Summary

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Summary
NS
Genetic Drift
Mutation
Gene Flow
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Genetic Basis for Evolution
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How does DNA work?
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Mutational Errors during DNA Replication Repair

• Most NB mutations are those occurring in
  production
• of gametes
• Various kinds of mutations occur e.g.
• 1. Point mutation
• 2. Frame-shift mutations
• 3. Chunks of chromosome
• The rates of mutation can be measured estimated

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Diploid organisms inherit a double set of genes

• DNA is physically carried on chromosomes
• Gene is smallest unit of inheritance
• 2 sets of chromosomes diploidy
• Gametes have one set i.e. haploid (formed by
  meiosis)
• Different forms of genes alleles

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Population Genetics is concerned with Genotype
Gene Frequencies

• Genes genotypes are symbolized by alphabetic
  letters
• e.g. 2 alleles (A a) occur at a locus then
  individual can
• have three genotypes AA, Aa aa
• If alleles same (e.g. AA or aa) homozygous
• If alleles differ (e.g. Aa) heterozygous
• Genotype at locus produces a phenotype

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Population Genetics is concerned with Genotype
Gene Frequencies

• Theory of pop genetics at one locus
• e.g. one genetic locus with 2 alleles (A a)
• and 3 genotypes (AA, Aa, and aa)
• Total population Aa AA aa aa AA Aa AA Aa
• Genotype frequencies
• Frequency of AA 3/8 0.375
• Frequency of Aa 3/8 0.375
• Frequency of aa 2/8 0.25
• Can symbolize genotype frequencies algebraically
• Genotype AA Aa aa
• Frequency P Q R

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Population Genetics is concerned with Genotype
Gene Frequencies

• Theory of pop genetics at one locus
• e.g. one genetic locus with 2 alleles (A a)
• and 3 genotypes (AA, Aa, and aa)
• Total population Aa AA aa aa AA Aa AA Aa
• Gene frequencies
• Frequency of A 9/16 0.5625
• Frequency of a 7/16 0.4375
• Can symbolize gene frequencies algebraically
• Genotype A a
• Frequency p q
• (In strict sense p q are allele frequencies)

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Population Genetics is concerned with Genotype
Gene Frequencies

• Gene frequencies can be calculated from
• genotype frequencies
• p P ½Q
• q R ½Q
• Genotype frequencies CANNOT be calculated
• from gene frequencies

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Genotype frequencies in absence of selection
Hardy-Weinberg equilibrium
Genotype AA Aa
aa Frequency p2 2pq
q2 (see page 94 97, M. Ridley)
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Assumptions of Hardy-Weinberg Equilibrium

• No selection
• Random mating
• 3. Large population size

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Test if genotypes in population are at
Harvey-Weinberg Equilibrium

• Count the genotype frequencies
• Calculate the gene frequencies
• If observed homozygote frequencies (gene
  freq.)2
• then population is in Hardy-Weinberg
  equilibrium
• (see p98, Ridley)

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Simplest Model of Selection One Favoured Allele
at a locus
Genotype Chance of
survival AA, Aa 1 aa
1 s s
selection coefficient (between 1 0) i.e. if s
0.1, then aa individuals have 90 chance of
survival The chance of survival fitness of
genotype
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Simplest Model of Selection One Favoured Allele
at a locus
NB alleles dont increase just because they are
dominant! Changes in gene frequency are set by
the fitnesses If the recessive allele has a
higher fitness, it will increase and the dominant
allele will decrease in frequency
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How Rapidly will the Population Change in Time?
Gene frequency of A (p) in 1 generation i.t.o.
its frequency in the previous generation (p)
?p p p

__spq2__ (1-
spq2)
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Three Methods to Estimate Fitness

• Measure the relative survival of the genotypes
• within a generation
• e.g. mark-recapture experiments

Peppered moth (Biston betularia)
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Three Methods to Estimate Fitness
2. Measure changes in gene frequencies between
generations then substitute measurements
into formula
?p p p

__spq2__ (1-
spq2)
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Three Methods to Estimate Fitness
3. Use deviations of genotype frequencies from
the Hardy-Weinberg ratios. NB can only
be used when the gene frequencies in the
population are constant between birth
adulthood Thus, cannot be used if expect
directional selection against a
disadvantageous gene
Sickle cell anemia
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Selection can Favour Polymorphism if Heterozygote
Fitter than Homozygote
Genotype AA Aa aa Fitness
1 - s 1 1 t chance of
death of A gene chance of death of a gene
ps qt
ps (1 p)t
p t / (s t) (See Table 5.9,
Ridley)
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Summary

• In absence of NS, and with random mating in large
  population the genotype frequencies at locus
  Hardy-Weinberg Equilibrium
• 2. In nature not H-W equilibrium because
  fitnesses of genotypes not equal, mating is
  non-random, and/or population small
• 3. Theoretical equation of NS at single locus
  frequency of gene in 1 generation as function of
  its frequency in next
• 4. The above relation is determined by fitnesses
  of genotypes

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Summary
5. Selection can maintain polymorphism when
heterozygote is fitter than homozygote