Variation and Selection

• Continuous variation


  A gradual change in a characteristic across a population, e.g. human height, mass and shoe size
• Continuous variation


  • Represented as a histogram
  • Shows normal distribution, with most individuals around the average value and a few at the extremes
• Discontinuous variation


  A characteristic of any species with only a limited number of possible values, e.g. gender, eye colour, blood group
• Discontinuous variation


  • Represented as a bar chart
  • No intermediate values
• How variation occurs in a genetic form

  1. Independent assortment during meiosis increases variation

  2. Random nature of sexual reproduction gives rise to variation
• Mutations


  Random changes in the number of chromosomes or the structure of a gene
• Genetic variation exists because of changes to chromosomes or genes (DNA)
• Environmental variation


  Actual height reached will depend on the nutrition available
• Natural selection


  1. Variation among phenotypes

  2. Competition causes struggle for existence

  3. Best-adapted individuals survive (differential survival)

  4. Surviving phenotype reproduces

  5. Beneficial gene passed to next generation
• How antibiotic resistance helps natural selection

  1. Variation in bacterial phenotypes

  2. Antibiotics provide competition

  3. Resistant individuals survive (differential survival)

  4. Resistant bacteria reproduce

  5. Resistant bacteria increase, non-resistant decrease

  6. Resistance gene caused by beneficial mutation
• Evolution


  Continuing process of natural selection that leads to gradual change of an organism over time
• Fossils


  Remains of living organisms preserved in rock for millions of years, provide evidence of changes in organisms
• Reasons for extinction


  • Climate change
  • Natural disaster
  • Hunted by humans
  • Hunted by non-native animals
  • Disease
  • Loss of habitat
• Ways to prevent extinction

  • Legislation preventing hunting of endangered species
  • International agreements to limit climate change
  • Creating nature reserves to protect habitats
  • Education to encourage environmental protection
• Wheat breeding


  • Shorter stalk length less likely to suffer wind damage
  • Uniform size easier to harvest
  • Larger yield