The effects of stabilising, directional and disruptive selection.

The idea

The three types of selection are distinguished by which part of the range of phenotypes is favoured. Stabilising selection favours the intermediate phenotype and selects against both extremes, so the mean stays where it is while the standard deviation falls and the distribution curve narrows; human birth mass is the standard example, since very small and very large babies historically had lower survival, and it is what you expect in a constant environment. Directional selection favours one extreme, so the whole curve shifts and the mean moves with it; antibiotic resistance in bacteria, where the least susceptible cells survive each course of treatment, is directional selection acting quickly because the generation time is short. Disruptive selection favours both extremes and selects against the intermediates, so a single curve becomes two peaks.

What earns the marks

• Identifying the type of selection from the shape and position of the distribution curve.
• Stabilising selection favouring the modal phenotype and reducing variation in a constant environment.
• Directional selection favouring one extreme, so that the mean shifts in that direction.
• Disruptive selection favouring both extremes against the intermediates, so that variation increases.
• Naming the selection pressure responsible for the change described.

Common mistakes and how to put them right

• Describing the curve moving without saying which phenotype had the selective advantage and why.

• Calling any change in a population directional selection, without checking whether the extremes or the intermediates were favoured.

• Saying that stabilising selection means no selection is happening, when it is active selection against both extremes.

• Forgetting that disruptive selection is the one type that increases variation.

• Misreading a distribution graph, so a taller peak is taken to mean a larger mean.

Exam tips

• Sketch the before and after curves in the margin, because deciding which type is acting is far easier from a picture.
• Name the type, then the favoured phenotype, then the selection pressure; three steps usually earn three marks.
• Antibiotic resistance, pesticide resistance and industrial melanism are safe directional examples, and birth mass is the safe stabilising one.