Selection on Quantitative Traits

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Selection on Quantitative Traits

• The number of loci that contribute to variation
  in a character may be less than the number that
  actually contribute to its development.
• Futuyma, p 301

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Swallowtail polymorphism
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Heliconius mimicry
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Heliconius genetics of mimicry

• Multiple loci
• H. erato (15) and H. melpomene (12)
• Loci are scattered on different chromosomes not
  linked
• Mating between forms produces diverse (and
  unsuccessful) intermediates

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How are genes influencing QT identified?

• QTL mapping
• More complex than on-off or single-gene
  approach
• Measures association between variation in a
  phenotype and a genetic marker
• Reveals
• Number of loci influencing a QT
• Magnitude of their effects on the QT
• Location in the genome

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Genetic markers (X,Y,Z) on different strains
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Which chromosome contributes to variation?
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Juenger et al., 2005. Quantitative trait loci
mapping of floral and leaf morphology traits in
Arabidopsis thaliana . . .. Evolution
Development 7259-271.
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Genomic positions of QTL for floral and leaf
characters Juenger et al.
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General conclusions about QTL

• Variation may be influenced by dozens of loci
• 53 QTL for Drosophila bristle number
• Loci vary in intensity of effect
• Diverse genetic mechanisms for affecting
  variation
• Additive effects
• Epistatic effects
• Gene-environment interactions

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How do we add this complexity to models of
population genetics?

• It depends
• How many loci are involved?
• Do the loci assort independently, or as a group?

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Questions about linkage disequilibrium

• What is linkage disequilibrium?
• Why is this phenomenon important?
• How is linkage disequilibrium measured?
• What factors influence change in linkage
  disequilibrium?
• How are linkage disequilibrium and natural
  selection related?

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What is linkage disequilibrium?

• LD addresses relations among different genes
  (loci) on same chromosome type (e.g., Chromosome
  1, X Chromosome)
• When genotypes of one locus are distributed
  independently of genotypes at another locus, the
  two loci are in linkage equilibrium
• Linkage disequilibrium is anything else

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Crossing-over disconnects loci on a chromatid,
creating new haplotypes
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How is L.-D. measured?

• Count each haplotype in the population
• e.g., AB Ab aB ab
• Calculate frequencies (the proportion of all
  chromosomes represented by each haplotype)
• gAB gAb gaB gab
• Coefficient of Linkage Disequilibrium D
• D gABgab gAb gaB

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Linkage disequilibrium tends to decline (loci
appear to assort independently)

• What reduces physical linkage?
• Crossing over during meiosis
• Sexual reproduction is required
• genetic recombination results from
• shuffling of chromosomes
• crossing over within chromosomes

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An experimental test of L.-D. theory
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Why is L.-D. observed in nature?

• New mutations in asexually reproducing
  populations
• Selection on multilocus genotypes
• e.g., genes influencing mimicry
• Population admixture
• Genetic drift

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How does L.-D. influence selection?

• What if . . . a new, beneficial mutation is
  linked with a deleterious allele?
• What if . . . a new, nasty mutation is linked
  with a beneficial allele?
• Can single-locus models predict evolutionary
  change?

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Response to selection for a simple QT
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Can we quantify the response of QT to selection?

• Selection differential the difference in average
  phenotype between the general population and the
  surviving subset
• Selection gradient the shift in relative fitness
  for the starting population and surviving subset

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Selection events for Eurosta
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Selection differential vs response high h
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Selection differential vs response low h
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Response to Selection

• The response to selection will depend on
• The selection differential (S)
• The heritability of the trait (h2)
• R h2S

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What about selection on correlated characters?

• What are correlated characters?
• Why might selection on one trait depend on the
  form of a different character?

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Correlated characters Selection Surfaces
Adaptive landscape