Natural Selection & Sickle-Cell Anaemia

• Organisms reproduce at a rate greater than can be supported by the environment
• Genotypic variation


  Exhibited in the phenotypes of individuals
• Selective agents


  Act on the alleles in a gene pool
• Natural selection acts on phenotype
• A lion chooses its prey based on colour or speed, not genotype
• Heritable traits that favour survival and reproduction will tend to produce more offspring
• Changes to Gene Pool Leading to Speciation

  • Mutation/trait is present
  • Variation
  • Isolation
  • Struggle for Existence
  • Survival of the Fittest
  • Selection
  • Speciation
• Variation


  Must be heritable (genetic)
• All members in a species vary due to the combination of alleles passed on to the offspring
• Isolation


  Barriers to gene flow inhibiting interbreeding between the two populations
• Populations are reproductively isolated
• Characteristics that are more suitable for the environment are present
• Organisms have excessive birth rates as well as limited resources
• Survival of the Fittest

  Creates competition for survival and ability to reproduce
• Some individuals do not live to reproductive age
• Selection


  Suitable genes are passed onto offspring
• Alleles for the traits are selected and would increase over time
• Traits that are favourable and enhance chances of survival and reproduction are selected over time in the environment
• Speciation


  Changes in gene frequency of several alleles over many generations produces more individuals with a particular phenotype
• Emergence of a new species occurs when they are no longer able to reproduce with the new species
• Sickle Cell Anaemia


  A mutated form of haemoglobin
• Function of Sickle Cell Anaemia

  Distorts RBCs into a crescent shape/reduces the surface area of RBC’s
• Sickle Cell Anaemia reduces RBC oxygen carrying capacity
• Sickle Cell Anaemia can be lethal/fatal
• Sickle Cell Anaemia has become more prominent due to natural selection
• Advantages of Sickle Cell Trait

  • Provides immunity against Malaria
  • Genes are passed onto next generation from heterozygous individuals
• Disadvantages of Sickle Cell Trait

  • May cause breathing problems when exercising at high altitudes/scuba diving/on low oxygen supply
  • (Rare): urinary tract infections/heat stroke/blood in eye
• Reproducing with a sickle-cell trait means offspring will have the trait
• Point Mutation on the HBB gene causes changes in the structure of haemoglobin
• Symptoms of Sickle Cell Anaemia

  • RBC’s that have a sickle shape
  • Reduces RBC oxygen carrying ability
  • Fatigue
  • Shortness of breath
  • Inflammation of hands or feet
• Inheritance of Sickle Cell Anaemia

  Recessive trait/passed from affected parents
• Effect on Gene Pool

  • Heterozygous individuals are malaria resistant
  • Increases sickle cell allele frequency in population
  • Individuals homozygous usually die early
• Malaria reduces reproduction of normal cell individuals
• Sickle-cell allele is favoured
• Carriers of sickle cell reproduce at greater rates due to early death
• There is a need for continuing population of sickle cell trait allele combinations
• Sickle cell trait allele combinations increase