Chapter 7 - Beyond Alleles p. 190


7.1 Genetics of Quantitative Traits p. 192

  •  Quantitative genetics focuses on phenotypic traits with continuous distributions and the underlying mechanisms of evolution that produce them.
  • Genetic variation, environmental variation, and the interaction between genetics and the environment are important components of models that examine the evolution of these traits.
  1. Continuous traits have a complex genetic basis
    1. Polygenic trait: influenced by many genetic loci
      1. Interaction between alleles (epistasis)
      2. Interaction with environment (phenotypic plasticity)
      3. Quantitative genetics: study of the genetic mechanisms of continuous phenotypic traits
      4. Hardy-Weinberg extended to polygenic traits (Fig. 7.2)
      5. Complex traits vary continuously (Fig. 7.3)
      6. Components of phenotypic variation 
        1. VP  = VG + VE
        2. Genetic and environmental influences create continuous distribution (Fig. 7.4)

9.3 Measuring Heritable Variation

  1. Heritability: proportion of phenotypic variation attributed to variation in genes
    1. VP = phenotypic variance: total variation in a trait
    2. VG = Genetic variation: variation due to genes
    3. VE = environmental variation
  1. Broad sense heritability
    1. Proportion of phenotypic variance explained by genetic differences among individuals
    2.  H2 = VG / VP  = VG / VG + VE

9.3 Measuring Heritable Variation

  1. broad-sense heritability = VG/VP = VG/(VG + VE)
  1. Narrow sense heritability
    1. Proportion of phenotypic variance explained by additive genetic variation
    2. Causes offspring to resemble parents
    3. h2 = VA / VP  = VA / VA + VD + VI + VE 
    4. Estimating heritability
      1. Slope = heritability (Fig. 7.5)
      2. Key Concepts
        1. When components of variation are additive, genetic and environmental variance sum to total phenotypic variance
        2. Heritability is the proportion of phenotypic variance due to genetic differences
        3. Broad sense heritability includes:
          1. Additive effects
          2. Dominance effects
          3. Epistatic effects
          4. Maternal/Paternal environmental effects–
  1.  Narrow-sense Heritability (h2) measures the additive genetic variation in a trait; allows prediction of how a population will respond to selection
    1. h2 is estimated by the slope of the regression line between the trait values of parents plotted against offspring [Fig. 9.13].
    2. h2 is an estimate of variation in the in parents as a result of genes. If h2 = 1, then all the variation is caused by VA.
    3. VA = additive genetic variation (variation from additive effects of genes)
    4. h2 can be also be estimated by other methods. e.g., in humans comparing monozygotic versus dizygotic twins; covariance of values among siblings
  2. Example: measuring heritability of beak size in song sparrows (fig. 9.14)
  3. Twin studies [Fig. 9.16]

7.2 The Evolutionary Response to Selection p. 198

  • The speed of evolution depends on the strength of selection and the proportion of the total phenotypic variance of the trait that is attributable to the additive effects of alleles.
  1. Modes of selection (Fig. 7.6 )
    1. Directional
    2. Stabilizing (more in chapter 8, gall flies)
    3. Disruptive
    4. Cumulative effects of directional selection can be large (Fig. 7.7)
      1. Tripling the oil content in corn by artificial selection over 100 generations
      2. Disruptive selection (Fig. 7.8,)

9.6 Modes of Selection and the Maintenance of Genetic Variation

  • three modes of selection
  1. directional
    1. selection favors one extreme of the frequency distribution of the trait
    2. over time, the mean value of the trait will change
    3. over time, the variance in the trait will decline
    4. beak size in Galapagos finches, flower size in skypilots
    5. Natural Selection Can Produce New Traits, Even Though It Acts on Existing Traits
      1. selection works on variation already present.
      2. tripling the oil content in corn by artificial selection over 60 generations [Fig. 3.18]
  2. stabilizing
    1. selection acts against the extremes of the frequency distribution of the trait
    2. fitness peaks at intermediate values of the trait
    3. over time, the mean value of the trait stays the same
    4. over time, the variance in the trait will decline
    5. selection on gall-making fly Eurosta solidaginis (fig. 9.26)
  3. disruptive
    1. rare
    2. selection acts against intermediate values of the trait, favoring both extremes
    3. fitness is lowest at intermediate values of the trait and high for extremes
    4. over time, mean value of the trait will stay the same
    5. over time, variance in the trait will increase
    6. black-bellied seedcrackers in Africa (fig. 9.27)
  1. Evolutionary response to selection
    1. How much the population changes depends on:
      1. Selection differential (S)
      2. Heritability
      3. Selection differential measures the strength of selection (Fig. 7.9)
        1. S =  strength of selection on a trait
        2. Strong Selection
        3. Weak Selection
      4. High heritability results in larger change (Fig. 7.10)
      5. Calculating the evolutionary response to selection
        1. R = h2S
        2. Key Concepts
          1. Evolution and selection are not the same
            1. Selection can occur without evolution
            2. The magnitude of change depends on:
              1. Strength of selection (selection differential) –Heritability

9.4 Measuring Differences in Survival and Reproductive Success

  1. measuring differences in survival and reproductive success = measuring differences in fitness
  2. Measuring differences in fitnesses among individuals measures the strength of selection
  3. note who survives/reproduces and who doesn't
  4. measure differences between winners and losers in the trait of interest
  5. if difference in fitness among individuals is low, then selection is not acting strongly
  6. selection for increased tail length in mice
    1. S = selection differential, a measure of the strength of selection on a trait [Fig. 9.17a]
    2. Selection Gradient---analogous to selection differential, but is related to fitness (e.g., survival to reproductive age or any other measure of fitness) [Fig. 9.17b]

9.5 Predicting the Evolutionary Response to Selection

  1. R = response to selection
  2. R = the difference in the means of the generation after selection and the original population.
  3. R = h2S
  4. Figure 9.18: mid-parent (x-axis) and offspring (y-axis) phenotypes. The response to selection is equal to the heritability times the selection differential.

7.3 Disecting Complex Traits: QTL

  1. Quantitative trait locus (QTL) analysis links traits with genes  (Fig. 7.11)
  2. QTL analysis of coat color in mice  (Fig. 7.12)
  3. QTL analysis of coat color in mice  (Fig. 7.13)
  4. Much of variation in coat color explained by differences in two genes  (Fig. 7.14)
  5. Expression of Agouti during development influences coat color  (Fig. 7.15)
  6. Genetic manipulation of dark mice makes them lighter  (Fig. 7.16)
  7. QTL analysis of traits in Mimulus  (Fig. 7.17)
  8. Key Concepts
    1. QTL analysis identifies regions of the genome associated with phenotypic variation

7.4 The Evolution of  PHENOTYPIC PLASTICITY p. 209

  • Organisms can differ in how they react to environmental situations
  • a reaction that is itself heritable and capable of evolving when the nature of plasticity changes due to shifts in the frequencies of alleles within the population
  1. Environmental influences on quantitative traits
  2. Phenotypic plasticity  (Fig. 7.18)
    1. A single genotype produces different phenotypes depending on the environment
    2. Reaction norm  (Fig. 7.19)
    3. All genotypes may not respond to the environment in the same way  (Fig. 7.20)
      1. Genotype x environment interaction
      2. Phenotypic plasticity in Caenorhabditis elegans  (Fig. 7.21)
      3. Plasticity can evolve  (Fig. 7.22)
      4. Rapid change can lead to mismatch between plastic traits and environment   (Fig. 7.23)
      5. Key Concepts
        1. When plasticity is heritable, the response can evolve



  1.  Same genotype produces different phenotypes in different environments. 
  2. May or may not be adaptive. 
  3. Phenotypic Plasticity in Behavior: Water Fleas and Fish 
    1. Variation in phototactic behavior in the asexual Daphnia [Figs. 10.17] 
      1. Three lakes [Fig 10.18], with populations showing considerable variation in phototactic behavior. Similar variability in all 3. 
      2. Fish select for Daphnia that avoid well-lit areas when fish are present 
    2. Phenotypic plasticity can evolve.  [Fig. 10.19]


  1. Alpine Skypilots and Bumblebees

    1. The alpine skypilot (Polemonium viscosum) [Fig. 9.19] grows in the Rocky Mountains at timberline and in the higher-elevation tundra. At timberline it is pollinated by a variety of insects, but in the tundra it is pollinated almost exclusively by bumblebees. Tundra flowers are 12% larger than timberline wildflowers. Galen (1996) tested whether the larger flower size of the tundra skypilots could be explained by selection for larger flower size (imposed by a bumblebee preference for larger flowers).
    2. heritability of flower size in a small-flowered timberline population was measured by regressing offspring corolla flare against maternal corolla flare [Fig. 9.20]. 
    3. Fig. 9.21 Estimating the Selection Gradient: Plot relative fitness as a function of maternal flower size
    4. Fig. 9.22 Measuring the evolutionary response to selection in alpine skypilots



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