Lecture 13: Speciation Continued

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Lecture 13 Speciation Continued

•  Hybrid zone
• area where differentiated populations interbreed
  (incomplete speciation)
• Stepped cline in allele freq.
• Introgressive hybridization
• cline widths differ among loci (selection varies)
•  

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Clines

• ? in NS ? cline width more abrupt ?s
• Cline width ? (SD of dispersal dist)
• ? s (selection coefficient against
  Aa)
• Hybrid Zone 2? contact or start of parapatric
  speciation??

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If clines are concordant

• 2? contact
• But
• Linkage Disequilibrium genes combine nonrandomly
•  Epistasis fitness of 1 allele depends on
  occurrence of a 2nd allele
• e.g. Mimetic butterflies

Papilio memnon
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Parapatric Speciation

• Adjacent Populations
•  

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Mechanism

• 1) CLINE evolves in hybrid zone
• 2) REINFORCEMENT
• Repro. isoln b/w incipient spp. by NS
• (assortative mating)
• if no selection against hybrid - zone is
  STABLE
• counteracted by gene flow elimination of rarer
  allele
• ? need fast strong reinforcement

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Parapatric contd

• Most hybrid zones no ? fitness of hybrid
• Most researchers think hybrid zones are 2?
  contact

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Sympatric Speciation

• No sepn of ancestral popns geog. range
• Need stable polymorphism assortative mating 

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A) Instantaneous Sympatric Speciation

• Polyploidy
• gt2 sets genes
• Immediate repro isoln
• Fertile
• Restores chromosomal segregation
• Need gt 1 ind. for repro
• Sometimes called
• STATISPATRIC SPECIATION
• e.g. Grasshoppers

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Examples

• 2N 2N
• ?
• 4N (close inbreeding)
• Plants
• Some parasitic Hymenoptera ( sib mating)
• ? diversity of spp.

Backcross 3N
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Inversions

• DNA segment reversed

Inversion Loop b/c homologous areas align
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Inversion results

• Inviable gametes
• - dicentric bridges acentric fragments
  (paracentric inversions)
• - duplications deletions (pericentric
  inversions)
• Result
• Non-viable gametes
• Duplicate some info
• Lose other info

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B) Gradual Models

• Disruptive Selection NS favours
• forms that deviate from popn mean
• If random mating generates phenotypes matched to
  resource distn
• NO selectn for assortative mating
• (e.g. seed beak sizes) 
• No speciation b/c equal fitness

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• But
• Nonnormal resource distn
• random mating unequal fitness
• assortative mating matches distn better ?
  speciation
• Heterogeneous Envts Selection maintains
  Diversity
• Multiple Niche Polymorphisms
• Coarse vs. Fine Grained
• Spatial vs. Temporal  

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eg. Papilio (Butterflies)

•  AA aa
• (Host 1) (Host 2)
• A a
• LOW FITNESS
• - selection for assortative mating
•  Locus B BB, Bb mate on host 1
• bb mate on host 2
• RIM (premating isolation)

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Conditions for Sympatric Speciation

• Strong linkage b/w A (resource)
• B (host choice)
• Strong selection against Aa (hybrid)
• ? gene flow b/c varn in host preference
• Few loci involved in mate preference  

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Why few mate preference loci?

• Recombination causes ? linkage disequilibrium
• ? right alleles for mate preference no longer
  linked with right alleles for host selection.
• These conditions are Exceptional Circumstances!!!

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e.g. Lacewings

• colour niche seasonal diffns
• (multiple niche polymorphisms)
• currently sympatric
• assortative mating b/c poor camouflage of
  heterozygote
• NOT proof of
• sympatric speciation
•  

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Host shifts

• e.g. Apple pest from Hawthorn
• breed on hatching fruit type
• different development times for 2 fruits
• Assortative mating but hybridize in lab
• What maintains Diversity?
• Envtl segregn, difft devt times
• ? maybe dont need more selection for isolation

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Evidence

• Little for Sympatric Speciation
• Parapatric Sympatric models require
  Reinforcement
• Character Displacement (increased difference in
  traits between related spp. in sympatry)
• ? suggests Reinforcement
• Isolating characters
• SYMPATRIC gt ALLOPATRIC b/c threat of
  hybridization lowers fitness

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e.g. Damselflies

• Wing Colour
• (Courtship ? diffn in colour with sympatry)
• Interpopulation comparisons convincing
• Interspecific comparisons .not convincing
• Sympatric spp. with low repro isoln already
  fused ? artificially inflates repro isoln

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Damselflies Contd
Past Present Past Present
1b
Allopatric w Recontact (no interbreeding) Sympatri
c w High Isoln
1a
1a 1b
2a
2b
Allopatric w Low Isoln (interbreeding)
Fused
2b
2b
2a
Sympatric sp. only ever show spp. with high
isolation
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But, doesnt explain
Hybrid Zone
If mate then allopatric w low isolation If wont
mate sympatric w high isoln
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Genetic Models of Speciation
a
Freq of x Fitness
b

•  1) Divergence model
• isolated popn
• Selectn for lower x
• divergence to equilibria a b

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2) Peak Shift
b
selection
drift
a
P2 P1

• small population (drift more likely)
• character moves past saddle by drift
• NS wont push into area of lower fitness
• moved to peak z by selection

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Recontact

• Differentiation in populations by adapting to
  different niches
• May incidentally confer repro isolation when
  later meet

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How do R.I.M. arise?

• Sexual Selection F pref. arise through drift
• Runaway Selection rapid divergence
• Coevolution
• drift in flower phenotype in local popns
• selecn on pollinator, isoln of flower, drives
  divergence

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Do R.I.M. arise to prevent hybridization?

• Evidence repro. isoln arises allopatrically by
  sex. selection, drift, ecol. selection
• e.g. Sticklebacks (predation vs. sexual
  selection)
• Intermediate b/w red/black (hybrid)
• ? fitness

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Rapid Speciation

• Can occur through
• strong sexual selection
• high trophic specialization
• few competitors

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Lake Malawi Cichlids

• Highest speciatn rate of any vertebrate
• group living or extinct (450 spp. in 2 MY)
• Hypothesis rapid divergence due to sexual
  selection

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Summary

• Reproductive isolation can evolve by selection
  drift whether threatened by hybridization or
  not
• Speciation need not be adaptive in itself
• Byproduct of selection drift