Variation and Evolution

• Variation


  Differences in the characteristics of individuals in a population
• Causes of variation within a species

  • Genetics
  • Environment
  • A mixture of both
• Genetic variation


  Variations in the genotypes of organisms of the same species due to the presence of different alleles, creating differences in phenotypes
• What creates genetic variation in a species

  • Spontaneous mutations
  • Sexual reproduction
• Mutation


  A random change to the base sequence in DNA which results in genetic variants, occurring continuously
• Types of gene mutation

  • Insertion
  • Deletion
  • Substitution
• How a gene mutation may affect an organism's phenotype

  • Neutral mutation does not change amino acid sequence, no effect on phenotype
  • Mutation may cause minor change in phenotype e.g. eye colour
  • Mutation may completely change amino acid sequence, resulting in non-functional protein, severe changes to phenotype
• A new phenotype caused by a mutation being suited to an environmental change


  There will be a rapid change in the species
• Evolution


  A gradual change in the inherited traits within a population over time, occurring due to natural selection which may result in the formation of a new species
• Theory of natural selection

  1. Genetic variation exists due to spontaneous mutations

  2. Selection pressures (e.g. competition, disease) exist

  3. Random mutation gives an organism a selective advantage

  4. Organism is better adapted to the environment and survives

  5. Organism reproduces, passing on its beneficial alleles

  6. Frequency of advantageous alleles increase
• Speciation

  When the phenotypes of two populations become different to the extent that they can no longer interbreed to produce fertile offspring
• Selective breeding


  The process by which humans artificially select organisms with desirable characteristics and breed them to produce offspring with similar phenotypes
• Steps in selective breeding

  1. Identify a desired characteristic

  2. Select parent organisms with desired traits and breed them

  3. Select offspring with desired traits and breed them

  4. Repeat process until all offspring have desired traits
• Characteristics selected for in selective breeding

  • Disease resistance in crops
  • Higher milk or meat production in animals
  • Gentle nature in domestic dogs
  • Large flowers
• Advantage of selective breeding

  Creates organisms with desirable features e.g. higher crop yields, greater milk supply, larger fruit, domesticated animals
• Other uses of selective breeding


  • In medical research
  • In sports e.g. horse racing
• Disadvantages of selective breeding

  • Reduction in gene pool
  • Inbreeding results in genetic disorders
  • Development of physical problems e.g. respiratory issues
  • Potential to select harmful recessive alleles
• Genetic engineering


  The modification of the genome of an organism by the insertion of a desired gene from another organism, enabling the formation of an organism with beneficial characteristics
• Uses for genetically modified plants

  • Disease resistance
  • Produce larger fruits
• Use for genetically modified bacteria cells

  To produce human insulin to treat diabetes mellitus
• Benefits of genetic engineering

  • Increased crop yields for growing population
  • Useful in medicine e.g. insulin-producing bacteria, anti-thrombin in goat milk, possibility to overcome some inherited disorders
  • GM crops produce scarce resources e.g. GM golden rice produces beta-carotene
• Risks of genetic engineering

  • Long-term effects of consumption of GM crops unknown
  • Negative environmental impacts e.g. reduction in biodiversity, impact on food chain, contamination of non-GM crops
  • Late-onset health problems in GM animals
  • GM seeds are expensive, LEDCs may be unable to afford them or become dependent on businesses
• Genetically modified crops


  Crops that have had their genes modified
• Genetically modified (GM) crops


  Crops that have had their genes modified
• GM crops


  • GM golden rice produces beta-carotene (source of vitamin A in the body)
• Long-term effects of consumption of GM crops unknown
• Negative environmental impacts of GM crops e.g. reduction in biodiversity, impact on food chain, contamination of non-GM crops forming 'superweeds'
• Late-onset health problems in GM animals
• GM seeds are expensive. LEDCs may be unable to afford them or may become dependent on businesses that sell them
• Bacillus thuringiensis (Bt)


  Insect larvae are harmful to crops. Bt is a bacterium which secretes a toxin that kills insect larvae
• How genetic engineering is used to protect crops against insects

  1. The gene for toxin production in Bt can be isolated and inserted into the DNA of crops

  2. Bt crops now secrete the toxin which kills any insect larvae that feed on it
• Benefits of Bt crops

  • Increased crop yields (fewer crops damaged)
  • Lessens the need for artificial insecticides
  • Bt toxin is specific to certain insect larvae so is not harmful to other organisms that ingest it
• Risks of Bt crops

  • Long term effects of consumption of Bt crops unknown
  • Insect larvae may become resistant to the Bt toxin
  • Killing insect larvae reduces biodiversity
• Process of genetic engineering

  1. DNA is cut at specific base sequences by restriction enzymes to create sticky ends

  2. Vector DNA cut using the same restriction enzymes to create complementary sticky ends

  3. Ligase enzymes join the sticky ends of the DNA and vector DNA forming recombinant DNA

  4. Recombinant DNA mixed with and 'taken up' by target cells
• Vector


  A structure that delivers the desired gene into the recipient cell e.g. plasmids, viruses
• How plants can be cloned

  1. Taking plant cuttings

  2. Tissue culture
• Tissue culture


  Using small groups of cells from part of a plant to grow identical new plants
• How plants are grown using tissue culture

  1. Select a plant that shows desired characteristics

  2. Cut multiple small sample pieces from meristem tissue

  3. Grow in a petri dish containing growth medium

  4. Transfer to compost for further growth
• Ensure aseptic conditions to prevent contamination by microorganisms when preparing tissue cultures
• Growth medium


  Contains nutrients and growth hormones