Variation & Evolution

• Reasons for Extinction

  • new disease / pathogen
  • flooding
  • competition for food
  • new predator kills them all
  • destruction of habitat
  • catastrophic event ; earthquake, volcanic eruption
• Define variation

  • Differences in the characteristics of individuals in a population
• Genetic causes leading to variation : 


  • the genes they have inherited
• Environmental causes leading to variation 


  • the conditions in which they have developed
• Genetic + Environmental causes to variation is known as 


  • Combination of genes + environment
• Example of characteristics inherited solely by genes

  • Eye colour
  • Blood group
  • Inherited disorders
• Environmental variation examples


  • Different accents + languages
  • Accidents = scarring, injuries
  • Plant grow more healthy in light rather than in the dark
• A combination of genes + environment examples 


  Height

  • How tall a human can be due to genes, inherited from its parents. If poor diet + lack of calcium, struggle to grow full capacity - shorter
  • Body weight
  • Skin colour
• What do all variants arise from?

  • Mutations
  • Most mutations have no effect on phenotype; some influence phenotype; very few determine phenotype
• Define evolution 


  • a change in the inherited characteristics of a population over time through a process of natural selection
• What may evolution lead to?

  • Formation of a new species
• What does the theory of evolution by natural selection state?

  • all species of living things have evolved from simple life forms that first developed more than three billion years ago
• Explain the process of natural selection

  • mutation occurs
  • variation between organisms within a species
  • those most suited to the environment /fittest survive reproduce
  • genes / favourable alleles passed on to offspring over the next generation
• Variants give rise to P..

  • Phenotypes best suited to the environment
• What happens when two populations of one species become so different in phenotype?


  • No longer interbreed to produce fertile offspring
  • forming two new species
• Speciation


  • formation of new species
• How can speciation occur?


  • Geographical isolation / physical barrier
  • Natural selection
• Describe the process of speciation (6 marks)

  • Isolation of different populations e.g by physical barrier : river and a mountain range.
  • habitat variations between these different populations.
  • mutation in each population
  • those better adapted to survive, reproduce + pass on favourable alleles to offspring
  • eventually cannot produce fertile offspring with other populations
• Alternative term for selective breeding

  • Artifical selection
• Describe the process of selective breeding

  • Humans breed plants + animals for particular genetic characteristics.
  • Done for thousands of years since they first bred food crops from wild plants + domesticated animals.
• What does selective breeding particularly involve?

  • Choosing parents with the desired characteristic from a mixed population.
  • They are bred together.
  • From the offspring, those with the desired characteristic are selected and bred together.
  • Continues over many generations until all offspring show the desired characteristic (in phenotype)
• Examples of where humans use selective breeding


  • Disease resistance in food crops
  • Animals which produce more meat + milk
  • Domestic dogs with gentle nature
  • Large or unusual flowers for aesthetic purposes
• Risks of selective breeding : Inbreeding

  • Lead to inbreeding
  • Some breeds particularly prone to disease / inherited defects
• Disadvantages of selective breeding

  • decreases gene pool; reduces genetic variability amongst population
  • increased chance of inbreeding where some breeds are are prone to disease / inherited genetic defects which could kill whole populations
  • undesirable traits from both parents may appear in offspring
  • Animal welfare concern
• Describe the process of Genetic Engineering

  • Involves modifying the genome of an organism by
  • introducing a gene from another organism to give a desired characteristic.
• How have plant crops been genetically engineered?

  • Disease-resistance
  • Produce bigger better fruits
• How have bacterial cells been genetically modified?

  • Produce useful substances
  • E.g human insulin to treat type 1 diabetes
• Main steps of genetic engineering

  • Enzymes used to isolate the required gene; this gene is inserted into a vector, usually a bacterial plasmid / virus.
  • The vector is used to insert the gene into the required cells.
  • Genes are transferred to the cells of animals, plants or microorganisms at an early stage
• Why are genes transferred at early stage?

  • receive all genes
  • develop all desired characteristics
• Brief description of genetic engineering


  • Genes from the chromosomes of humans + other organisms
  • 'cut out' + transferred to cells of other organisms
• What are genetically modified (GM) crops?

  Product of genetic engineering :

  • usually resistant to insect attack / herbicides
  • generally show increased crop yield
• Concerns about GM crops

  • Effect on populations of wild flowers + insects
  • Effects of eating GM crops not fully explored
  • Reduce genetic variability in ecosystem
  •  genes transferred to wild plants by pollinators. E.g gene for herbicide resistance transferred to weeds, herbicides no longer be effective.
  • GM seeds can be expensive so smaller farms cannot afford
• Use of modern medical research for GM crops

  • Exploring the possibility of genetic modification to overcome inherited disorder - GENE THERAPY
• 4 Types of cloning

  1. Tissue culture
  2. Cuttings
  3. Embryo transplants
  4. Adult cell cloning
• Cuttings


  • Older, simple method
  • Used by gardeners
  • Produce many identical new plants from a parent plant
• Embryo transplants


  • Splitting apart cells from developing animal embryo before they become specialised
  • transplanting identical embryos into host mother uterus
• Process of adult cell cloning

  1. Nucleus is removed from unfertilised egg cell
  2. Nucleus from an adult body cell, e.g skin cell, is inserted into the egg cell.
  3. An electric shock stimulates the egg cell to divide to form an embryo.
  4. Embryo cells contain the same genetic information as the adult skin cell.
  5. When embryo has developed into ball cells, inserted into the uterus of an adult female to continue its development .
• Tissue culture


  • using small groups of cells from part of a plant to grow identical new plants, provided with nutrients and auxins.
  • this is important for preserving rare plant species or commercially in nurseries.
• What did Charles Darwin do and discover?

  • Proposed the theory of evolution by natural selection
• How did Charles Darwin propose the theory of evolution?


  • As a result of observations on a round the world expedition
  • Backed by years of experimentation and discussion
  • Linking to developing knowledge of geology and fossils