6.2 Variation and Evolution

• What is variation?


  Differences between individuals of the same species
• What is phenotypic variation?


  Differences in features between individuals of the same species
• Give 2 ways phenotypic variation can be caused

  • Genetic - controlled entirely by genes
  • Environmental - caused entirely by environment
• Where do all genetic variants arise from?

  Mutations (random genetic changes that occur continuously)
• What effect do mutations have on phenotypes?


  • Most mutations have no effect on the phenotype as the protein that a mutated gene produces may work just as well as the protein from the non-mutated gene
  • Occasionally, the new allele gives the individual a survival advantage over other members of the species
• Explain the mutation of darker colours in moths

  1. A moth develops a mutation, leading to a change in colour
  2. This makes it blend in better with the tree bark it lives on, and less visible to predators
  3. The moth has a survival advantage and breeds more frequently, increasing its chances of passing on the mutated phenotype to the generation
  4. The new colour quickly spreads through the species
• What is evolution?


  A a change in the inherited characteristics of a population over time through a process of natural selection which may result in the formation of a new species
• Explain the process of natural selection

  1. Individuals in a species show a range of variation caused by differences in genes
  2. Organisms reproduce to produce more offspring, leading to competition for food and other resources ('struggle for survival')
  3. Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce
  4. Therefore the alleles resulting in these characteristics are passed to their offspring and more of the species with this characteristics become common
• Outline the main stages in natural selection

  1. Variation and chance mutation
  2. Survival advantage
  3. Live longer
  4. Reproduce and breed more
  5. Alleles are passed on
  6. Repeated over generations, so the phenotype will become more common
• When are two new species formed?

  When two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring
• What is selective breeding (artificial selection)?

  Process by which humans breed plants and animals for particular genetic characteristics

  • Humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals
• Explain how selective breeding is done

  1. Breed individuals with the desired characteristics together
  2. Offspring that show the desired characteristics are selected and bred together
  3. This continues over many generations until all the offspring show the desired characteristic
• Give 4 examples of characteristics chosen for selective breeding

  • Disease resistance in food crops
  • Animals which produce more meat or milk
  • Domestic dogs with a gentle nature
  • Large or unusual flowers
• What are some problems with selective breeding?

  • Selective breeding can lead to inbreeding
  • This results in a reduced gene pool - a reduction in the number of alleles in a population
  • Organisms inheriting harmful genetic defects
  • Organisms being vulnerable to new diseases
• What is genetic engineering?


  A process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic
• What are genetically modified plants?

  Plants that have had foreign DNA inserted into their genome, done to improve food production in some way
• Give 3 ways plants have been genetically modified

  • Tomatoes have been genetically modified to make them grow larger fruit
  • Wild rice has been genetically modified to produce beta carotene (needed by humans to make vitamin A)
  • Crop plants have been genetically modified to be resistant to diseases or resistant to certain herbicides
• Describe the process of genetic engineering

  1. Restriction enzymes are used to isolate or 'cut out' the required DNA, leaving sticky ends
  2. Vector DNA (bacterial plasmid or virus) is cut using the same restriction enzyme to create complementary sticky ends. The gene is transferred to the vector.
  3. Ligase enzymes join the sticky ends of DNA and vector DNA forming recombinant DNA
  4. The vector is used to insert the DNA into the required cells
  5. Genes are transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with desired characteristics
• Describe 3 benefits of genetic engineering

  • Increased crop yields for growing population (e.g. disease-resistance)
  • Useful in medicine (e.g. insulin-producing bacteria)
  • GM crops produce scarce resources (e.g. GM golden rice produces beta-carotene)
• Describe the risks of genetic engineering

  • Long-term effects of consumption of GM crops are unknown
  • Increased cost of seeds to cover cost of development
  • Some research has show that plants with genes inserted into them do not grow as well as non-GM plants
• How can gene therapy be used to cure diseases?

  As inherited genetic diseases are caused by faulty genes, it is possible to treat these by inserting working versions of these genes into people with the genetic disease
• What is tissue culture and what is it used for?

  Using small groups of cells from part of a plant to grow identical new plants

  • Important for preserving rare plant species or commerically in nurseries
• What are the advantages of tissue cultures?

  • Allows a variety of plant with desirable characteristics to be produced:
  • Cheaply
  • With a greater yield
  • Quickly
  • At any time of year
  • Tissue culture can also preserve rare plant species
• What is cuttings?


  • An older but simple method used by gardeners to produce many identical new plants from a parent plant
  • Gardeners take cuttings from good parent plants and plant them to grow into genetically identical versions of the original plant
• What are the advantages of cuttings?

  Plants can be produced cheaply and quickly
• Explain how a tissue culture is made

  1. Divide the desired plant into hundreds of tiny pieces, each containing a small number of cells
  2. The cells are transferred to be incubated in a sterile petri dish containing nutrient agar
  3. Hormones (e.g. auxins) are added to encourage plants to grow into small masses of tissue
  4. Tissue continues to grow and forms plantlets that can be transferred to individual potting trays and develop into plants
• What is embryo cloning?

  Splitting apart cells from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers
• Explain the process of embryo cloning with cows

  1. Egg cells from the best cow are artificially fertilised using sperm cells taken from the best bull to develop into an early stage embryo
  2. The developing animal embryo is then split apart many times before the cells of the embryo become specialised - this forms many separate genetically identical embryos
  3. These cloned embryos are then transplanted into host mothers.
  4. The caves born from the host mothers are all genetically identical
• Explain the process of adult cell cloning

  1. The nucleus is removed from an unfertilised egg cell
  2. The 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. These embryo cells contain the same genetic information as the adult skin cell
  5. When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue development
• What are the benefits of cloning

  • Could be used to help preserve endangered species of plants and animals
  • Makes it possible to quickly and cheaply produce commercial quantities of consistently high quality plants at any time of the year
  • Allows farmers to ensure consistently high quality livestock
• What are the risks of cloning?

  • Can result in a reduced gene pool - fewer different alleles - smaller chance of having resistance to new diseases
  • Some evidence that cloned animals may not be as healthy as normal ones
  • Worries of humans being cloned in the future - deeply unethical
• The scientists want to produce a type of cattle that makes large volumes of low-fat milk. Describe how they would do this.

  1. Find females with lowest fat in milk and highest milk yield
  2. Find males whose female offspring have highest milk yield and lowest fat in milk
  3. Cross the best female with the best male
  4. Select the best offspring from each generation and repeat for several generations
• Explain how the structure of enzyme means that it is only able to cut the gene for Gene 1 and not Gene 2


  • The enzyme has a specifically-shaped active site
  • The 2 genes have different bases
  • Only Gene 1 fits into the active site of enzyme
• Explain why all bacteria in a clone are able to produce the same hormone

  • Clone produced by asexual reproduction / mitosis
  • All DNA are copied
  • Every cell receives a copy to form genetically-identical cells
• Give 3 reasons for selectively breeding animals

  • So they do not have specific genetic defects
  • To produce docile animals / so they're not aggressive
  • For aesthetic reasons
• Name and explain the conditions used for a tissue culture

  • Several groups of cells are scraped off the leaf
  • So many / several plants can be produced
  • Nutrients are added to the agar jelly
  • For making protein / amino acids
  • Make chlorophyll
  • Hormones are added to agar jelly
  • To stimulate cell division and differentiation
  • Plants are kept in sterile conditions
  • To prevent growth of microorganisms
  • Plants are kept at 20C
  • Optimum growth as enzymes work well