Evolution as Genetic Change

• Natural Selection in Polygenic Traits

  • Directional selection
  • Stabilizing selection
  • Disruptive selection
• Allelic frequency


  Frequency of a particular allele in a population
• Phenotypic frequency


  Frequency of a particular phenotype in a population
• Genetic equilibrium


  A state where allele frequencies remain constant
• Genetic drift


  1. Bottleneck effect

  2. Founder effect
• Mechanisms that can determine if a population is evolving or in genetic equilibrium

  • Non-random mating
  • Genetic drift
  • Migration (gene flow)
  • Mutations
  • Natural selection
• Evolution on a genetic level

  Natural selection on single-gene traits can lead to changes in allelic frequency and thus to evolution
• Plumage color in Turkeys


  • Organisms of one color might be more visible to predators and may produce fewer offspring
  • The allelic frequency for that allele will change
• Natural selection on polygenic traits

  • Can affect the distribution of phenotypes in three ways: directional, stabilizing, or disruptive selection
• Stabilizing selection


  The average individuals in the population have higher fitness and are favored
• Directional selection


  Individuals at one of the ends of the curve have higher fitness and are favored more than individuals in the middle or at the other end
• Disruptive selection


  Individuals at both extremes (upper and lower ends) of a trait are both more fit and are favored over the average individuals in the population
• Genetic drift


  • Occurs in small populations that are isolated
  • Individuals that carry a particular allele may leave more descendants than other individuals, just by chance
• Fixation


  The rise to 100% frequency of some alleles
• Bottleneck effect


  A change in the allelic frequency caused by a dramatic reduction in size of the population
• Founder effect


  A change in the allelic frequency caused by the migration of a small subgroup of a population
• Hardy-Weinberg principle


  Allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change
• Genetic equilibrium


  The situation in which allele frequencies remain constant
• Things that can cause evolution to occur

  • Non-random mating
  • Genetic drift
  • Migration (gene flow)
  • Mutations
  • Natural selection
• Hardy-Weinberg equation


  Used to determine allele frequency of a particular allele or genotype frequency of a particular genotype when information is missing
• The Hardy-Weinberg equations are p^2, 2pq, q^2 = 1