EVOLUTION AT MULTIPLE LOCI: QUANTITATIVE GENETICS
Chapter 9 in the 4th edition, Chapter 8 in the 3rd.
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9.1 The NATURE of QUANTITATIVE TRAITS
- qualitative traits
- characters that show discrete variation
- quantitative traits
- show continuous variation e.g., human height [Fig.
9.1], like
many quantitative traits, has a normal distribution
- result of combined effect of many loci [Fig.
9.2] plus the environment
[Fig. 9.4]
- The same genotype can express various phenotypes depending on
environmental condition.
- The more genes influencing a
given trait, the more continuous/quantitative the trait.
- the first step in quantitative genetics is to partition phenotypic
variation into environmental and genetic components
- quantitative genetics measures and studies heritability, the strength
and direction of selection, and the response of populations to selection
9.3 Measuring Heritable Variation
- Heritability: proportion of phenotypic variation attributed to variation
in genes
- VP = phenotypic variance: total variation in a trait
- VG = Genetic variation: variation due to genes
- VE = environmental variation
- broad-sense heritability = VG/VP = VG/(VG
+ VE)
- Narrow-sense Heritability (h2) measures the additive genetic
variation in a trait; allows prediction of how a population will respond
to selection
- h2 is estimated by the slope of the regression line between
the trait values of parents plotted against offspring [Fig.
9.13].
- 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.
- VA = additive genetic variation (variation from additive
effects of genes)
- h2 can be also be estimated by other methods. e.g., in
humans comparing monozygotic versus dizygotic twins; covariance of
values among siblings
- Example: measuring heritability of beak size in song sparrows (fig.
9.14)
- Twin studies [Fig. 9.16]
9.4 Measuring Differences in Survival and Reproductive Success
- measuring differences in survival and reproductive success = measuring
differences in fitness
- Measuring differences in fitnesses among individuals measures the
strength of selection
- note who survives/reproduces and who doesn’t
- measure differences between winners and losers in the trait of
interest
- if difference in fitness among individuals is low, then selection is
not acting strongly
- selection for increased tail length in mice
- S = selection differential, a measure of the strength of selection on a
trait
[Fig. 9.17a]
- 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
- R = response to selection
- R = the difference in the means of the generation after selection and
the original population.
- R = h2S
- 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.
Alpine Skypilots and Bumblebees
- 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).
- heritability of flower size in a small-flowered timberline population was
measured by regressing offspring corolla flare against maternal corolla
flare [Fig. 9.20].
- Fig. 9.21 Estimating the Selection Gradient: Plot
relative fitness as a function of maternal flower size
- Fig. 9.22 Measuring the evolutionary response to selection in alpine
skypilots
9.6 Modes of Selection and the Maintenance of Genetic Variation
- three modes of selection [Fig. 9.25]
- directional
- selection favors one extreme of the frequency distribution of
the trait
- over time, the mean value of the trait will change
- over time, the variance in the trait will decline
- beak size in Galapagos finches, flower size in skypilots
- stabilizing
- selection acts against the extremes of the frequency distribution of the
trait
- fitness peaks at intermediate values of the trait
- over time, the mean value of the trait stays the same
- over time, the variance in the trait will decline
- selection on gall-making fly Eurosta solidaginis (fig.
9.26)
- disruptive
- rare
- selection acts against intermediate values of the trait, favoring
both extremes
- fitness is lowest at intermediate values of the trait and high
for extremes
- over time, mean value of the trait will stay the same
- over time, variance in the trait will increase
- black-bellied seedcrackers in Africa (fig. 9.27)
9.7 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. 9.31], 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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