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Genes Within Populations
Chapter 20
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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 to its present
environment.
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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 JeanBaptiste 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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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.
2.
3.
4.
5.
No mutation takes place
No genes are transferred to or from
sources
Random mating is occurring
The population size is very large
No selection occurs
other
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Hardy-Weinberg Principle
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 (that is the total population)
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Hardy-Weinberg Principle
Using Hardy-Weinberg equation to predict frequencies in
subsequent generations.
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Agents of Evolutionary Change
A population not in Hardy-Weinberg equilibrium indicates that
one or more of the five evolutionary agents are operating in a
population.
• 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.
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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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Selection
Pocket mice from the Tularosa Basin
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Selection to match climatic conditions
Selection for pesticide resistance
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Fitness and Its Measurement
• 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
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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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
Positive frequencydependent selection.
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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;
Heterozygotes may exhibit greater fitness than
homozygotes.
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Maintenance of Variation
• Homozygous
dominant phenotype:
no anemia; susceptible
to malaria.
• Heterozygous
phenotype: no
anemia; less
susceptible to malaria
Frequency of sickle cell allele.
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Maintenance of Variation
Disruptive selection: acts to eliminate intermediate
types.
Disruptive selection for large and small beaks in blackbellied seedcracker finch of west Africa.
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Maintenance of Variation
Directional selection: acts to eliminate one extreme from
an array of phenotypes
Directional selection for negative phototropism in
Drosophila.
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Maintenance of Variation
Stabilizing selection: acts to eliminate both extremes.
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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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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