Lec 8

• Hardy-Weinberg Equilibrium


  When conditions are not met
• Hardy-Weinberg (HW) Equilibrium is when in a large random mating population and in the absence of selection, mutation and migration, gene and genotypic frequencies are constant from generation to generation
• Population in HW Equilibrium

  Has constant gene and genotypic frequencies
• Conditions for HW Equilibrium

  • No mutation
  • No migration
  • Random mating
  • Large population size
  • No selection
• Hardy-Weinberg Equilibrium fails to apply when the four forces that change gene frequency occur: mutation, migration, selection, genetic drift
• Mutation


  Alterations of genetic material that occur naturally as a result of occasional errors in DNA replication during cell division
• Mutation must appear in the phenotype of individuals for natural selection to act on
• Effect of Mutation on Gene Frequency

  Dp = -up + v(1-p)
• Migration


  Individuals from one population mating with immigrants from another population, introducing new genes into the population
• Migration has the effect of increasing genetic variation
• Effect of Migration on Gene Frequency

  Dp = m (pm - po)
• Natural Selection


  A force of nature where the environment prefers individuals who are better adapted than the ones who are poorer adapted
• Natural Selection is a powerful mechanism in the evolutionary process
• Selection against one of the homozygotes

  A kind of directional selection selecting against a homozygous recessive genotype
• Selection against one of the homozygotes

  • Albinism
  • Juvenile diabetes
• Selection against both homozygotes

  A kind of stabilizing selection selecting against AA & aa, leaving Aa
• Selection against heterozygotes and one of the homozygotes

  Selecting against Aa & aa, discouraging Aa to reproduce, reducing gene frequency (a) in the offspring
• Artificial Selection

  Selection imposed by man when selecting certain animals to become parents to produce the next generation's offspring
• Continued selection for certain genes would lead to fixation of the genes in the populations or lines, increased homozygosity
• Genetic Drift


  Change in gene frequency due to chance, especially in small isolated populations