Lecture 12: Speciation

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Lecture 12 Speciation

• Geographic variation
• genetic, behavioural, developmental differences
  over geographic range
• important to the study of evolution
• describes the course of evolutionary change
• new species formed by same processes that give
  rise to variation among conspecifics

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Patterns in Geographic Variation Among Species

• Parapatric different forms meet interbreed
• Sympatric different forms meet but dont
  breed
• Allopatric geographically separated never get
  the chance to mate
• Polytypic species spp. with several defined
  subspecies (geographic races)
• Superspecies monophyletic group of closely,
  mostly allopatric species (i.e. used to be one
  spp., usually post-mating isolation)

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Polytypic species (Rassenkreis)

• Heliconius butterflies Cougars

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Superspecies (Artenkreis)

• Parus spp. Gasterosteus aculeatus

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Forms of Geographic Variation

• Cline gradient of variation in
  genotype/phenotype
• Types of Clines
• Concordant gt 1 characters vary along the same
  transect
• Discordant characters vary independently
• Shallow gradual changes in character
• Stepped abrupt changes in character

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Concordant Clines

• e.g. Orioles
• Colour
• Enzyme frequencies
• West to east
• Pleiotropy may cause concordance

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Discordant Clines

• e.g. Rat Snake
• Blotching
• Colour
• Striping
• Due to geographic variation in selection pressure

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More Examples

• Gradual Cline
• CLOVER
• cyanide production
• ? N to S cline
• Balance costs benefits
• Benefit protection against herbivory
• Cost frost sensitivity

• Discordant Cline
• RABBIT TICKS
• body size ? S to N
• appendage size ? W to E
• Trade-off b/w fat storage desiccation

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Famous Clines

• Bergmans rule
• warm-blooded spp.
• larger in colder environments
• Allens rule
• warm-blooded species
• shorter protruding body parts relative to body
  size in colder
• Glogers rule
• More darkly pigmented in more humid climates

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• Variation Reproductive Isolation ? Speciation
• speciation requires isolating mechanism
• eventual genetic barrier to interbreeding

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Speciation

• Speciation can be classified by geographic
• characteristics or genetics
• Allopatric speciation
• Parapatric speciation
• Peripatric speciation
• Sympatric speciation

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Allopatric Speciation Model
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Allopatric Speciation

• Best known easiest understood mode of
  speciation
• Lots of evidence
• Ring Species (Ensatina spp.)

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Model Island Archipelagoes

• Invasion
• Divergence
• Reinvasion

1. 2. 3.
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Recontact of Divergent Species

• Speciation complete no hybridization
• Speciation incomplete hybridization
• Hybrid Zones regions of 2? contact b/w
  previously isolated popns w viable hybrids
• Fitness of hybrids determines incidence of hybrid
  zones
• If reduced fitness isolation reinforced by
  selection

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Reinforcement model

• AA Aa aa
• selected against
• (low fitness)
• ?
• AA ? A
• Aa
• aa ? a

Fixation Speciation
Assortative mating
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Problem

• If a is rare, selection against Aa removes it
  from the pool
• AA
• Aa ? AA
• aa

Fixation No Speciation
This genotype is uncommon
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• Gene flow between divergent populations will
• - equalize gene frequencies
• - reduce isolation
• - make one species (hybrid zone)
• Reinforcement must act quickly because of
  competing effects of gene flow isolation

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Peripheral Isolate Model

• Mayr 1954
• founder effect
• peripatric speciation
• Observed local, isolated populations peripheral
  to the main range are often divergent

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Peripatric Speciation

• new environment homogeneous
• (few conflicting selective pressures)
• population small
• founded by few individuals
• low genetic diversity
• genetic drift
• must be no gene flow

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Mechanism

• Genetic change in large popns is slow (gene
  flow)
• Faster in small populations (genetic drift)
• Epistasis synergistic effect of 2 loci on
  phenotype (may help pass fitness valley)
• Rapid speciation
• Evidence flycatcher
• lizards
• Model allows for non-adaptive speciation

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Speyeria spp.