Speciation and Hybridization

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Speciation and Hybridization
Avise (2004) Chapter 7 - Speciation
history - Speciation Concepts -
Reproductive isolation - Genetics of
Speciation - Hybridization
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Speciation and Hybridization
Wed. March 7, Fri. March 9 - Speciation Speciation
Reading for Monday March 12, 900 950 am
discussion http//www.mun.ca/biology/dinnes/B4250
/Biol4250.html Wed. March 14, Fri. March 9
Hybridization Hybridization reading for Monday
March 19, 900 950 am discussion
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Speciation Reading

• What does Drosophila genetics tell us about
  speciation? James Mallet (2006)
• Drosophila and the understanding of
  speciation
• Reproductive isolation and speciation
• Comparative methods
• What we do not know
• Conclusions

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Speciation Concepts

• Avise (2004) Box 7.1
• BSC Biological species concept
• ESC Evolutionary species concept
• PSC Phylogenetic species concept
• RSC Recognition species concept
• CSC Cohesion species concept
• CP Concordant principles

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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
• Box 7.2
• 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

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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 D.
pseudoobscura x D. persimilus ?
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)

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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
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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