Fig. 19-1

1
Chapter 19 Population genetics
Fig. 19-1
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• Population genetics
• Population interbreeding members of a species
• Three major principles of Darwinian evolutionary
  theory
• variation for traits exists within populations
• selection applies to a subset of those traits
• (selection can act only upon variations)
• traits are genetically transmitted

3

• Polymorphism multiple forms of a gene are
• commonly found in a population
• (all studied populations are wildly
  polymorphic)
• chromosomal polymorphisms
• immunological polymorphisms
• protein polymorphisms
• nucleic acid sequence/structure polymorphisms

4
p (2 x MM) (1 x MN) frequency of M
allele q (2 x NN) (1 x MN) frequency of N
allele Therefore, frequency of MN reflects the
genetic variation in a population
5
Fig. 19-2
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• Factoids regarding protein polymorphisms
• in most species
• structural polymorphisms are displayed by
• about one-third of all proteins
• typically, about 10 of the individuals in
• a large population are heterozygous for
• polymorphisms of an average gene
• Therefore, enormous protein-level variation
• exists in most populations

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Electrophoretic allelic variants of esterase-5 in
Drosophila
Fig. 19-2
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Electrophoretic variants of hemoglobin A in
humans
Fig. 19-3
9
Enormous naturally-occuring variation
(polymorphism) in protein sequence
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Enormous naturally-occuring variation
(polymorphism) in chromosomal rearrangements
11
Restriction sites within Drosophila xdh gene (58
wild chromosomes sampled)
4-base sites in 4.5 kb DNA Site present in
minority of chromosomes 0/1 Site in ½ of
chromosomes
Enormous naturally-occuring variation
(polymorphism) in nucleotide sequence
Fig. 19-5
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Enormous naturally-occuring variation
(polymorphism) in tandem repeat arrays (VNTRs)
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Hardy-Weinberg Equilibrium Random mating
within a large population assures a stable
equilibrium of genetic diversity in subsequent
generations provided that certain assumptions
apply Mating is random (no biased mating,
infinite population size) Allele frequencies
do not change (no selection, no migration,
etc.)
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Hardy-Weinberg Equilibrium For a
two-allele system, all genotypes exist as a
simple product of the frequency of each
allele homozygotes p2 or q2
heterozygotes 2pq p2 2pq
q2 1
Box 19-2
15
Box 19-2
16
Allele frequencies determine frequencies of
homozygotes and heterozygotes
Rare alleles are almost always found in
heterozygotes, almost never homozygous
Fig. 19-6
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Another measure of heterozygosity is haplotype
diversity
Haplotype combination of non-allelic alleles on
a single chromosome
18
MN allele and genotype frequencies reflect
Hardy-Weinberg assumptions
Allele and genotype frequencies can vary between
populations, while exhibiting H-W equilibria
within each population
19
Non-random mating inbreeding (mating among
relatives)
Positive inbreeding Mating among relatives is
more common than random Increases
frequencies of homozygotes in a population
Fig. 19-7
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Extreme inbreeding self-fertilization results
in loss of heterozygosity
No change in p or q change only in
heterozygosity and diversity
Fig. 19-8
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Negative inbreeding (enforced outbreeding) -barr
iers to inbreeding are common attributes
of successful populations Positive
assortative mating - individuals chose like
mates (not necessarily
relatives) Negative assortative mating -
individuals choose dissimilar mates
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• Sources of variation
• Mutation very slow

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Mutation is the ultimate source of variation
But spontaneous mutations occur at extremely low
frequencies
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Mutation frequency is influenced by allele
frequency
Mutation alone is a very slow evolutionary force
and cannot directly account for diversity
observed in populations.
Box 19-3
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• Sources of variation
• Mutation very slow
• Recombination rapidly mixes genes to
• provide new genetic combinations in
• a population
• Migration gene flow among different
• populations changes gene frequencies

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Selection directed change in genotypes
in a population Fitness survival and
reproduction success function of
genotype and environment
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• Fitness can be obvious (mortality, sterililty)
• HbS/HbS severe anemia, low survival
• HbS/HbA apparent resistance to malaria
• or more subtle/partial/conditional

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Fitness (viability) of various homozygotes
as a function of temperature
Drosophila pseudoobscura
Fig. 19-9
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Enhanced fitness of a genotype will enrich those
genes in subsequent generations of that population
Frequencies of positively selected genes increase
over time Frequencies of negatively selected
genes decrease over time Change in A frequency
(?p) is greatest where p q
Fig. 19-11
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For a two-allele system, mean fitness (W) in a
population is the proportional contribution of
fitness by each genotype (A/A, A/a, a/a)
W p2WA/A 2pqWA/a q2Wa/a WA/A and WA/a
gt Wa/a p should increase q should
decrease Wa/a gt WA/A and WA/a q should
increase p should decrease

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Fitness can account for allele frequency changes
over time
?p for malic dehydrogenase electrophoretic
mobility variant MDHF where WS/S1, WS/F0.75,
WF/F0.4
Fig. 19-12
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Selection directed change in genotypes
in a population Fitness survival and
reproduction success function of
genotype and environment Frequency
independent selection fitness is
independent of genotype frequency Frequency
dependent selection fitness changes
as genotype frequency changes
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• Random genetic drift random changes in gene
• frequency that can lead to extinction/fixation
• of genes
• Requires no selection
• Essentially sampling error inherent in
• each generation in achieving Hardy-
• Weinberg equilibrium
• Most exaggerated in small populations
• (especially founder effects)
• Allows isolated populations to diverge
• without differential selection (each
• experiences its own drift history)

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Model history of emergence of ten mutations
and their drift in a population over time
Drift to extinction for nine drift to p1 in one
Fig. 19-13
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Drift explains differences in unselected
allele frequencies in isolated populations
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Genetic change is directed by diverse
evolutionary forces which tend to increase (blue)
or decrease (red) variation
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Recommended problems in Chapter 19 3, 5, 12, 17