BIOL100 Lecture Notes - Lecture 14: Disruptive Selection, Directional Selection, Phenotypic Trait

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31 Jan 2013
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Measuring Heritable Variation
Basic tenets of Darwin’s theory of evolution by natural selection:
oIf there is heritable variation among the individuals in a population, and if there are differences in
survival and/or reproductive success among the variants, then the population will evolve.
Quantitative genetics includes tools to measure heritable variation, tools for measuring differences in
survival and/or reproductive success, and tools for predicting the evolutionary response to selection
Quantitative genetics allows us to analyze evolution by natural selection in traits controlled by many loci
The first step in a quantitative genetic analysis is to determine the extent to which the trait in question is
heritable. That is, we must partition the total phenotypic variation (Vp) into a component due to genetic
variation (VG) and a component due to environmental variation (VE)
Heritability- fraction of the total variation in a trait that is due to variation in genes
oIn the broad sense, that fraction of the total phenotypic variation in a population that is caused by
genetic differences among individuals; in the narrow sense, that fraction of the total variation that is
due to the additive effects of genes.
Phenotypic Variation (VP)- total variation in a trait
Genetic Variation (VG)- variation among individuals that is due to variation in their genes
Environmental Variation (VE)- variation among individuals due to variation in their environments
Broad-Sense Heritability or Degree of Genetic Determination:
Heritability = VG/ VP = VG/ (VG + VE)
Estimating Heritability from Parents and Offspring
If the variation among individuals is due to variation in their genes, then offspring will resemble their
parents
Figure 9.13:
o[9.13a] If offspring do not resemble their parents, then the slope of the best-fit line through the data
will be near 0; this is evidence that the variation among individuals in the population is due to
variation in their environments, not variation in their genes
o[9.13c] If offspring strongly resemble their parents, the slope of the best-fit line will be near 1; this is
evidence that variation among individuals in the population is due to variation in their genes, not
variation in their environments
o[9.13b] Most traits in most populations fall somewhere in the middle, with offspring showing a
moderate resemblance to their parents; this is evidence that the variation among individuals is
partly due to variation in their environments and partly due to variation in their genes
Narrow-sense Heritability (h2)- that fraction of the total phenotypic variation in a population that is due
to the additive effects of genes
oIs an estimate of the fraction of the variation among the parents that is due to variation in their
genes
h2= VA/VP = VA/ (VA + VD + VE)
Heritability, h2, is a measure of the (additive) genetic variation in a trait
Additive genetic variation (VA) is variation among individuals due to the additive effects of genes
Dominance genetic variation (VD) is variation among individuals due to gene interactions such as
dominance
ototal genetic variation is the sum of the additive and dominance genetic variation
VG = VA + VD
Narrow sense heritability, h2, allows us to predict how a population will respond to selection
Estimating Heritability from Twins
Monozygotic (identical) twins share their environment and all of their genes
Dizygotic (fraternal) twins share their environment and half of their genes
If heritability is high, and variation among individuals is due mostly to variation in genes, then monozygotic
twins will be more similar to each other than are dizygotic twins
If heritability is low, and variation among individuals is due mostly to variation in environments, then
monozygotic twins will be as different from each other as dizygotic twins
Measuring Differences in Survival and Reproductive Success
Second tenet of Darwin’s theory:
oThere are differences in survival and/or reproductive success among individuals
When measuring the differences in success among individuals, it usually means measuring the strength of
selection
If we can measure heritable variation and strength of selection, we will be able to predict evolutionary
change in response to selection
On average, individuals with some values of a trait survive at higher rates, or produce more offspring, than
individuals with other values of a trait
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