Types of Selection

• Natural selection


  The change in the composition of a gene pool in response to a differentially selective environmental pressure
• The frequency of one particular phenotype in relation to another


  Will be a product of the type of selection that is occurring
• Stabilising selection


  • An intermediate phenotype is favoured at the expense of both phenotypic extremes
  • Results in the removal of extreme phenotypes (phenotypic distribution becomes centrally clustered to reflect homogeneity)
  • Operates when environmental conditions are stable and competition is low
• Stabilising selection


  • Human birth weights (too large = birthing complications ; too small = risk of infant mortality)
• Directional selection


  • One phenotypic extreme is selected at the cost of the other phenotypic extreme
  • Causes the phenotypic distribution to clearly shift in one direction (towards the beneficial extreme)
  • Operates in response to gradual or sustained changes in environmental conditions
  • Typically followed by stabilising selection once an optimal phenotype has been normalised
• Directional selection


  • Development of antibiotic resistance in bacterial populations
• Disruptive selection


  • Both phenotypic extremes are favoured at the expense of the intermediate phenotypic ranges
  • Causes the phenotypic distribution to deviate from the centre and results in a bimodal spread
  • Occurs when fluctuating environmental conditions (e.g. seasons) favour the presence of two different phenotypes
  • Continued separation of phenotypic variants may eventually split the population into two distinct sub-populations (speciation)
• Disruptive selection


  • Proliferation of black or white moths in regions of sharply contrasting colour extremes