2.4 bio- variation and evolution

• Variation


  Differences in organisms of the same species
• Types of variation


  • Genetic
  • Environmental
• Genetic variation


  Differences in the genotypes of organisms due to different alleles, creating differences in phenotypes
• Environmental variation


  Differences in phenotype acquired during lifespan due to environmental factors
• Some variation may be due to a combination of genetics and environment
• Types of variation


  • Continuous
  • Discontinuous
• Continuous variation


  Characteristics that do not fall into distinct categories, showing a continuous range
• Discontinuous variation


  Characteristics that fall into distinct categories
• Genetic variation


  Can be due to sexual reproduction or spontaneous mutations
• Sexual reproduction


  1. Meiosis produces genetically different gametes

  2. Fertilisation mixes genetic information from both parents to form a unique individual
• Asexual reproduction does not create genetic variation as it involves mitosis producing genetically identical daughter cells
• Mutation


  A random change to the base sequence in DNA resulting in genetic variants
• Effects of gene mutations

  • Neutral mutation (no change to amino acid sequence)
  • Minor change to phenotype
  • Completely change amino acid sequence, resulting in non-functional protein and severe phenotypic changes
• Inherited disease


  Harmful alleles present in sex cells can be inherited by offspring
• Inherited disease


  • Cystic fibrosis
• Genotypes for cystic fibrosis

  • FF (healthy)
  • Ff (carrier)
  • ff (cystic fibrosis)
• Family trees can be used to investigate the inheritance of heritable diseases
• Gene therapy


  Techniques used to counteract the effects of a defective allele
• Methods of gene therapy

  • Insertion of a functional allele to replace the faulty allele
  • Turning off the faulty allele
• Ethical issues with gene therapy


  • Expensive
  • Religious objections to genetic manipulation
  • Health implications of new gene having unintended side effects
• Evolution


  The gradual change in the inherited traits within a population over time, due to natural selection
• Natural selection


  1. Genetic variation exists due to spontaneous mutations

  2. Competition between organisms for limited resources

  3. Beneficial mutations give organisms a selective advantage

  4. Better adapted organisms survive and reproduce, passing on advantageous alleles

  5. Frequency of advantageous alleles increases
• The theory of natural selection was proposed by Charles Darwin and Alfred Russel Wallace
• When a species becomes extinct, all members of the species have died
• Reasons for extinction include organisms not being adapted to their environment, not adapting rapidly enough to changing conditions, and being outcompeted by better adapted species
• Modelling natural selection


  1. Disperse coloured paper squares on a background

  2. Volunteer collects as many squares as possible in 30 seconds (representing predation)

  3. Record number of each colour remaining
• Limitations of the paper square model

  • Prey do not move
  • Extreme difference in straw colours
  • Background is one colour
  • Potential bias by volunteer
  • Not affected by other factors
• Development of antibiotic resistance

  1. Genetic variation exists due to spontaneous mutations

  2. Mutation gives bacterium antibiotic resistance

  3. Resistant bacteria survive when antibiotic is administered

  4. Resistant bacteria reproduce, passing on resistant variant

  5. Frequency of resistant allele increases
• Bacterial resistance to antibiotics is increasing due to overprescription and antibiotic misuse
• Human Genome Project


  Scientific research project that successfully mapped the entire human genome
• Importance of the Human Genome Project

  • Enables understanding of how lifestyle factors interact with genes, allowing identification of disease predisposition and potential preventions
  • Allows more rapid identification of disease-causing alleles and earlier prescription of appropriate treatments
  • Enables prediction of individual response to drugs, allowing development of tailored treatments
• Scientists now aim to identify the function of every gene in the human genome