EVOLUTION AT MULTIPLE LOCI:  QUANTITATIVE GENETICS

Chapter 9 in the 4th edition, Chapter 8 in the 3rd.

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SELECTION on QUANTITATIVE TRAITS

Measuring Heritable Variation

1. qualitative traits
   1. characters that show discrete variation
2. quantitative traits
   1. show continuous variation e.g., human height [Fig. 7.19], like many quantitative traits, has a normal distribution
   2. result of combined effect of many loci plus the environment
   3. The same genotype can express various phenotypes depending on environmental condition.
   4. The more genes influencing a given trait, the more continuous/quantitative the trait.
3. the first step in quantitative genetics is to partition phenotypic variation into environmental and genetic components
4. quantitative genetics measures and studies heritability, the strength and direction of selection, and the response of populations to selection
5. Heritability: proportion of phenotypic variation attributed to variation in genes
   1. V_P = phenotypic variance: total variation in a trait
   2. V_G = Genetic variation: variation due to genes
   3. V_E = environmental variation
   4. broad-sense heritability = V_G/V_P = V_G/(V_G + V_E)
6.  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. 7.20].
   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 V_A.
   3. V_A = 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
7. Example: measuring heritability of beak size in song sparrows (fig. 7.22)

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
   • S = selection differential, a measure of the strength of selection on a trait [Fig. 7.23a]
   • Selection Gradient---analogous to selection differential, but is related to fitness (e.g., survival to reproductive age or any other measure of fitness) [Fig. 7.23b]

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 7.26: mid-parent (x-axis) and offspring (y-axis) phenotypes. The response to selection is equal to the heritability times the selection differential.

Alpine Skypilots and Bumblebees

• The alpine skypilot (Polemonium viscosum) [Fig. 7.27] 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).
• heritability of flower size in a small-flowered timberline population was measured by regressing offspring corolla flare against maternal corolla flare [Fig. 7.28]. 

Modes of Selection and the Maintenance of Genetic Variation

• three modes of selection [Fig. 7.34]

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
2. stabilizing [Fig. 7.35]
   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. 7.35)
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. 7.36)

The Bell-Curve Fallacy and IQ

In The Bell Curve, Murray and Herrnstein (1994) argued that the difference in average IQ scores of African Americans and European Americans is at least partly due to genetic differences between the groups.  Read in your text why their argument is erroneous. Heritability tells us nothing about the causes of phenotypic differences between populations that live in different environments.

How to properly test the contention that the differences between groups is genetic

• Common garden experiments have been done with a variety of other species, including the classic studies of yarrow (Achillea) by Clausen, Keck, and Hiesey [Fig. 7.33], who found that plants from low-elevation populations make more stems than plants from high-elevation, when grown in their respective natural environments. When both types are grown at low elevation, the low-elevation population plants still grow more stems than the high-elevation population plants. This might seem to suggest that low-elevation plants have been genetically "programmed" to produce more stems. But when both types grown at high elevation, the high-elevation population plants produced more stems than the low-elevation population plants
• Thus, there are genetic differences between low- and high-elevation plants in the manner in which each responds to the environment. means that plants vary in the way their genes interact with their environments – each is "superior" in its environment of origin

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