18. Variation and selection

• Variation - differences between individuals of the same species

  • phenotypic variation results from genetic variation
• Discontinuous variation - individuals display a characteristic which falls into a distinct group and there are no intermediate values

  • mostly cause by genes alone
• Continuous variation - results in a range of phenotypes between two extremes, e.g height...
• Causes of variation:

  • gender: genes
  • accent: environment
  • height: genes, environment
  • mass: genes, environment
  • hair colour: genes, environment
  • intelligence: genes, environment
• Mutation - a random change to the sequence of bases on DNA

  • natural selection
  • ionising radiation and some chemicals increase the rate of mutation
• Sickle cell anemia (recessive):

  • happens with a mutation in allele that make haemoglobin - changes its shape
  • less oxygen is carried, can block blood vessels

  Symptoms:

  • yellowish skin colour
  • episode of extreme pain
  • swelling of hands and feet
  • frequent infections

  Found where:

  • those who are heterozygous with on sickle-cell allele are immune to malaria
  • found commonly in places with lots of malaria
• Adaptive features - the inherited functional features of an organism that increase its fitness (ability to suvive)
• Fitness - the probability of an organism surviving and reproducing in the environment which it is found
• Xerophytes adaptive features (live in dry places):

  • deep roots - reach underground water
  • leaves have reduce spines - minimises surface area for transpiration
  • shallow spreading roots - competition to get precipitation
  • rolled leaves, leaf hairs, stomata sunk in pits - trap moist air
  • stomata open at night, closes at midday - reduce transpiration rate
• Hydrophytes adaptive feature (live in fully/partly submerged water):

  • leaves are highly divided - create large surface area for absorption and photosynthesis
  • minimal cuticle formation - water does not need preserve
  • lack of xylem tubes, no stomata underside of leaves
  • stomata on upper surface, have thick waxy layer - repel water & keep stomata open and clear
  • roots are often reduced, roots hair are often absent - water can be transported easily
• Evolution - the change in adaptive features of a population over time as a result of natural selection
• Adaptation - the process resulting from natural selection, by which a population become more suited to their environment over many generations
• Natural selection process (dark-wing moth/long neck giraffe):

  1. a mutation occurred in the moth/giraffe
  2. advantage of mutation
  3. state what the advantage mean
  4. how the moth/giraffe with genes is reproducing and passing it down to offspring's
  5. offspring survive and reproduce, passing the gene again and again --> population with the advantageous mutation

  
• Selective breeding process:

  1. decide which characteristic is most important
  2. select parent which exhibit high levels of this characteristic
  3. breed the selected individuals (could be siblings with the allele)
  4. select the offspring with the selected characteristic and breed them together
  5. repeat the process over many generations until all of the offspring show the selected characteristic
• Natural selection in bacteria:

  • a mutation occurs causing the bacteria to develop resistance to antibiotic - since they breed quick = bacteria offspring would eventually be immune
• Why is selective breeding chosen for crops/animals (benefit):

  • can produce high yield
  • can lead them to be disease resistant
  • enhance sustainability
  • global food security

  Drawback of selective breeding:

  • reduces gene pool (n. of alleles) - reduce variation
  • give features which leads to health problems (pug: shorter noses --> breathing problems)