Quantifying Darwins postulates

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Quantifying Darwins postulates

• Individuals within species vary.
• Some of these variations are heritable.
• More offspring are produced than can survive.
• Survival and reproduction are nonrandom.
• The individuals that survive reproduce the
  most are those
• with variations that best suit their environment.

Quantitative genetics allows us to measure the
degree to which variation in a trait is heritable
(and therefore can respond to selection).
We can also measure the strength of selection.
Combining heritability and strength of selection
allows us to predict evolutionary change in
response to selection.
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How can we study the genetic basis of
quantitative variation?
VP Total variance in the trait (phenotype)
VG variance in the trait thats due to
variation in genes
VE variance in the trait thats due to
variation in the environment
This is simply a mathematical way of saying
that both genetic and environmental variation can
contribute to phenotypic variation.
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Requirements for evolution Heritability
Heritability the degree to which variation in a
quantitative trait can be passed from parent
to offspring how well parents trait predicts
offsprings trait
NOT how much a trait is based on genes or
environment, but how much the variation in a
trait within a population is based on
variation in genes or environment In other
words, are the differences in the trait based on
a genetic or environmental differences, or both
NOT genes for milk production, but for variation
in milk production
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Requirements for evolution Heritability
Broad-sense heritability
H2 VG / VP
In words broad-sense heritability is the
fraction of the total variation that can
be explained by variation in genes.
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Requirements for evolution Heritability
Narrow-sense heritability
h2 VA / VP
What does h2 1 mean? h2 0? h2
0.5?
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Estimating narrow-sense heritability
Slope ½ h2
Heritability means that offspring should resemble
their parents and relatives
offspring
one parent
midparent average of trait in mom dad
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Slope h2
Slope h2
offspring
offspring
midparent
midparent
Slope ½ h2
Slope h2
offspring
offspring
midparent
one parent
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How can we make sure that the resemblance between
relatives is due to shared genes and not due to
shared environment? Why is this important to
control? How can this be controlled?
Common garden experiment grow several families
under the same controlled experiment. Everyone
has the same environment. Therefore differences
should be genetic.
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Requirements for evolution Selection
Mean before selection
Mean after selection
number of individuals
Selection differential
body size (or other trait)
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mean trait value of selected individuals
mean trait value before selection
S -
S 6 5 1