presentation_16 - Lecture 16 Natural selection on...

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1 Lecture 16 Natural selection on quantitative (polygenic) traits Polygenic traits • Many traits of presumed evolutionary signiFcance are polygenic (determined by few to many independent gene loci). • These are continuously varying or meristic (countable) traits • Examples: body size, growth rate, age of Frst reproduction, number of vertebrae, etc.
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2 Continuous variation: body size +++-+- ---+-+ +++--- +++--- +-+-+- +-+-+- Six loci, each with the + and - alleles. If + and - alleles at all six loci contribute equally to overall size, then any genotype with 6 + and 6 - alleles will be of the same size. Each of the 6 loci will be a QTL for size with presence of the + allele associated with larger size. Possible multi-locus genotypes for loci contributing to size
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3 Mean and variance of a distribution variance is given by 1/( n ! 1) ( x i ! x " ) 2 mean x i phenotype No. individuals No. individuals phenotype X 1 X 2 V P1 V
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Partition observed (phenotypic) variance into genetic and environmental components V P = V G + V E V G = V A + V D + V I Interested in the relative magnitude of V G + V E V G can be further partitioned into additive, dominance and interaction variance V A , V D , V I ? +++-+- ---+-+ +++--- +++--- +-+-+- +-+-+- Each + allele contributes +1 cm to overall size height of 3 genotypes is 6 Effect of + allele at locus 1 depends depends on there being a + allele at locus 5 +++-+- ---+-+ +++--- +++--- +-+-+- +-+-+-
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presentation_16 - Lecture 16 Natural selection on...

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