Directional selection

  • Occurs when one extreme phenotype is favoured over other phenotypes
  • This happens when the environment changes in a particular way such as :
Biotic  Abiotic 
Predators Temperature
Food pH
Competition Weather

phylogenetic_tree
The original population, in this example are non-resistant bacteria. The Population after selection are resistant to bacteria.

The remaining bacteria of the original bacteria has a mutation (extreme phenotype) which allowed them to become resistant

The new bacteria replicate rapidly, thus overtaking the population of the old bacteria whilst also having the resistant phenotype.

Stabilising (normalising) selection

  • Occurs when the immediate phenotype is selected over extreme phenotypes
  • Extreme phenotypes such as overly small or overly big do not likely carry their genes on
  • The result is a ‘normal range’ is produced
  • This could be, for example birth mass, where a low mass has an increase chance of mortality but equally a high mass also has an increased chance of mortality. As a result the middle (being the normal) survives and thus the extreme phenotypes are removed.

The original population, for example had a wide range of birth masses. However the extreme phenotypes do not carry their gene onwards due to an increased chance of mortality.

The result is the population after selection have a ‘normal range’ where mortality rate decreases as there is a reduced number of ‘extreme’ birth masses.