Biology 4250 Evolutionary Genetics

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Biology 4250 Evolutionary Genetics

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

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Speciation
Fri. March 7 Discussion - Speciation Large
Punctuational Contribution of Speciation to
Evolutionary Divergence at the Molecular
Level Mark Pagel,Chris Venditti, Andrew Meade
(2006) Phylogenetic signatures Avise
(2004) http//www.mun.ca/biology/dinnes/B4250/Biol
4250.html
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Announcements

• Monday March 10 afternoon lab
• Please pass in a page listing your term paper
  topic with a brief outline and a few references.
  Presentations begin March 17.

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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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Speciation
- Speciation history -
Speciation Concepts - Reproductive
isolation - Genetics of Speciation
- Phylogenetic Signatures
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Speciation
Summary most biologists agree that species are
real Coyne and Orr (2004)
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Speciation Concepts

• BSC Biological species concept
• ESC Evolutionary species concept
• PSC Phylogenetic species concept
• GSC Genealogical species concept
• RSC Recognition species concept
• CSC Cohesion species concept

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Speciation Concepts

• Biological Species Concept (BSC)
• Species are systems of populations gene
  exchange between these systems is limited or
  prevented by reproductive isolating mechanisms or
  perhaps by a combination of several such
  mechanisms
• Dobzhansky 1937

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Reproductive Isolation

• RIBs Reproductive isolating barriers
• RIMs Reproductive isolating mechanisms
• Prezygotic - ecological or habitat isolation
• - temporal isolation
• - ethological isolation
• - mechanical isolation
• - gamete incompatibility

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Reproductive Isolation

• Postzygotic
• 1. Intrinsic (independent of the Env.)
• - F1 inviability
• - F1 sterility
• - hybrid breakdown (F2 and
• backcrosses)
• 2. Extrinsic (depends on Env.)
• - ecological inviability
• - behavioural sterility

Lab. experiments
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Speciation

• Usefulness of Genetic markers
• Distinct genetic signatures related to
  demographic events during speciation
  (bottlenecks)
• Genetic differentiation at different stages of
  RIB acquisition temporal aspect of speciation
  process
• Assess magnitude and pattern of genetic exchange
  among related forms (hybridization) understand
  the intensity and nature of RIBs

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Genetic differences between species

• Traditional approaches
• Hybrids Compare F1, F2 to parental species
• - large variation in F2 due to
  recombination species differ for a large number
  of genes with small effects
• 2. Chromosome mapping of genes involved in
  pre- and postzygotic reproductive isolation

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Genetics of Reproductive Isolation
Hybrid Sterility in Drosophila (sibling species)
D. pseudoobscura x D. persimilis
Hybrid males ? sterile Hybrid females
?fertile Backcross
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Testis length
Sibling species
D. persimilis
D. pseudoobscura
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Genetics of Reproductive Isolation
Hybrid Sterility in Drosophila D.
pseudoobscura x D. persimilus all the
chromosomes carry genes affecting fertility of
males - within the same
species - sterility of
hybrid males
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Genetic differences between species

• Limitations of traditional approaches
• 1. Can only be applied to hybridizing taxa
• 2. Can only be used examine genes that differ
  between species cannot determine the proportion
  of genes distinguishing species

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The Genetics of Speciation

• Species - reproductively isolated,
• independently evolving
• Genetics differences at different stages
• of the speciation process
• - among populations before speciation
• - during speciation
• - after speciation

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The Genetics of Speciation

• Example Drosophila willistoni Complex
• - geographic populations
• - subspecies/semispecies
• - sibling species/species (F.J.
  Ayala)
• N. South America
• C. America
• Caribbean

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The Genetics of Speciation

• Geographic populations fully compatible
• Subspecies allopatric, incipient RIB (hybrid
  male sterility)
• Semispecies distributions overlap, partial pre-
  and postzygotic RIBs
• Sibling species nearly identical, mostly
  reproductively isolated
• Species phenotypically distinct, reproductively
  isolated

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The Genetics of Speciation

• Measuring genetic differences between species
• - allele frequency for several protein
  loci
• genetic identity (I) range 1
  to 0
• genetic distance (D) range 0 to
  gt 1
• D - ln(I)
  (Neis)

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Drosophila willistoni complex 36 allozyme loci
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(No Transcript)
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The Genetics of Speciation

• Process of divergence continuous
• (gradual speciation).
• 2. Substantial degree of genetic differentiation
  after the first stage. Genetic differentiation
  can become more pronounced after reproductive
  isolation. (drift, selection)

Identity 0.970 0.795 0.517
0.352
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The Genetics of Speciation

• 3. Speciation does not involve a major
• reorganization of the genome
• 4. Reproductive isolation may involve only a
• few loci (gamete recognition loci)

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Reproductive Isolation

• Stages of speciation (allopatric)
• 1. Isolation
• 2. Genetic Divergence
• 3. Secondary contact - reinforcement

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Reproductive Isolation

• Divergence
• - drift, selection, mutation
• - postzygotic mechanisms
• evolve as a by-product

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Reproductive Isolation

• Secondary Contact
• - postzygotic mechanisms
• waste gametes
• - evolution of prezygotic mechanisms
• favoured
• (reinforcement)

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Evolution of Pre- and Postzygotic Isolation
Expect 1. Increased postzygotic isolation with
time 2. Increased prezygotic isolation for
sympatric compared to allopatric species
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Evolution of Pre and Postzygotic Isolation
- 171 species pairs of Drosophila - genetic
distance (D) between species - postzygotic
isolation hybrid viability/fertility -
prezygotic isolation mate choice
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• lots of scatter
• both pre- and postzygotic isolation increase
  with time
• Prezygotic increases quicker than postzygotic

D time since divergence (clock)
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Prezygotic greater for sympatric compared to
allopatric taxa
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Evolution of Pre- and Postzygotic Isolation

• Conclusions
• Strength of prezygotic and postzygotic isolation
  increases gradually
• Prezygotic isolation enhanced in sympatric
  species (supports reinforcement)

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Speciation in Drosophila
Total Isolation
Speciation takes about 1.5 3.5 MY
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Genetics of Speciation

• Genetic difference at various stages of
  speciation
• Patterns similar to D. willistoni
• Sunfishes Lepomis
• Sunflowers Helianthus

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Genetics of Speciation

• Summary based on several species
• postzygotic isolation increases slowly with time
• Hybrid sterility evolves faster than inviability
• Postzygotic isolation evolves at different rates
  in different groups
• frogs, Drosophila gt Lepidoptera gt birds

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Genetics of Reproductive Isolation

• What genes are involved in intrinsic postzygotic
  reproductive isolation ?
• Observations from hybridizations
• Hybrid inviability
• Hybrid sterility

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Genetics of Reproductive Isolation

• Darwins dilemma
• Evolution of intrinsic
• Postzygotic
• isolation

Inviable hybrid
aa
AA
Aa
aa
Aa
aa
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Genetics of Reproductive Isolation

• Hybrid inviability and sterility appear to be due
  to between-locus incompatibilities.
• Dobzhansky-Muller
• model

Hybrid AaBb
aaBB
AAbb
Aabb
aaBb
A incompatible with B
aabb