Variation and Evolution

• Types of variation

  • Genetic
  • Environmental
• Genetic variation

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

  Differences in phenotype that are acquired during the lifespan of an organism due to environmental factors e.g. diet, lifestyle, climate, exposure to light etc.
• Some variation may be due to a combination of both genetics and the environment
• Types of variation

  • Continuous
  • Discontinuous
• Continuous variation


  Produces characteristics which do not fall into distinct categories, instead showing a continuous range e.g. height, weight
• Discontinuous variation


  Produces characteristics that fall into distinct categories e.g. gender, eye colour, blood group
• Causes of genetic variation

  • Sexual reproduction
  • 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 which produces genetically identical daughter cells known as clones
• Mutation


  A random change to the base sequence in DNA which results 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 changes to phenotype
• Inherited disease


  Mutations may result in the formation of harmful alleles which can be inherited by offspring
• Genotypes for cystic fibrosis

  • FF (healthy)
  • Ff (carrier)
  • ff (cystic fibrosis)
• The inheritance of heritable diseases such as cystic fibrosis can be investigated using family trees
• Gene therapy


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

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

  • Expensive
  • Religious objections to genetic manipulation
  • Health implications - new gene may have unintended side effects
• Natural selection


  The gradual change in the inherited traits within a population over time due to genetic variation, competition, survival of the fittest, and reproduction
• The theory of natural selection was proposed by Charles Darwin and Alfred Russel Wallace
• Darwin studied a variety of organisms whilst travelling on the HMS Beagle and proposed the idea of 'survival of the fittest'
• Reasons for extinction

  • Organisms are not adapted to their environment
  • Organisms have not adapted rapidly enough to changing environmental conditions
  • 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 predator)

  3. Record number of each colour remaining
• Limitations of modelling natural selection

  • Straws do not move (prey move)
  • Extreme difference in straw colours (prey more similar in colour)
  • Paper is one colour (environment is not one colour)
  • Potential bias by volunteer (no bias in reality)
  • Not affected by other factors (other factors affect survival in reality)
• Development of antibiotic resistance

  1. Genetic variation exists due to spontaneous mutations

  2. A mutation may give a bacterium antibiotic-resistance

  3. If an antibiotic is administered, the resistant bacterium survives

  4. Bacterium reproduces, passing on its resistant variant

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


  A scientific research project which successfully mapped the entire human genome
• Importance of the Human Genome Project

  • Enables understanding of how lifestyle factors interact with genes, allowing identification of predisposition to disease and potential preventions
  • Disease-causing alleles identified more rapidly and appropriate treatments prescribed earlier
  • Scientists can predict an individual's response to certain drugs, and develop new drugs tailored to specific alleles
• Scientists now aim to identify the function of every gene in the human genome
• Types of Reproduction


  • Sexual
  • Asexual
• Sexual reproduction


  • 2 Parents
  • Gametes are produced with half of the genetic information of both parents
  • A combination of these gametes at fertilisation produces new offspring
• Asexual reproduction


  • 1 Parent
  • Offspring are produced from 1 parent
  • No energy is wasted on gametes and finding a mate meaning a lot of offspring can be produced quickly
  • Offspring show no genetic variation and are clones of the parent
• Offspring from sexual reproduction are more likely to survive sudden environmental change
• Genetic variation

  Differences between members of the same species caused by genes inherited from parental DNA
• Environmental variation


  Differences between members of the same species caused by environmental factors
• Most variation is not caused by just one gene but by groups of genes or a combination of genetic and environmental factors
• Evolution


  Variations in species that can be inherited (are heritable) are the basis of evolution
• Theory of evolution by natural selection

  1. Mutation

  2. Variation

  3. Competition

  4. Survival of the fittest

  5. Breeding
• If environmental change is too quick for species to adapt by natural selection extinction may occur
• Types of variation

  • Continuous variation
  • Discontinuous variation