Download Population Genetics

Genes Within
Populations
Chapter 20
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Charles Darwin
Served as naturalist on mapping
expedition around coastal South
America.
Used many observations to develop his
ideas
Proposed that evolution occurs by
natural selection
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Voyage of the Beagle
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Darwin’s Evidence
Population growth vs. availability of resources
-population growth
is geometric
-increase in food
supply is arithmetic
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Darwin’s Evidence
Population growth vs. availability of resources
- Darwin realized that not all members of a
population survive and reproduce.
-Darwin based these ideas on the writings of
Thomas Malthus.
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Genetic Variation and Evolution
•  Darwin: Evolution is descent with
modification
•  Evolution: changes through time
1.  Species accumulate difference
2.  Descendants differ from their
ancestors
3.  New species arise from existing
ones
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Natural selection: mechanism of
evolutionary change
Natural selection: proposed by Darwin as
the mechanism of evolution
•  individuals have specific inherited
characteristics
•  they produce more surviving offspring
•  the population includes more individuals
with these specific characteristics
•  the population evolves and is better adapted
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to its present environment
Darwin’s
theory for
how long
necks
evolved in
giraffes
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Natural selection: mechanism of
evolutionary change
Inheritance of acquired characteristics:
Proposed by Jean-Baptiste Lamarck
•  Individuals passed on physical and
behavioral changes to their offspring
•  Variation by experience…not genetic
•  Darwin’s natural selection: variation a result
of preexisting genetic differences
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Lamarck’s theory of how giraffes’ long
necks evolved
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Population Genetics
•  The study of the properties of genes in
populations
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Hardy-Weinberg Principle
Godfrey H. Hardy: English mathematician
Wilhelm Weinberg: German physician
Concluded that:
The original proportions of the genotypes in a
population will remain constant from
generation to generation as long as five
assumptions are met
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Hardy-Weinberg Principle
Five assumptions :
1.  No mutation takes place
2.  No genes are transferred to or from
other sources
3.  Random mating is occurring
4.  The population size is very large
5.  No selection occurs
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Hardy-Weinberg Principle
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Calculate genotype frequencies with a
binomial expansion
(p+q)2 = p2 + 2pq + q2
p = individuals homozygous for first allele
2pq = individuals heterozygous for both
alleles
q = individuals homozygous for second
allele
because there are only two alleles:
p plus q must always equal 1
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Hardy-Weinberg Principle
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Hardy-Weinberg Principle
Using Hardy-Weinberg equation to predict
frequencies in subsequent generations
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A population not in Hardy-Weinberg
equilibrium indicates that one or more of the
five evolutionary agents are operating in a
population
Five agents of evolutionary change
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Agents of Evolutionary Change
•  Mutation: A change in a cell’s DNA
–  Mutation rates are generally so low they
have little effect on Hardy-Weinberg
proportions of common alleles.
–  Ultimate source of genetic variation
•  Gene flow: A movement of alleles from
one population to another
–  Powerful agent of change
–  Tends to homogenize allele frequencies
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Agents of Evolutionary Change
•  Nonrandom Mating: mating with specific
genotypes
– Shifts genotype frequencies
– Assortative Mating: does not change
frequency of individual alleles; increases
the proportion of homozygous
individuals
– Disassortative Mating: phenotypically
different individuals mate; produce
excess of heterozygotes
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Genetic Drift
•  Genetic drift: Random fluctuation in
allele frequencies over time by chance
• important in small populations
– founder effect - few individuals
found new population (small allelic
pool)
– bottleneck effect - drastic
reduction in population, and gene
pool size
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Genetic Drift: A bottleneck effect
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Selection
•  Artificial selection: a breeder selects for
desired characteristics
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Selection
•  Natural selection: environmental
conditions determine which individuals in a
population produce the most offspring
•  3 conditions for natural selection to occur
– Variation must exist among individuals in
a population
– Variation among individuals must result
in differences in the number of offspring
surviving
– Variation must be genetically inherited
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Fitness and Its Measurement
•  Fitness: A phenotype with greater
fitness usually increases in frequency
– Most fit is given a value of 1
•  Fitness is a combination of:
– Survival: how long does an
organism live
– Mating success: how often it mates
– Number of offspring per mating that
survive
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Fitness and its Measurement
Body size and egg-laying in water striders
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Maintenance of Variation
•  Frequency-dependent selection:
depends on how frequently or infrequently
a phenotype occurs in a population
– Negative frequency-dependent
selection: rare phenotypes are favored
by selection
– Positive frequency-dependent selection:
common phenotypes are favored;
variation is eliminated from the
population
•  Strength of selection changes through time
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Maintenance of Variation
•  Oscillating selection: selection favors
one phenotype at one time, and a
different phenotype at another time
•  Galápagos Islands ground finches
– Wet conditions favor big bills
(abundant seeds)
– Dry conditions favor small bills
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Maintenance of Variation
•  Heterozygotes may exhibit greater fitness
than homozygotes
•  Heterozygote advantage: keep
deleterious alleles in a population
•  Example: Sickle cell anemia
•  Homozygous recessive phenotype: exhibit
severe anemia
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Maintenance of Variation
•  Homozygous dominant phenotype:
no anemia; susceptible to malaria
•  Heterozygous phenotype: no anemia;
less susceptible to malaria
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Maintenance of Variation
Frequency of sickle cell allele
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Maintenance of Variation
Disruptive selection acts to eliminate
intermediate types
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Maintenance of Variation
Directional selection: acts to eliminate one
extreme from an array of phenotypes 34
Maintenance of Variation
Stabilizing selection: acts to eliminate
both extremes
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