mechanisms of evolution-ii
TRANSCRIPT
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Mechanisms of Evolution II
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• Overproduction & competition
• Phenotypic variation
• Some variation is heritable
• Differential survival & reproduction
*Leads to changes in allele frequencies over time
Natural Selection Reminder
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The Equilibrium Population:Assumptions of Hardy-Weinberg
• No mutation (no new genetic variation)
• No genetic drift (infinitely large population)
• No migration (no individuals entering or leaving the pop)
• No selection (genotypes have equal fitness)
• Random mating (dealing with a single population)
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Example of Hardy-Weinbergp2 +2pq + q2 = 1
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The Equilibrium Population:Assumptions of Hardy-Weinberg
• No mutation (no new genetic variation)
• No genetic drift (infinitely large population)
• No migration (no individuals entering or leaving the pop)
• No selection (genotypes have equal fitness)
• Random mating (dealing with a single population)
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Recall: DNA replicates at every cell division => Opportunity for error and repair (mutation)
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Recall: DNA replicates at every cell division => Opportunity for error and repair (mutation)
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Recall: DNA replicates at every cell division => Opportunity for error & repair (mutation)
mismatchmust berepaired
if C =>T,a mutation
if A => G,no mutation
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Mutation• alterations of the base DNA sequence
• ultimate source of all genetic variation• many types of mutation (e.g. point, chromosomal)• often reduce fitness (deleterious), but can be beneficial or neutral too
• weak force in changing allele frequencies over time (evolution)
• many genes per genome (~30,000 in humans), so odds of hitting any particular one is very low per
generation
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The genetic code is degenerate => many mutations are silent
GUUGUCGUAGUG
Valine, so 3rd position mutations don’t affect protein sequence
Recall: DNA => mRNA => protein => => => phenotype
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If mutation changes protein sequence, it may affect phenotype (e.g. Sickle cell anemia)
Recall: DNA => mRNA => protein => => => phenotype
single amino acid change
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The Equilibrium Population:Assumptions of Hardy-Weinberg
• No mutation (no new genetic variation)
• No genetic drift (infinitely large population)
• No migration (no individuals entering or leaving the pop)
• No selection (genotypes have equal fitness)
• Random mating (dealing with a single population)
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Genetic drift: an extreme example• Imagine a population of only 1 M and 1 F per generation (N = 2)
• Start with 2 heterozygotes (Aa), p & q = 0.5
• Simulate allele formation & random mating with coin flip
1.0
.50
.75
.25
alle
le fr
eq.
0 1 2 3 4 5 6 7 8 9 10
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Genetic drift
• change in allele frequency between generations due to the random sampling of alleles
• large population, allele and genotype frequencies predictable
• smaller populations, random chance (drift) becomes important
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Genetic drift...1. Changes allele frequencies of populations
2. Reduces genetic variation of populations
3. Is random: same starting point => different outcomes
4. Depends on population size (small pops have strong drift)
N = 4 N = 40 N = 400
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Bottlenecks & founder effects
Bottleneck = drift due to a drastic reduction in population sizeFounder effect = bottleneck associated with the founding of a
new population
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The Equilibrium Population:Assumptions of Hardy-Weinberg
• No mutation (no new genetic variation)
• No genetic drift (infinitely large population)
• No migration (no individuals entering or leaving the pop)
• No selection (genotypes have equal fitness)
• Random mating (dealing with a single population)
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Migration = movement of individuals between populations
Gene flow = transfer of alleles from one population to another
Connectivity
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AA
aaAAAA AA
AAAA aaAAAAAA
aaaaAA
aa
AAaa
Freq A = p = 1Freq a = q = 0
Freq A = p = 0Freq a = q = 1
Gene flow can be a strong evolutionary force
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Gene flow can be a strong evolutionary force
One migration event => deviation from H-W expectations
AAaaAAAA AA
AAAAaa
AAAAAA
aaaaAA aa
AAaa
Freq A = p = 0.82Freq a = q = 0.18Would predict 2pq = 0.30Actual frequ of Aa = 0
Freq A = p = 0.33Freq a = q = 0.67Would predict 2pq = 0.44Actual frequ of Aa = 0
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AAAaAAAA Aa
AAAAAAAa
Aaaa
aa
Ongoing gene flow prevents population divergence & (eventually) homogenizes allele frequencies
AaaaAAaa
AAAa
Gene flow can be a strong evolutionary force
Freq A = p = 0.67Freq a = q = 0.33Would predict 2pq = 0.44Actual frequ of Aa = 0.33
Freq A = p = 0.5Freq a = q = 0.5Would predict 2pq = 0.5Actual frequ of Aa = 0.33
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The Equilibrium Population:Assumptions of Hardy-Weinberg
• No mutation (no new genetic variation)
• No genetic drift (infinitely large population)
• No migration (no individuals entering or leaving the pop)
• No selection (genotypes have equal fitness)
• Random mating (dealing with a single population)
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Natural selection
beneficial mutation
(should increase in frequency)
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Natural selection happensMany pathogens evolve resistance to antibiotics
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Many pests evolve resistance to pesticidesNatural selection happens
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Adaptation/diversification in higher eukaryotes - slower, but still going on
before selection
after selection(1977 drought)
mediumground
finch
Natural selection happens
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Patterns of phenotypic selectionDirectional selection
trait value
frequ
ency
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Stabilizing selection
trait value
frequ
ency
Patterns of phenotypic selection
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Diversifying selection
trait value
frequ
ency
Patterns of phenotypic selection
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Frequency-dependent selection
Morphs of a single Heliconius speciesNon-poisonous mimics of poisonous butterflies
=> each has higher fitness when rare
Patterns of phenotypic selection
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Patterns of phenotypic selectionHeterozygous Advantage (Overdominance)