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• 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
Genetic Variation and Evolution
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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
Natural selection: mechanism of evolutionary change
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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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• 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
Gene Variation in Nature
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Godfrey H. Hardy: English mathematicianWilhelm 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
Hardy-Weinberg Principle
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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
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
Hardy-Weinberg Principle
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Using Hardy-Weinberg equation to predict frequencies in subsequent generations
Hardy-Weinberg Principle
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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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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 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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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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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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• 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
Interactions Among Evolutionary Forces
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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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• 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
Maintenance of Variation
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• Fitness of a phenotype does not depend on its frequency
• Environmental changes lead to oscillation in selection
Maintenance of Variation
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• 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
Maintenance of Variation
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• Homozygous dominant phenotype: no anemia; susceptible to malaria
• Heterozygous phenotype: no anemia; less susceptible to malaria
Maintenance of Variation
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Disruptive selection for large and small beaks in black-bellied seedcracker finch of
west Africa
Maintenance of Variation
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Directional selection: acts to eliminate one extreme from an array of phenotypes
Maintenance of Variation
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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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• 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.
Experimental Studies
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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
Experimental Studies
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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
Experimental Studies
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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