1. CHAPTER 24
LECTURE
SLIDES
Prepared by
Brenda Leady
University of Toledo
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2. Genetic drift
 Changes allelic frequency due to random
chance
 Random events unrelated to fitness
 Favors either loss or fixation of an allele
 Frequency reaches 0% or 100%
 Faster in smaller populations
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1.0
N = 10
All BB
0.75
Frequency of B allele
N = 1,000
0.5
N = 10
0.25
0.0
0 10 20 30 40 50 All bb
Generations
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4. Bottleneck
 Population reduced dramatically and then
rebuilds
 Randomly eliminated members without
regard to genotype
 Surviving members may have allele
frequencies different from original
population
 Allele frequencies can drift substantially
when population is small
 New population likely to have less genetic
variation
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6. Founder effect
 Small group of individuals separates from
a larger population and establishes a new
colony
 Relatively small founding population
expected to have less genetic variation
than original population
 Allele frequencies in founding population
may differ markedly from original
population
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8. Neutral theory of evolution
 Non-Darwinian evolution
 Neutral variation
 Much of the variation seen in natural populations is
caused by genetic drift
 Does not preferentially select for any particular allele
 Most genetic variation is due to the accumulation
of neutral mutations that have attained high
frequencies due to genetic drift
 Neutral mutations do not affect the phenotype so
they are not acted upon by natural selection
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9.  Main idea is that much of the modern
variation in gene sequences is explained
by neutral variation rather than adaptive
variation
 Sequencing data supports this idea
 Nucleotide substitutions much more likely
in 3rd base of codon (usually doesn’t
change amino acid) than 1st or 2nd (usually
does change amino acid)
 Changing the amino acid is usually
harmful to the coded protein
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