Introgressive Hybridization and Endangered Species

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Introgressive Hybridization and Endangered
Species
Joey Hinton Louisiana State University
Photo by Barron Crawford
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Introgressive hybridization as a conservation
problem

• Conservation biologists have underestimated the
  role hybridization can play in biotic
  homogenization.
• Genetic hybridization due to human disturbance
  can compromise the genetic integrity of existing
  species.
• Habitat modifications break down reproductive
  isolation between native species.
• Mixing of gene pools of formerly distinct taxa
  through introductions.

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Northern spotted owls hybridizes with barred owls

• Northern spotted owls (NSO) decline to low
  numbers due to loss of habitat.
• Barred owl (BO) range expanding due to the NSO
  decline.
• NSO and BO produce fertile hybrids.
• If NSOBO favors BO, genetic swamping a risk.

Owl-pictures.com
Northern Spotted Owl
Barred Owl
Joey Hinton
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New Zealand grey duck hybridizes with introduced
mallards.

• New Zealand grey ducks (NZG)are rare but mallards
  are common.
• NZG mate with mallards when conspecifics are not
  available.
• Pure NZG are disappearing rapidly via genetic
  swamping.

New Zealand Grey Duck
Teara.govt.nz
Mallard
Joey Hinton
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Introgressive Hybridization

• Definitions

• Introgressive hybridization is a critical and
  contentious issue within evolutionary biology.
• Advances in genetic techniques have revitalized
  the issue of hybridization now that it is
  recognized to be more widespread.

• Hybridization breeding between individuals from
  genetically distinct populations.
• Introgression gene flow between distinct
  populations by hybrid individuals backcrossing
  with parental taxa.
• Hybrid swarm population of hybrids.
• Hybrid Zones an area of contact between two
  genetically distinct populations where
  hybridization occurs.

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Why Study Hybridization

• Used to determine systematic relationships.
• Decipher mechanisms that may lead to speciation.
• Reproductive barriers are seen as equal to the
  process of speciation.
• Process is evolutionarily important in its own
  right.
• Generate novel genotypes that lead to adaptive
  evolution.

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Historical Perspective

• Botanists

• Zoologists

• Emphasized evolutionary potential of hybrid
  genotypes to occupy novel habitats and act as
  progenitors of new clades.
• Adopted a phylogenetic perspective.

• Viewed hybridization as maladaptive because
  individuals involved produced fewer and/or
  less-fertile progeny.
• Postzygotic barriers
• Adopted a process-oriented perspective to test
  the Biological Species Concept.

Lotsy 1931 Anderson and Hubricht 1938 Anderson
and Stebbins 1954
Dobzhansky 1937, 1940 Mayr 1942
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Detection of hybridization
Red Wolf

• Allendorf et al. (2001) argued for use of
  genetic rather than morphological data.
• Hybrids may be hard to indentify w/o genetic
  data.
• Morphology may not distinguish F1 hybrids from
  backcross hybrids.
• Smooth variation in degree of hybridization.

Joey Hinton
Coyote
Joey Hinton
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Hybridization can have three general outcomes

• Hybrid zone area of contact between two
  divergent populations where hybridization occurs.
• Hybrid swarm a population of individuals that
  are all hybrids that backcross with the parental
  stock and mate among other hybrids.
• Hybrid taxon independently evolving stable
  population with a unique set of heritable traits,
  distinct from the parental taxa.

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Six types of hybridization
Allendorf et al. 2001
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Introgressive hybridization in Darwins finches
Cactus finch (Geospiza scandens)

• Grants have been studying Galapagos finches for
  over 30 years.
• Interspecific hybridization appears to be an
  important source of genetic variation for the
  cactus finch.
• Grant et al. 2004

Nick Athanas
Medium ground finch (Geospiza fortis)
Nick Athanas
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Introgressive hybridization in Darwins finches

• Between 1973 and 2003 mean morphological features
  in both finch species were subject to fluctuating
  directional selection.
• Increase in bluntness in the beak of G. scandens
  after 1990 can only partly be explained by
  selection.

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Introgressive hybridization in Darwins finches

• Grant et al. (2004) used 16 microsatellite loci
  to test predictions of the hypothesis that
  introgressive hybridization contributed to the
  trend.
• Identified F1 hybrids and backcrosses from
  pedigrees where known, supplemented by the
  results of genetic tests.
• Analyzed the changes in morphology and allelic
  composition in the two populations over a 15-20
  year period.

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Introgressive hybridization in Darwins finches

• G. scandens population became more similar to the
  G. fortis population both genetically and
  morphologically.
• Gene flow between the species was estimated to be
  3x greater for G. fortis to G. scandens than vice
  versa.
• Resulted in a 20 reduction in the genetic
  difference between the species.

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Introgressive hybridization in Darwins finches

• Removing indentified F1 hybrids and backcrosses
  from the study and reanalyzing the traits did not
  eliminate the convergence.
• A combination of introgressive hybridization and
  selection jointly provide the best explanation.
• Postmating barriers to gene exchange can
  alternate between convergence and divergence when
  environmental conditions oscillate.

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Introgressive hybridization in Darwins finches

• Closely related populations in sympatry may
  undergo merge-and-diverge oscillations in a
  strongly fluctuating environment.
• Despeciating when the population evolves in the
  direction of their ancestral population.
• Divergence when the population moves in a
  evolutionary trajectory independent of its
  ancestral population.

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Hybridization between the threatened Canada lynx
and bobcat
Canada lynx
FWS

• Schwartz et al. (2004) designed and tested an
  assay that could detect hybridization between the
  two cat species.
• mtDNA analysis revealed that all hybrids had lynx
  mothers.

Bobcat
Joey Hinton
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Hybridization between the threatened Canada lynx
and bobcat
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Hybridization between the threatened Canada lynx
and bobcat

• Reproductive status of the hybrid offspring is
  unclear.
• Lynx females are wasting reproductive effort if
  offspring are sterile or less fit.
• Hybrid swarm may form if hybrids are fertile.
• The extent, rate, and nature of hybridization is
  unclear.
• Federal agencies will need to reevaluate the
  conservation needs of Canada lynx.

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Six types of hybridization
Allendorf et al. 2001
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Six types of hybridization
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Why is hybridization a growing concern for
conservation biologists?

• Observed in congeneric species where populations
  have some overlap.
• Densities of one species is low, mates are hard
  to find, and interaction strengths weakened.
• Endangered species appear to have the demographic
  conditions to favor hybridization.
• Key may be to understand the proximate ecological
  conditions that facilitate hybridization.

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Future Directions Assessing Biological Risk

• Widespread management response is to cull
  hybridization.
• Hybrids may have some ecological function.
• Models to test whether hybridization affects the
  rate of species displacement.
• Quantitative genetic models to predict the
  potential for hybrid invasions.
• Provide resource agencies with information when
  considering the translocation of species.
• Biological risk assessments

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Future Directions Policy Issues

• Previously, hybrids were not protected under the
  Endangered Species Act.
• Hybrid policy abandoned in 1990.
• Allendorf et al. (2001) recommendations.
• Guidelines for hybridization scenarios.
• Controlled intercrosses when necessary.
• Florida panther
• Incorporate realistic science into policy
  decisions.

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References

• Allendorf, FW, RF Leary, P Spruell, and JK
  Wenburg. 2001. The problems with hybrids setting
  conservation guidelines. Trends in Ecology and
  Evolution 16 613-622.
• Anderson, E. and L Hubricht. 1938. Hybridization
  in Tradescantia. III. The evidence for
  introgressive hybridization. American Journal of
  Botany 25 396-402.
• Anderson, E. and GL Stebbins, Jr. 1954.
  Hybridization as an evolutionary stimulus.
  Evolution 8 378-388.
• Dobzhansky, T. 1937. Genetics and the origin of
  species. Columbia University Press, New York.
• Dobzhansky, T. 1940. Speciation as a stage in
  evolutionary divergence. The American Naturalist
  74 312-321.
• Grant, PR, BR Grant, JA Markert, LF Keller, and K
  Petren. 2004. Convergent evolution of Darwins
  finches caused by introgressive hybridization and
  selection. Evolution 58 1588-1599.
• Lotsy, JP. 1931. On the species of the taxonomist
  in its relation to evolution. Genetica 13 1-16.
• Mayr, E. 1942. Animal species and evolution.
  Columbia University Press, New York.
• Schwartz, MK, KL Pilgrim, KS McKelvey, EL
  Lindquist, JJ Claar, S Loch, and LF Ruggiero.
  2004. Hybridization between Canada lynx and
  bobcats genetic results and management
  implications. Conservation Genetics 5 349-355.