Download Evolution and variation - Anoka

4/5/09
Chapter 20
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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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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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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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Interactions Among Evolutionary
Forces
•  Mutation and genetic drift may counter
selection
•  The magnitude of drift is inversely related to
population size
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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
– 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
Negative frequency - dependent selection
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Maintenance of Variation
Positive frequency-dependent selection 28
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
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
– Compound eyes of insects: same
genes affect both eyes
– Control of ommatidia number in left
and right eye
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Experimental Studies
Selection for increased speed in
racehorses is no longer effective
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Experimental Studies
Phenotypic variation in insect ommatidia
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