worlds, growing
Commons
into a 12­foot­long, 900­pound carnivore with the
capability 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”, but
can 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 hybrid
displays 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 identified
by 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 lineage
may end with this newborn, the first member of the F1 (first hybrid) generation.
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For successful generations of hybrids to occur, the two parent species must
share at least some breeding traits, be geographically and reproductively
available 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 Anthropogenic
Hybridization 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 could
possibly result in decreased genetic diversity among species as unique alleles
are 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 to
carbon emissions. The influence of the greenhouse effect, coupled with levels
of carbon approaching 400ppm, has initiated the migration of species from
warmer to cooler climates (and vice versa) as organisms attempt to maintain
their ideal temperature ranges.7 As organisms encounter similar species in
their new habitats, they may reproduce with them. A substantial number of
natural hybridizations occurred during the post­glacial Pleistocene era when
carbon levels exceeded those of today.8
The most recent example of temperature­ mediated
hybridization is between polar bears (Ursus maritimus), and
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grizzlies (Ursus arctos)in a hybrid tentatively referred to as a
“pizzly” or “grolar” bear. In 2006, an Inuvialuit hunter from
Victoria Island shot what appeared to be polar bear.
However, the animal possessed characteristics of a
grizzly, with brown patches of fur, long claws, a concave
facial profile, and a humped back. DNA testing confirmed
the individual to be first documented case of a polar­
grizzly hybrid in the wild, a genetically possible but
improbable phenomenon given the differing habitat ranges
and breeding rituals of the two related species.7Human­
The pizzly bear is a polar
bear­grizzly bear hybrid.
induced climate change, which has caused some animals
Photo Credit | Messybeast
via Wikimedia Commons
to shift their range northward, may have driven grizzly
bears into polar bear territory and grizzly bears have been
recorded in Canada’s western Arctic Nunavut province periodically throughout
the last 50 years. An alternate theory proposes that polar bears have been
driven southward by the melting of the ice caps, bringing them into closer
contact with grizzly bears. Three more hybrid individuals were reported after
2006, and scientists expect more encounters between grizzlies and polar bears
in the future.
Habitat Destruction and Fragmentation
Deforestation and urbanization result in decreased habitat area, leading to
increased competition between individuals and smaller populations. Habitat
destruction may either isolate a singular region from a larger range, or can
combine 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 borders
where one territory overlaps another, individuals are forced to share resources
including food, shelter, and potential mates. Natural selection predicts that the
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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 Anubis
baboons (Papio anubis) in Kenya did not interbreed 30 years
ago; however, the proportion of hybrids has increased
over time with the construction of tourist
lodges.2 Historically, the yellow baboons occupied the
south and east territories and the neighboring Anubis
lived in the north and west. Typically, baboons stay close
their natal locations, separated from other populations by
Yellow baboons (top) and
anubis baboons (bottom) only
physical obstacles like long stretches of waterless land.
started interbreeding recently.
Credit | Wikimedia Commons
However, researchers observed an alternate immigration
pattern of Anubis baboons that were foraging from tourist
lodges. The immigration of Anubis coupled with their successful reproduction
led to hybridization zones between the two baboon species. The hybrids are
not selected against as would be if they had a genetic fitness lower than their
predecessors. Male hybrids undergo puberty earlier in life than the purebred
males and as a result, have higher reproductive and competition success.
Researchers noted an increase in overall Anubis alleles in yellow baboon
populations as well as in the number of offspring resulting from mating between
hybrids. By 2000, almost 1/3 of the previously homogenous population had a
mixed genome.2 Clearly, hybridization poses a great threat to the genetic
integrity of indigenous populations, especially if interbreeding occurs with a
non­native species.
Aliens among Us
Non­native, or invasive, species are particularly threatening to genetic diversity
because they carry foreign alleles and often lack predators and diseases that
would naturally eliminate them from the population. An invasive species may be
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more aggressive than its indigenous cousins, and thus more successful in
finding mates. Genetic pollution from the invasive species may lead to
homogenization or even extinction of the local population as its natural alleles
are replaced.3 An invasive species, which tends to increase exponentially when
colonizing a new territory, also has the potential to outnumber the area’s native
individuals.
Introduction of an alien species may be accidental, as was the case of the
marsh frog Pelophylax esculentus family, which is derived from a cross between P.
lessonae and P. ridibuntus.
9Translocations of water frogs into new habitats have been
associated with stocking of fish populations and garden ponds. The genetic
integrity of native frog population has been aggravated by the recurrent
introductions of alien species, and hybridization persists at a steady rate. As
the number of hybridogenetic individuals increases, scientists have been able
to identify three subsets and potential new species of water frog, each adapted
to a different geographic location. The hybrids have been shown to succeed in
different environments given its heterozygotic nature, suggesting that the hybrid
may eventually overtake one or both of its parent species.9 What does this
mean for the future of this species, and so many others that have been
genetically altered as a result of anthropogenic hybridization?
Positives and Negatives
Not all effects of hybridization are negative; in fact, many individuals can benefit
from reproducing with other species. An intermediate amount of genetic drift in
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a population corresponds to increased genetic diversity, allowing for new
fitness characteristics including disease resistance and higher habitat
tolerance.9 Hybridization can help to prevent extinction of rare and endangered
species by preserving their unique homozygous alleles within the
heterogeneous genome of the F1 generation. Yet the strong adaptive capability
of hybrids can also work against the parent species if environmental conditions
favor hybrids and thus change the relative fitness of the entire population. In
this 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, much
less live to reproduce and replace its parent species. Many individuals suffer
from 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 to
mutations, hybrids may also inherit less­than­desirable traits from the weaker of
the parent species, resulting in a reduced capability to handle physical stress. A
poorly adapted hybrid may also display lower disease resistance, ineffective
foraging ability, or reduced predation survival compared to its parents.10 Even
though 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 process
of natural selection certainly will not help anyone: humans, natural species, or
anthropogenically­created hybrids.
A Mixed Future
What constitutes an organism as a hybrid? Biologists still lack an explicit
operational rule to define species boundaries on genetic tree
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information.8 While some hybrids have been classified as new species, such as
the 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 is
largely influenced by the method of hybridization, while natural hybrids are
eligible for protection, anthropogenic hybrids are not.9 Yet in the case of
endangered species, perhaps conserving hybrid offspring will be crucial in
preserving their distinct alleles for the future. In either case, management of
population biodiversity must be sustainable.10 Going forward we must ask
ourselves what type of selection­natural or anthropogenic – will ensure the
continuation of a species.
References
1Mott, M. (2005, August 5). Ligers make a “dynamite” leap into the limelight.
National Geographic. Retrieved March 10, 2012, from
http://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 wild
baboon population. Molecular Ecology, 17, 1998­2011.
3Largiader, C. (2007). Hybridization and introgression between native and alien
species. Ecological Studies, 193, 275­288
4Paugy, D & Leveque, C. (1999). Taxonomy and systematics. ASFA 1: Biological
Sciences and Living Resources. pp. 97­119.
5Lamont, B., He, T., Enright, J., Krauss, S., & Miller, B. (2003). Anthropogenic
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disturbance promotes hybridization between Banksia species by altering their
biology. J. Evol. Biol., 16, 551­557.
6Keller, B., Wolinska, J., Manca, M., & Spaak, P. (2008). Spatial,
environmental, and anthropogenic effects on the taxon composition of
hybridizing Daphnia. Philosophical Transactions of the Royal Society, 363, 2943­2952.
7Roach, J. (2006, May 16). Grizzly­Polar Bear hybrid found — but what does it
mean? National Geographic, 2006. Retrieved October 28, 2011, from
http://news.nationalgeographic.com/news/2006/05/polar­bears_1.html.
8Randi, E. (2010). Wolves in the Great Lakes region: a phylogeographic
puzzle. Molecular Ecology,19, 4386­4388.
9Holsbeek, G., & Jooris, R. (2010). Potential impact of genome exclusion by
alien species in the hybridogenetic water frogs (Pelphylax esculentus complex). Biol
Invasions, 12, 1­13.
10Aguiar, J. B., Jara, P. G., Ferrero, M., Sánchez­Barbudo, I., Virgos, E.,
Villafuerte, R., et al. (2008). Assessment of game restocking contributions to
anthropogenic hybridization: the case of the Iberian red­legged partridge. Animal
Conservation, 11, 535­545.
11Lingle, S. (1993). The hybrid’s dilemma. Discover, 14, 14. Retrieved November 1
2011, from General Science Full Text database.
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