types of selection

• Stabilising selection is the process where natural selection favours an average phenotype.
• Selection pressure

  • Natural selection acts on an individual by imposing a selection pressure.
  • The selection pressure is an external factor that influences the reproductive success of an individual.
  • E.g. Disease acts as a selection pressure that increases the reproductive success for individuals who are immune to the disease.
• Stabilising selection

  • In stabilising selection, natural selection favours an average phenotype.
  • The selection pressures in stabilising selection select against the extreme phenotypes.
  • Stabilising selection takes place in environments that do NOT change.
• describe and explain an example of stabalising selection

  E.g. Mouse camouflage

  • A population of mice live on the floor of the woods.
  • Natural selection will favour individuals who best match the brown colour of the floor. This helps avoid detection by predators.
  • Mice with brown fur are most likely to survive and reproduce, passing on their genes for their brown coat.
  • Mice that carry alleles that make their fur lighter or darker will stand out against the ground and be more likely to be eaten by predators. Stabilising selection favours the average phenotype: brown.
• Directional selection is the process where natural selection favours an extreme phenotype.
• Directional selection

  • In directional selection, natural selection favours one extreme phenotype.
  • The selection pressures in directional selection select against all other phenotypes.
  • Directional selection takes place after an environment has experienced a change.
• describe and exlain an example of directional selection

  E.g. Peppered moth

  • Before the Industrial Revolution, peppered moths were predominantly white. This allowed them to camouflage with the light-coloured trees and lichens in their habitat.
  • The environment changed when the Industrial Revolution darkened the trees with soot.
  • The white moths became easier for predators to spot and the white colour was now selected against.
  • Natural selection caused the frequency of the black-coloured moth to increase because the black colouration became more favourable.
• Disruptive selection is the process where natural selection favours the two extremes of a phenotype.
• Disruptive selection

  • In disruptive selection, natural selection favours the two extremes of a phenotype.
  • The selection pressures in disruptive selection select against the average phenotype.
  • Disruptive selection takes place in an environment that favours more than one phenotype.
  • Disruptive selection can eventually lead to the production of two new species.
• Multiple male forms

  • Some populations of animals have multiple male forms.
  • Large, dominant alpha males obtain mates by brute force.
  • Small males can sneak in for secret copulations with the females in an alpha male’s territory.
  • The two phenotypes both have a selective advantage.
  • Natural selection selects for both phenotypes.
  • Medium-sized males are selected against because they are not big enough to obtain mates using size but they aren't small enough for "sneaking".
• Directional selection

  • Natural selection favours an extreme phenotype.
  • The selection pressures select against all other phenotypes.
  • Takes place after an environment has experienced a change.
• Stabilising selection

  • Natural selection favours the average phenotype.
  • The selection pressures select against the extreme phenotypes.
  • Takes place in environments that do not change.
• Disruptive selection

  • Natural selection favours the two extremes of a phenotype.
  • The selection pressures select against the average phenotype.
  • Takes place in an environment that favours more than one phenotype.
• • Types of selection
  • Stabilising
  • Selection favours average phenotype
  • Environment does not change
  • Directional
  • Selection favours extreme phenotype
  • Environment has just experienced a change
  • Disruptive
  • Selection favours two extremes of a phenotype
  • Environment favours more than one phenotype