Lecture 13

• Qualitative traits


  Often simple Mendelian traits inherited as a single gene (e.g., eye colour)
• Quantitative traits


  Have continuously distributed phenotypes (e.g., height, mass)

  
• One of the cornerstones of the modern synthesis was the discovery that quantitative traits are also compatible with Mendelian genetics

  
• This is the basis of the field of quantitative genetics
• Phenotypic variance (VP)


  Can be decomposed into genetic variation (VG), environmental variance (VE), and genotype-by-environment interaction variance (VG×E)
• Genetic variation in quantitative traits can be caused by few or by many loci
• When quantitative traits are influenced by many loci (and environmental effects), the trait is generally normally distributed

  
• Normal distribution


  Mean (μ) determines the location, standard deviation (σ) determines the spread

  
• The normal distribution is ubiquitous in Biology due to the central limit theorem
• Quantitative traits compatible with Mendelian inheritance

  1. Short- and long-flowered plants crossed

  2. F1 offspring have intermediate flower length

  3. F1 individuals are heterozygous at flower-length genes

  4. F2 offspring are more variable

  5. Some flower-length genes are homozygous in the F2 generation

  6. Original parental phenotypic range can rapidly be recovered by F5

  
• Genetic variance (VG)


  Can be decomposed into additive genetic variance (VA), dominance variance (VD), and epistasis variance (VI)
• Broad-sense heritability (H2)


  Genetic component of variation (VG/VP)

  
• Narrow-sense heritability (h2)


  Additive genetic component of variation (VA/VP)
• In Evolutionary Biology, unless told otherwise, "heritability" means narrow-sense heritability (h2)
• Additive genetic effects


  Adding each copy of an allele has the same effect on the trait, and the mean heterozygote is exactly in the middle
• Additive genetic variance (VA)

  Depends on genetic variation at the locus (expected heterozygosity, 2pq) and the squared magnitude of additive effects of alleles on the phenotype (a2)
• Modes of selection on quantitative traits

  • Directional selection
  • Stabilizing selection
  • Disruptive selection
• Estimating heritability from parents and offspring

  1. Regress midoffspring height on midparent height

  2. Heritability (h2) is the slope of the line

  
• High heritability of a trait does not mean that differences in the trait are entirely genetic or that the trait is unalterable by the environment
• Low heritability of a trait does not mean the trait is not genetically controlled, just that genetic variation is small compared to environmental variation
• Selection differential (S)


  Difference between the mean trait value of the selected individuals and the mean of the entire population
• Selection gradient (b)


  Slope of the regression line of relative fitness on the trait value
• Predicting the evolutionary response to selection

  In artificial selection, the response (R) is given by the breeder's equation: R = h2S
• Quantitative traits can evolve beyond their original range of variation due to standing genetic variation
• Standing genetic variation


  Allelic diversity that is already available for selection to act on
• Genetic correlations between traits

  Selection on one trait can cause another trait to evolve by "dragging it along for the ride"
• Genetic correlations can cause trade-offs and constraints on evolution