Types of Selection

Biology: Cambridge International A Level, topic 17.2.2

Stabilising Selection

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

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.

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.

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.

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.

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.

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.

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".