Population Genetics: Genetic Drift

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Population Genetics Genetic Drift
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What is drift?

• Results from the violation of the assumption of
  infinite population size
• It is equivalent to sampling error or choosing
  too small of a sample size
• In a small population, chance outcomes differ
  from theoretical expectations
• It does not result in adaptation, but does change
  allele frequencies

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Possible outcome when choosing the 10 mice each
generation
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As sample size increases, values meet expectations
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Founder effect

• Small founder populations probably have a
  different frequency of alleles
• This is a result of sampling error
• For example, if a continental population of
  lizards has 35 alleles at a single locus, then
  the probability0 that 15 lizards floating away
  from that population contains all of the alleles

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Founder effect example

• Sonya Clegg studied the effects of migration of
  Silvereyes from Tazmania to Islands surrounding
  New Zealand.
• Six Microsatellite loci were examined on each
  population

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Latest colonization
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Founder effects in humans

• Pingelapese people are descended from 20
  individuals following a typhoon and famine
• An individual carrying the LOF for the CNGB3
  (protein crucial for normal color vision)
• Normally, only 1/20,000 people are effected with
  Achromatopsia
• Of the 3,000 Pingelapese, 1/20 have this
  condition

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Fixation and loss of diversity are related to
sample size
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Loss of heterozygosity

• As alleles drift to fixation or loss,
  heterozygosity also declines
• If we start with Allele A at 73 and allele a at
  27, then each of them proceeds to fixation tat
  the probability equal to their allele frequency
• Hg1Hg1-1/2N. Heterozygosity is always being
  lost between 1/2 and 1
• If you had only 50 breeding pairs of animals in
  the world, and you bred them randomly, then you
  still see a loss of 1 heterozygosity per year

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Experimental Study on Loss of Heterozygosity

• Buri (1956) kept 107 populations of fruit flies
  for 19 generations.
• He only bred 8 males and 8 females from each
  generation.
• A particular allele began 0.5

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By the end of the experiment, 28 pop had become
fixed at 0. 30 Pop fixed at 1 Overall they
remained symmetric around 0.5
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Random fixation in natural populations

• Templeton (1990) studied collared lizards in
  Missouri
• This lizard normally inhabits desert land.
• In the past 8,000-4,000 years, Missouri has
  become wetter and isolated these desert habitats
  into glades
• Fire suppression is further isolating the glades
  and lizardsthey will not migrate through woods
  to other glades

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Most are fixed for one type of mtDNA haplotype
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Problems with loss of heterozygosity

• If a pathogen kills one lizard in a glade, then
  it could kill them all. Why? because they are
  all have identical genotypes
• Lizards would not be able to evolve a response as
  biological or physical environment changes
• In fact 2/3 of 130 glades in Missouri contained
  no lizards
• How to save them?
• Introduce new genetic lines of collared lizards
• Restore fire regimesthus increasing migration

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Genetic Diversity Increases with Population size
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Mutation vs Substitution
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Genetic Drift and Molecular Evolution

• Neutral Theory (Kimura) says that advantageous
  mutations are rare and that most genes are
  neutral. Therefore evolution generally occurs by
  drift
• Selectionists (Gillespie) advantageous mutations
  are common and that the rate of substitution
  occurs by selection

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Neutral Theory

• By the mid-60s, amino acid sequences for
  hemoglobin and cytochrome c were determined.
• It was found by Kimura that when comparing rate
  of substitution of AA for horses and humans
  (using fossils to calibrate) was extremely high.
  Too many changes if we expect that beneficial
  mutations are rare
• Zuckerland and Pauling also determined that the
  rate AA substitution was clock-like. Not what
  you would expect if selection should act rapidly
  during times of environmental change

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Kimuras Neutral Theory

• Beneficial mutations are largely inconsequential.
• Rate of molecular evolution is equal only to the
  rate of mutation
• Strangely, in spite of drift, Kimura says that
  population size does not matter in terms of
  fixation
• Positive natural selection is excluded, because
  the vast number of mutations are neutral

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Patterns in DNA sequence divergence

• Use pseudogenes as the paradigm of neutral
  evolution
• The divergence rates in pseudogenes should be
  equal to the neutral ratethe highest observed in
  genomes.

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Do the number synonymous substitutions support
the neutral theory?
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Selection should get rid of most non-synonymous
changesTherefore, drift is the only way that
neutral (non-syn) changes could be spread