(18) variation and selection

• Variation


  Differences between individuals of same species
• Continuous variation


  • Results in a range of phenotypes between two extremes
  • Caused by genes and environment
  • Examples: body length and mass
• Discontinuous variation


  • Results in a limited number of phenotypes with no intermediates
  • Caused by genes
  • Examples: ABO blood groups, seed shape and colour in peas
• Mutation


  • Genetic change, new alleles are formed
  • Caused by ionising radiation and some chemicals
• Gene mutation


  Random change in base sequence of DNA
• Genetic variations


  • Mutation
  • Meiosis
  • Random mating
  • Random fertilisation
• Adaptive feature


  • Inherited feature that helps organism survive and reproduce in its environment
• Hydrophytes


  • Wide and flat leaves, increase surface area, float on water surface, absorb more sunlight for photosynthesis
  • Small roots, can absorb water directly through roots and stems
  • Stomata found on upper side, allow gas exchange
• Xerophytes


  • Thick waxy cuticle, prevent water loss
  • Sunken stomata, reduce water loss
  • Small leaves, reduce surface area for evaporation
  • Extensive roots, maximise water uptake
  • Thick stems and leaves, to store water
• Natural selection


  1. Genetic variation within populations

  2. Production of many offspring

  3. Struggle for survival, competition for resources

  4. Greater chance of reproduction by better adapted individuals

  5. These individuals pass on their alleles to next generation
• Selective breeding


  1. Selection by humans of individuals with desirable features

  2. Crossing these individuals to produce the next generation

  3. Selection of offspring showing the desirable features
• Selective breeding by artificial selection is carried out over many generations to improve crop plants and domesticated animals
• Adaptation


  Process resulting from natural selection, populations become more suited to their environment over many generations
• Development of antibiotic resistant bacteria

  1. By chance, some bacteria may have genetic mutation that makes them resistant to antibiotics

  2. Resistant bacteria survive and reproduce

  3. Pass resistant allele to offspring

  4. Increased amount of resistant allele

  5. This is natural selection