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Genes Within
Populations
Chapter 20
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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
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evolved
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
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•
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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 to its
present environment
Evolution requires study of Population Genetics
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Darwin’s
theory for
how long
necks
evolved in
giraffes
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Gene Variation in Nature
• Measuring levels of genetic variation
– blood groups
– enzymes
• Enzyme polymorphism
– A locus with more variation than can be
explained by mutation is termed
polymorphic.
– Natural populations tend to have more
polymorphic loci than can be accounted
for by mutation.
• DNA sequence polymorphism
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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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•
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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
Since there are 16 white (bb) their freq is q2=0.16.
q=0.4
p=1.0 - 0.4=0.6
p2=0.36 2pq=0.24
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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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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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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 (1/100,000
divisions) they have little effect on HardyWeinberg 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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Bottleneck effect: case study
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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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Selection
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Selection
Pocket mice from the Tularosa Basin
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Selection to match climatic
conditions
• Enzyme allele frequencies vary with latitude
• Lactate dehydrogenase in Fundulus
heteroclitus (mummichog fish) varies with
latitude
• Enzymes formed function differently at
different temperatures
• North latitudes: Lactate dehydrogenase is a
better catalyst at low temperatures
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Selection for pesticide resistance
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Selection for pesticide resistance
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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
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Body size and egg-laying in water striders
Fitness and its Measurement
Body size and egg-laying in water striders32
Fitness and its Measurement
Body size and egg-laying in water striders
Favors medium body size
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Interactions Among Evolutionary
Forces
• Mutation and genetic drift may counter selection
• The magnitude of drift is inversely related to
population size
• In very small populations drift can be more
important than natural selection.
• Usually natural selection is predominate force
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Interactions Among Evolutionary
Forces
• Gene flow may promote or constrain
evolutionary change
– Spread a beneficial mutation
– Impede adaptation by continual flow of
inferior alleles from other populations
• Extent to which gene flow can hinder the
effects of natural selection depends on the
relative strengths of gene flow
– High in birds & wind-pollinated plants
– Low in sedentary species
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Interactions Among Evolutionary
Forces
Degree of copper tolerance
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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
• Prey are used to looking for more common types
– Positive frequency-dependent selection:
• common phenotypes are favored
• variation is eliminated from the population
• Strength of selection changes through time
(Oscillating Selection)
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Maintenance of Variation
•Negative frequency - dependent selection
•Most common water boatman are eaten because fish
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are more used to identifying these.
Maintenance of Variation
•Positive frequency-dependent selection
•Off type is selected & eaten
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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
• Fitness of a phenotype does not depend
on its frequency
• Environmental changes lead to
oscillation in selection
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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
Disruptive selection for large and small
beaks in black-bellied seedcracker finch of
west Africa
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Maintenance of Variation
Directional selection: acts to eliminate one
extreme from an array of phenotypes
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Maintenance of Variation
Directional selection for negative
phototropism in Drosophila
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Maintenance of Variation
Stabilizing selection: acts to eliminate
both extremes
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Maintenance of Variation
Stabilizing selection for birth weight in humans
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Experimental Studies of Natural
Selection
• In some cases, evolutionary change can
occur rapidly
• Evolutionary studies can be devised to test
evolutionary hypotheses
• Guppy studies (Poecilia reticulata) in the lab
and field
– Populations above the waterfalls: low
predation
– Populations below the waterfalls: high
predation
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Experimental Studies
• High predation environment - Males
exhibit drab coloration and tend to be
relatively small and reproduce at a younger
age.
• Low predation environment - Males
display bright coloration, a larger number of
spots, and tend to be more successful at
defending territories.
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Experimental Studies
The evolution of protective coloration in
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guppies
Experimental Studies
The laboratory experiment
–10 large pools
–2000 guppies
–4 pools with pike cichlids (predator)
–4 pools with killifish (nonpredator)
–2 pools as control (no other fish
added)
–10 generations
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Experimental Studies
The field experiment
– Removed guppies from below the
waterfalls (high predation)
– Placed guppies in pools above the
falls
– 10 generations later, transplanted
populations evolved the traits
characteristic of low-predation
guppies
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Experimental Studies
Evolutionary change in spot number
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The Limits of Selection
• Genes have multiple effects
– Pleiotropy: sets limits on how much
a phenotype can be altered
• Evolution requires genetic variation
– Thoroughbred horse speed
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Experimental Studies
Selection for increased speed in racehorses
is no longer effective
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