10.3.6

10.3.6: Identifying examples of directional, stabilizing, and disruptive selection

Directional Selection:

When a phenotype is favoured by another by natural selection.

-The frequency of one phenotype increases while the other’s decreases.

-Occurs when the environment changes.

-Can be considered “selection away from one extreme”

Example:

Industrial melanism - Noticed in the lighter-coloured pepper moth’s (Biston betularia) frequency decreased and the frequency of the darker phenotype increased during the industrial revolution.

Stabilizing Selection:

When one phenotype is favoured over two extreme phenotypes. i.e. an intermediate phenotype will be favoured.

-Can be considered “selection away from two extremes” or “selection towards a mean”

Example:

If a plant has two types of phenotypes in its flowers: 1. More nectar 2. Some nectar. Flowers with more nectar will drain the plant’s sugar resources and flowers with less nectar will not attract insects or discourage insects from returning. Therefore, for survival, the plant will favour a phenotype in which the flowers have an intermediate quantity of nectar, balancing the two extremes.

Disruptive Selection:

When two phenotypes are favoured over one intermediate phenotype.

- This can be advantageous to the species’ survival.

- If the two disruptive phenotypes are extreme and the two populations occupy different niches, speciation can occur.

- Can be considered “selection against the mean”  

Example:

Tadpoles in spadefoot toads have two different morphologies:

1. One with an omnivorous diet (Spea multiplicata)

2. One with a strictly carnivorous diet (Spea bombifrons) which includes cannibalism during food scarcity.

Two separate morphologies (hence, two different phenotypes) give the species a better chance of survival in places where water and food supplies are constantly changing.