Genetic Drift: 2nd Mechanism of Evolution

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Genetic Drift 2nd Mechanism of Evolution

• Alleles can change in a population as a result
  of directional selection

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Genetic Drift 2nd Mechanism of Evolution

• Alleles can change by chance in non-directional
  way
• ? random sampling of gametes during
• reproduction
• Simple examples e.g. Only half of females find
  males
  half of females fertile

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Random Genetic Drift

• When genotypes in a pop have the same number of
  offspring (i.e. identical fitness) selective
  neutrality
• Genotype AA Aa aa
• Fitness 1 1 1
• Then random sampling can cause relative
  frequency in population to change
• genetic drift / random drift

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Influence of Population Size

• Role of drift depends on population size
• Random sampling events more NB in small
  populations

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Influence of Population Size

• Founder effects establishment of new pop by few
  founders ? carry small fraction of total
  genetic variation of parental pop

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Influence of Population Size

• Population bottleneck size of pop ?
  dramatically

Bottleneck
Cheeta
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Fixed
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Fixed
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Now imagine 1000
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Random Genetic Drift Influence of Population
Size
e.g. Huntingtons disease
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Random Genetic Drift Influence of Population
Size

• Chance that new population has ? genetic
  variation?
• chance of homozygotes (p2)N (q2)N

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Random Drift produces march to Homozygosity?

• Over long term pure random drift causes pop to
  homozygosity at locus? No ? pop ? homozygous
  because new variation by mutation
• Heterozygosity balance between elimination by
  drift creation by mutation
• equilibrial heterozygosity (H) 4Nu / (4Nu
  1)
• where u neutral mutation rate
• Thus, heterozygosity is lower if low mutation
  rate small population

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Population Size Effective Population Size

• In ecological sense N can be measured by
  counting e.g. number of adults
• However, what matters is the chance that two
  copies of a gene will be sampled in next
  generation
• ? this is affected by the breeding structure of
  the pop
• Population geneticists use Ne can be affected
  by
• Sex ratio
• Population fluctuations
• Small breeding groups
• Variable fertility

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Neutral Drift vs Natural Selection
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Neutral Evolution
Motoo Kimura (1924 1994)
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Neutral Evolution

• There is finite probability that one of neutral
  alleles will be fixed other lost
• ? occurs regardless of initial frequency of
  allele
• ? rare alleles take longer to fixation than
  common ones
• Rate of this neutral evolution can be calculated
• ? is independent of population size
• ? neutral evolution rate neutral mutation rate

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Assumptions of Neutral Theory

• Favourable mutants are sufficiently rare so we
  can ignore them
• Unfavourable mutants are quickly removed by
  selection
• Hence, most segregating mutants are selectively
  neutral
• Likewise, most fixed differences between species
  are due to drift fixing neutral alleles