Stabilising, directional and disruptive selection

Stabilising selection

when phenotypes are at the extremes are selected against making the middle range to be favoured

shape of stabilising graph

narrower, higher peak

same mean

Directional selection

when conditions gradually change and reproduces causing the frequency of those alleles to increase

shape of directional selection

moves left or right of original population

approx. same, height and width

examples of stabilising selection

birth mass

examples of directional selection

antibiotic resistance in bacteria

Disruptive selection

In fluctuating environments when two extremes of a phenotype are favoured, this leads to an increase in alleles causing these distinct phenotypes

examples of disruptive selection

light or dark shelled oysters better camouflaged over mid-coloured shelled oysters