Variation and evolution

• Continuous variation


  Variation that cannot be categorised, produces a continuous range (e.g. height, weight)
• Discontinuous variation


  Variation that can be categorised into distinct groups e.g. eye colour, blood group
• Types of variation


  • Continuous
  • Discontinuous
• Genetic variation


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


  Variations in phenotype that are acquired during the lifespan of an organism, due to environmental factors e.g. diet, lifestyle, climate, exposure to light etc.
• Genes and environment


  An individual's genes may make them predisposed to being tall, however lack of nourishment during childhood may stunt growth
• Causes of variation within a species

  • Genetics
  • Environment
• Sexual reproduction


  Meiosis produces genetically different gametes, during fertilisation one gamete from each parent fuses to form a zygote with genetic information from both parents
• Asexual reproduction does not create genetic variation
• Mutation


  A random change to the base sequence in DNA which results in genetic variants
• How a gene mutation may affect an organism's phenotype

  • Neutral mutation does not change the sequence of amino acids, no effect on phenotype
  • Mutation may cause a minor change in an organism's phenotype e.g. change in eye colour
  • Mutation may completely change the sequence of amino acids, resulting in a non-functional protein, severe changes to phenotype
• Mutagens


  Substances that increase the mutation rate of DNA, e.g. ionising radiation
• Mutation results in a harmful allele


  Allele inherited by offspring
• Ways to illustrate inheritance

  • Punnett square
  • Family tree
• Cystic fibrosis


  A recessive condition resulting in the production of sticky mucus that affects the lungs and digestive system
• Genotype of individuals with cystic fibrosis

  Homozygous recessive (ff)
• Genotype of carriers of cystic fibrosis

  Heterozygous (Ff)
• Inheritance of cystic fibrosis

  Female homozygous recessive (ff) + male heterozygous (Ff)
• Gene therapy


  A number of techniques used to counteract the effects of a defective allele within DNA
• Types of gene therapy


  • Insertion of a functional allele into DNA which replaces the faulty allele, a healthy dominant allele can counteract the recessive faulty allele
  • 'Switching off' the faulty allele
• Ethical issues surrounding gene therapy

  • Expensive - money could be better invested elsewhere
  • Religious groups do not agree with genetic manipulation
  • Health implications - new gene may produce an immune response
• Evolution


  A gradual change in the inherited traits within a population over time
• Evolution


  • Occurs due to natural selection
• Theory of natural selection

  1. Genetic variation exists due to spontaneous mutations

  2. Competition between organisms

  3. A mutation may give 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
• Competition between organisms


  The resources within a habitat required for survival are limited
• Competition between animals


  • Competition for food, shelter, mates etc.
• Competition between plants


  • Competition for light, water, minerals etc.
• Role of Darwin


  • Studied a variety of organisms whilst travelling on the HMS beagle
  • Noted that traits can be passed from parents to offspring
  • Proposed the idea of 'survival of the fittest'
  • Established the theory of natural selection and published his ideas in 'On the Origin of Species'
• Role of Wallace


  • Proposed a theory of natural selection that was similar to Darwin's, although the mechanisms were different
  • Gathered greater evidence (e.g. studying warning colouration in butterflies) to support the theory
• Modelling the effect of camouflage on predator-prey populations

  1. Take a piece of blue paper

  2. Disperse an equal number of blue and white straws across the paper (straws represent prey populations)

  3. Set a stopwatch for 30s and instruct a volunteer to collect as many straws as possible (volunteer represents predator)

  4. Record the number of blue and white straws remaining

  5. Repeat three times
• Limitations of the camouflage model
• Extinct


  All members of the species have died
• Reasons why species may become extinct

  • Organisms not adapted to their environment
  • Organisms have not adapted rapidly enough to changing environmental conditions
  • Outcompeted by better adapted species
• How antibiotic resistance in bacteria illustrates the process of evolution

  1. Genetic variation exists due to spontaneous mutations

  2. A mutation may give a bacterium antibiotic-resistance

  3. If an antibiotic is administered, the bacterium is better adapted and survives, whilst other bacteria are killed

  4. Bacterium reproduces, passing on its resistant variant

  5. Frequency of antibiotic-resistant allele increases
• Why antibiotic resistance in bacteria is a good study for evolution

  Bacteria reproduce very rapidly, allowing the first-hand observation of evolution
• Why bacteria are becoming increasingly resistant to antibiotics

  Due to overprescription and antibiotic misuse e.g. not completing the entire course
• How to reduce the spread of antibiotic-resistant bacteria

  • Prescribe antibiotics only when necessary
  • Ensure patients complete their antibiotic courses
  • Reduce the use of antibiotics in farming
  • Improve hygiene in hospitals
• Genome


  The entire genetic material of an organism
• Human Genome Project


  • Scientific research project involving thousands of scientists across the globe which successfully mapped the entire human genome
  • Scientists now aim to identify the function of every gene in the human genome
• How the results of the Human Genome Project can be applied to medicine

  • Enables scientists to understand how lifestyle factors interact with genes - identifying predisposition to disease and possible preventions
  • Disease-causing alleles identified more rapidly and the appropriate treatments prescribed earlier on
  • Scientists can predict an individual's response to certain drugs. New drugs can be developed which are tailored to a specific allele.