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Evolution of Populations
Lecture 2
Winter 2014
1
Darwin’s dilemma
• How do organisms
pass heritable
traits to their
offspring?
2
The Modern Synthesis
1844 - Darwin essay on Natural Selection
1859 – On the Origin of Species
1866 - Mendel published
1875 - Mitosis worked out
1890s - Meiosis worked out
Early 1900’s - Chromosomal Theory of
Inheritance
1953 – Structure of DNA discovered
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Non-adaptive Evolution
• Evolution:
– a genetically based change in the
characteristics of a population over time
– a change in the frequency of alleles in a
population over generations
• Natural Selection
Non-adaptive Evolution
• Genetic Drift
• Gene Flow
• Mutation
The Florida Manatee
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Non-adaptive Evolution
Genetic Drift
• A change in a population's allele frequencies due to
chance
• Significant in small populations
Fig. 23.9
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Non-adaptive Evolution
Genetic Drift: The Founder Effect
Non-adaptive Evolution
Genetic drift - The bottleneck effect
– Sudden severe reduction in population size
– Loss of alleles
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Non-adaptive Evolution: Genetic Drift
• Low mitochondrial DNA diversity
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Non-adaptive Evolution: Genetic Drift
• Loss of genetic
variation makes
population more
vulnerable
• Inbreeding
depression
• Increase frequency
of harmful alleles
Fig. 23.11
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Non-adaptive Evolution: Genetic Drift
Fig 23.11
Non-adaptive Evolution: Gene Flow
Gene Flow
• Transfer of alleles from one population to
another
• Reduces genetic differences between
populations
13
Non-adaptive Evolution: Gene Flow
Reproductively isolated populations
14
Non-adaptive Evolution: Gene Flow
Gene Flow: Increases genetic diversity
15
Gene flow as a conservation tool
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Non-adaptive Evolution
Mutation
• Random change in
the nucleotide
sequence of DNA
• Ultimate source of
new alleles
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Mutations & Variation
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Mutations & Variation
• Review types of mutations
(pgs. 344-346), alterations
of chromosome structure
(pgs. 298-300) and gene
regulation (Ch. 18)
Mechanisms of Microevolution
• Genetic drift, gene flow and mutations &
natural selection all lead to changes in
variation within a population
• Natural selection leads to adaptive
evolution
20
The Hardy Weinberg Principle
• How do we know whether a population is
evolving?
• Hardy-Weinberg equilibrium
– Calculates the frequency of genotypes you would
expect to find in a non-evolving population
– frequencies of alleles and genotypes in a population
will remain constant over generations,
• unless acted upon by agents other than Mendelian
segregation and recombination of alleles
21
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Conditions for Hardy-Weinberg
1.
2.
3.
4.
5.
Random mating
No natural selection
No genetic drift (large population size)
No gene flow
No mutation
The Hardy Weinberg Principle
• If the frequency of the alleles in a
population are given by p + q = 1,
• Then the frequency of genotypes will be
given by p2 + 2pq + q2 = 1,
p= frequency of one allele
q = frequency of other allele
• Hardy Weinberg Equation
p2 + 2pq + q2 = 1
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The Hardy Weinberg Principle
• Gene pool
– The total aggregate of genes in a population
at a any one time (All of the alleles at all loci)
• Population of wildflowers
– 500 individuals
• 320 reds
• 160 pinks
• 20 whites
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Fig 23.8