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 (abstract) 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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Phylogenetic Signatures of Speciation

• Molecular phylogenies
• - inferences about nature and tempo of
• speciation
• - utilize the shapes of evolutionary trees
• - test if cladogenesis through time departs
• from specific hypotheses

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Phylogenetic Signatures

• Lineage-through-time model
• Analogous to population dynamics
• Speciation birth (gain)
• Extinction death (loss)
• Net diversification Speciation Extinction
• Changing number of total lineages over time

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Lineage-through-time model

• Changing number of total lineages
• Could indicate
• Constant uniform rate of speciation
• Accelerated speciation
• Decelerated speciation

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Time
Decelerated speciation (or recent increase in
extinction)
Accelerated speciation
Can distinguish clustering of ancient speciation
events (Sebastes rock fishes) from recent
explosive speciation (African cichlids)
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Speciation Rates

• Speciation events might accelerate evolutionary
  differentiation
• Consequence divergence proportional to the
  number of speciation events rather than time
  since common ancestor (produces a correlation
  between speciation and divergence rates)
• Predicted by punctuated equilibrium
• - morphological stasis except during
• speciation

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Speciation Rates

• Test of hypotheses
• 1. Genetic divergence proportional to time
• Mean genetic distance similar in
  species-rich and species-poor clades
• Genetic divergence proportional to number of
  speciation events
• Mean genetic divergence greater in
  species-rich clades

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Species Rich
D avg. Genetic distance
Phyletic gradualism
Punctuated equilibrium
Test Minnow 200 sp. Sunfish 30
sp. Inconsistent with punctuated equilibrium
Species Poor
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Estimating diversification rates from
phylogenetic information

• R. E. Ricklefs (2007) TREE
• Variation in species richness
  (latitude)
• of Sp. Speciation (S)
  Extinction (E)
• Use phylogenetic reconstruction to estimate
  speciation
• and extinction rates

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Estimating diversification rates from
phylogenetic information

• Problems - phylogeny based on extant taxa.
  Extinct taxa
• not represented
• - different combinations of S
  and E produce the
• same expected clade size
• Assumptions - completeness of phylogeny
  (presence of
• cryptic species)
• - accuracy of branch
  lengths calibrated to time

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Sister Clade Analysis

• Variation in rates of diversification
• Comparison of the number of species
  in
• sister clades (have the same age)
• Comparison of sister clades at higher and lower
  latitudes
• 10/11 (passerine
  birds)
• 10/13 (butterflies)
• clades were larger for lower latitude clades

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Independent Clades

• Variation in rates of diversification
• Passerine birds
• 
  
  extinction rate
• 
  speciation rate (proportion of
  speciation rate)
• Temperate 3.16
  0.995
• Tropical 5.32
  0.954

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Latitudinal Variation in Diversity

• The Latitudinal Gradient in Recent Speciation and
  Extinction Rates of Birds and Mammals
• Jason T. Weir and Dolph Schluter Science March
  2007
• Reconstructed phylogeny
• Individual branch length estimate
• 1. inverse of diversification rates
  (1/(speciation extinction) )
• deep in phylogeny
• 2. inverse of speciation rate
  (1/speciation)
• toward the present
• Using sister species of New World birds and
  mammals
• recent speciation and extinction rates at
  higher latitudes gt tropics
• But, net diversification rates lower

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• Large Punctuational Contribution of Speciation to
  Evolutionary Divergence at the Molecular Level
• Mark Pagel, Chris Venditti, Andrew Meade Science
  2006

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(No Transcript)
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As the number of taxa approaches infinity, the
curve has an asymptote of 0.22, indicating that
punctuational effects contribute
approximately 20 of the total molecular
diversity on the tree. Plants (green), Animals
(red) Fungi (yellow)
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Summary

• Whatever the mechanisms of the effects we have
  characterized, relatively rapid and punctuational
  bursts of evolution driven by speciation appear
  to make a substantial contribution to molecular
  divergence.
• By comparison, we found no molecular counterpart
  to the periods of stasis noted for morphological
  traits the other half of
• the conventional punctuated-equilibrium
  description of morphological evolution.