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Introgressive Hybridization and Endangered Species


Conservation biologists have underestimated the role hybridization can play in ... Lynx females are wasting reproductive effort if offspring are sterile or less fit. ... – PowerPoint PPT presentation

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Title: Introgressive Hybridization and Endangered Species

Introgressive Hybridization and Endangered
Joey Hinton Louisiana State University
Photo by Barron Crawford
Introgressive hybridization as a conservation
  • Conservation biologists have underestimated the
    role hybridization can play in biotic
  • Genetic hybridization due to human disturbance
    can compromise the genetic integrity of existing
  • Habitat modifications break down reproductive
    isolation between native species.
  • Mixing of gene pools of formerly distinct taxa
    through introductions.

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
  • NSO and BO produce fertile hybrids.
  • If NSOBO favors BO, genetic swamping a risk.
Northern Spotted Owl
Barred Owl
Joey Hinton
New Zealand grey duck hybridizes with introduced
  • New Zealand grey ducks (NZG)are rare but mallards
    are common.
  • NZG mate with mallards when conspecifics are not
  • Pure NZG are disappearing rapidly via genetic

New Zealand Grey Duck
Joey Hinton
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.

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
  • Generate novel genotypes that lead to adaptive

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
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
  • Morphology may not distinguish F1 hybrids from
    backcross hybrids.
  • Smooth variation in degree of hybridization.

Joey Hinton
Joey Hinton
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.

Six types of hybridization
Allendorf et al. 2001
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
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

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
  • 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.

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

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.

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.

Hybridization between the threatened Canada lynx
and bobcat
Canada lynx
  • 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

Joey Hinton
Hybridization between the threatened Canada lynx
and bobcat
Hybridization between the threatened Canada lynx
and bobcat
  • Reproductive status of the hybrid offspring is
  • 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
  • Federal agencies will need to reevaluate the
    conservation needs of Canada lynx.

Six types of hybridization
Allendorf et al. 2001
Six types of hybridization
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.

Future Directions Assessing Biological Risk
  • Widespread management response is to cull
  • 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

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

  • 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.
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