Brent Cornell

    
Natural selection is 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

• Where an intermediate phenotype is favoured at the expense of both phenotypic extremes
• This 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
• An example of stabilising selection is human birth weights (too large = birthing complications ; too small = risk of infant mortality)

Directional Selection

• Where one phenotypic extreme is selected at the cost of the other phenotypic extreme
• This 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
• Directional selection will typically be followed by stabilising selection once an optimal phenotype has been normalised
  
• An example of directional selection is the development of antibiotic resistance in bacterial populations

Disruptive Selection

• Where both phenotypic extremes are favoured at the expense of the intermediate phenotypic ranges
• This causes the phenotypic distribution to deviate from the centre and results in a bimodal spread
• This 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)
• An example of disruptive selection is the proliferation of black or white moths in regions of sharply contrasting colour extremes

Types of Selection – Stabilising, Directional and Disruptive