Evolution

• Variation


  Differences between organisms of the same species
• Causes of variation


  • Genetic (sexual reproduction, mutations)
  • Environmental (environment, lifestyle)
  • Combination of both
• Continuous variation


  Gradual change in a feature with no clearly distinct groups - no clear boundaries
• Discontinuous variation


  Individuals can be placed into distinct groups easily, with no overlap
• Discontinuous variation is usually genetic, continuous variation is often both genetic and environmental
• Natural selection


  • All living organisms are adapted for living in their normal environment
  • Some are better adapted than others due to variation in phenotypes
  • The better adapted individuals are more likely to survive and pass their genes on to the next generation
• Genetic and environmental factors are the causes of variation. Continuous and discontinuous are the types of variation.
• In exam questions, continuous variation is often represented by a histogram and discontinuous variation by a bar chart.
• Natural selection


  • Differences between phenotypes
  • Differential survival
  • Differential reproductive success
• Antibiotic resistance in bacteria

  • Resistant phenotypes survive, non-resistant are killed
  • Resistant bacteria pass on resistant genes
• Antibiotics do not cause bacteria to become resistant, they create conditions where resistant bacteria are better adapted
• Evolution


  The process of natural selection that leads to gradual changes in organisms over time, which may include the formation of new species
• Evolution is a continuing process - natural selection is always happening and all species change very gradually over a long time period
• Some reasons why not everyone accepts the theory of evolution include that it contradicts some religious beliefs and the very long timescales involved mean it is difficult to see evolution actually happening
• Extinction


  A species is extinct if there are no living members of that species left
• Many organisms are endangered (at risk of extinction) due to climate change, hunting by humans, habitat destruction and many other reasons
• Selective breeding


  The selection and subsequent breeding of organisms chosen by humans for the desirable properties
• Selective breeding in wheat

  1. Select plants with short stalks and large heads of grain

  2. Cross-pollinate selected plants

  3. Repeat selection and cross-pollination over many generations

  4. Eventually all plants have the desired characteristics
• Selective breeding normally takes many generations (breeding cycles) and a long time to reach the stage where all the plants have the desired characteristics
• Inbreeding in selectively bred organisms can lead to genetic problems, e.g. some dog breeds are prone to health issues due to inbreeding