Population Genetics

1

• Chapter 23
• Population Genetics

2

• Darwin provided the first convincing case for
  evolution
• On the Origin of Species by Means of Natural
  Selection provided a conceptual framework to
  explain the unity and diversity of life
• Darwin made two important points
• species evolve from ancestral species
• natural selection is the mechanism that could
  cause evolutionary change

3

• Evolution
• descent with modification
• genetic change over time
• Natural Selection
• differential reproductive success
• different phenotypes result from interaction of
  organisms with environment
• Natural selection causes changes in relative
  frequencies of alleles gene pool

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• Mendel's work provided the underlying mechanism
  for variation
• populations are the basic units of study for
  evolutionary biology
• Microevolution outlines changes in the genetic
  makeup of a population from one generation to the
  next
• Population genetics
• Population

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• Fisher, Haldane, Wright
• developed almost all of the mathematical
  foundations of evolution
• Allele frequencies are directly related to
  genotype frequencies
• To calculate the allele frequencies at a
  particular loci, you must know
• mode of inheritance
• number of alleles involved

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• Hardy and Weinberg
• developed a mathematical model for estimating
  allele frequencies
• single locus, two allele system that demonstrates
  dominant/recessive inheritance
• (pq) X (pq) p2 2pq q2
• male female next generation
• Hardy-Weinberg equilibrium p2 2pq q2 1
• Assumptions of HW (an ideal population)
• large population size
• no gene flow between populations
• no mutation
• no natural selection
• random mating

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• Allele frequencies will remain constant from one
  generation to the next
• non-evolving population
• said to be at Hardy-Weinberg Equilibrium
• Factors that alter allele frequencies
• mutation
• migration
• natural selection 

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• 4. genetic drift
• bottleneck effect
• founder effect
• 5. Non-random mating
• inbreeding
• assortative mating

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• Population Evolution

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• Genetic variation within a population is
  generated by mutations and recombination
• Adaptive Evolution - chance events produce
  variation and natural selection favors some forms
  more than others
• Most species exhibit geographic variation
• Differences between gene pools of separate
  populations
• Natural selection contributes to this phenomenon

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• What preserves genetic variation?
• Diploidy
• Balanced Polymorphism
• Heterozygote advantage
• Neutral Variation

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• Effect of selection
• 1. Stabilizing selection
• 2. Directional selection
• 3. Diversifying selection
• Sexual selection
• May oppose natural selection.
• Sexual dimorphism
• Relative fitness

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• Natural Selection can not fashion perfect
  organisms
• 1. historical constraints
• 2. adaptations are usually compromises
• 3. not all evolution is adaptive
• 4. selection can only edit existing variation