Dr. David Innes

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Biology 4250 Evolutionary Genetics

• Dr. David Innes
• Dr. Dawn Marshall
• W 2008

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Outline of
topics 1. Introduction/History of Interest in
Genetic Variation 2. Types of Molecular
Markers 3. Molecular Evolution 4.
Individuality and Relatedness 5. Population
Demography, Structure Phylogeography 6.
Phylogenetic Methods Species Level
Phylogenies 7. Speciation, Hybridization and
Introgression 8. Human Evolutionary
Genetics 9. Conservation Genetics
Background
Applications
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Hybridization

• Topics
• Historical background
• Natural hybridization
• Genetic distance and hybridization
• Hybrid zones
• - geography
• - theoretical models
• - examples
• - sexual asymmetries (FA x MB gt FB
  x MA)
• - cytonuclear disequilibria
  (mtDNA/nucDNA)
• Evolutionary significance

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Hybridization

• G. L. Stebbins (1950) Evolutionary role of
  hybridization
• while hybridization is certainly less
  common in animals than plants, and is
  correspondingly less important as a factor in
  evolution, its influence in certain groups may be
  considerable.

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Hybridization in Plants
Higher frequency of hybridization in plants than
animals due to Sex determination
Mating system variation (pollination)
Vegetative reproduction
Ecotypes (genetic-environment associations)
Polyploidy
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Experimental Hybridization
Scarlet Gilia
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Louisiana IrisesNatural Hybridization
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Iris hexagona
Iris fulvia
Iris brevicaulis
Louisiana Irises
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Genetic markers

• Nuclear RAPD, microsatellites,
• allozymes
• Cytoplasmic cpDNA
• Additional information
• Floral morphology, habitat

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I. fulva I. brevicaulis

• absence of intermediate
• hybrids (F1)
• fulva ? forest
• brev. ? pasture

Nuclear and cpDNA
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• Environmental Variables
• Elevation
• Light
• Veg. Comp.

Mosaic hybrid zone Hybrids associated particular
environments
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Hybridization in Plants
Uncoupling of male and female components to gene
flow Pollen nuclear genes
Seed nuclear genes cpDNA
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Iris Hybrid Zone
fulva hexagona
fulva hexagona
Nuclear Markers (pollen flow)
cpDNA (seed dispersal)
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Evolutionary Significance of Hybridization

• Evolution of reproductive isolation
  (speciation)
• (reinforcement)
• Introgression source of genetic variation
  for
• adaptation
• Hybrid speciation

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Evolutionary Significance of Hybridization

• Introgression source of genetic variation for
• adaptation
• Requires viable and reproductive hybrids for
  gene flow between species

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Evolutionary Significance of Hybridization

• Are natural hybrids fit or unfit relative to
  their parents? Arnold and Hodges (1995)
• - Viable and fertile F1 hybrids may be rare,
  but repeated opportunities to form F1
• - wide range of hybrid fitness values
• lt , , gt parental taxa
• - importance of defining separate hybrid
  classes to evaluate fitness (based on genetic
  markers)

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Hybrid Speciation in Plants
Hybrids between species if
fertile ? introgression if sterile ?
allopolyploid Reproductively isolated from parent
species
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Hybrid Speciation (introgression)
F1
Time
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Iris hexagona
Iris fulvia
Iris brevicaulis
Iris nelsonii
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Hybrid Speciation in Irises(introgression)
Example Iris fulvia (Saltwater) Iris
hexagona (Freshwater) Iris brevicaulis (Pasture)
Iris nelsonii (Swamp, Ecotone)
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Hybrid populations Iris nelsonii
Lack of foreign markers in I. nelsonii (genetic
stability) (Homoploid not polyploid)
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Hybrid Speciation in Plants
Example Clarkia spp. (Reticulate evolution)

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allopolyploids



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(No Transcript)
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Sunflower Reticulate evolution
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Hybrid Speciation

• The same sexual processes that formed the hybrid
  can breakup the hybrid genotype
• Stability of hybrid (limits to gene flow between
  species)
• - genetic incompatibility (gene, chromosome)
• - distinct ecological preference
• - polyploidy
• - agamospermy (asexual) asexual vertebrates

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Speciation and Hybridization
http//www.nature.com/nature/journal/v446/n7133/fu
ll/nature05706.html
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High genetic among progeny of hybrids. Evolve
to new adaptive peak
3rd dimension fitness
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Hybrid Zones

• Natural experiments
• - many generation of
  hybridization and
• recombination
• - areas of strong selection
• - ecological context
• - adaptive evolution
• - processes that cause divergent
  evolution
• 
  (speciation)
• Windows on evolutionary process