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Chapter 4

BIO120H1 Chapter Notes - Chapter 4: Genetic Drift, Electrophoresis, Natural Selection

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Benjamin Wright

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Lecture 4
Genetic Variation: Models and Measurement
Key Concepts/ Terms:
Population genetic structure
Selection response
Polymorphic locus
Genotype frequency
Monomorphic locus
Allele frequency
1) Models of population genetic variation: What is population genetic variation? Classical and balance
2)Struggle to measure genetic variation: Selection experiments, Electrophoresis revolution, DNA
What can we learn from studying genetic variation?
Genetic variation is the raw material upon which evolution acts
We can study genetic diversity to help understand the processes of evolution
Fishers Fundamental Theorem of Natural Selection:
The rate of increase in fitness of a population at any time is equal to its genetic variance in fitness
at that time.
What factors influence patterns of genetic diversity?
What factors influence patterns of genetic diversity?
1) Mutation
2)Random genetic drift
4)Natural selection
5) Migration
Polymorphism (P) - Proportion of gene loci that are polymorphic
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Heterozygosity (H )- Average frequency of heterozygous individuals per gene locus
1)Number One:
a.Two contrasting theoretical models of the genetic variation of populations were developed
during the early-mid parts of last century
b.Called classical and balance hypothesises
c.Classical Hypothesises:
i.Propose that gene pool of population consists at each gene locus of a wild type allele
with a frequency approaching one
ii.Mutant alleles at very low frequencies man also exists at each locus
iii.A typical individual would be homozygous for the wild type allele at most
oat a very small portion of its loci the individual would be heterozygous for a
wild and mutant allele
iv.Normal genotype would be homozygous for the wild type allele at every locus
v.Mutant alleles are continuously introduced into population by mutation but are
generally deleterious and thus removed by selection
vi.Sometimes a beneficial mutant allele might arise, conferring higher fitness
vii.Would gradually replace wild allele to become new wild type
viii.Evolution consists of replacement at an occasional locus of the pre-existing wild type
allele by new one
ix.Largely developed by T.H. Morgan and H.J. Muller working with fruit fly mutants
d.Balance Model:
i.Developed largely by population and ecological geneticists, T. Dobzhansky and E.B.
ii.Proposed natural selection typically favours heterozygotes over homozygotes,
maintains high genetic diversity
iii.Gene pool of population consists at most loci of an array of alleles with individual
heterozygotes at significant proportion of its gene loci
iv.Generally no single wild type or normal allele at given locus and no normal ideal
v.Gene pools are co-adapted systems
vi.Sets of alleles favoured at one locus depends on the sets of alleles that exsist at toehr
Maintenance of Genetic Variation:
1) Mutation-selection balance
2)Diverse selective forces favouring diversity
1) Mutation-selection balance
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