Natural selection can be stabilising, directional or disruptive. Explain each with a labelled graph.

1. 1 · Set up the idea. Most traits show a bell-shaped distribution around a mean; selection changes this curve.
2. 2 · Explain stabilising selection. Mean value favoured, extremes removed; curve becomes narrower and taller (extra example: birth weight).
3. 3 · Explain directional selection. One extreme favoured; peak shifts that way and the mean changes (extra example: dark moths).
4. 4 · Explain disruptive selection. Both extremes favoured; curve splits into two peaks; may give two species.
5. 5 · Draw the three graphs. [Diagrams: number of individuals vs phenotype; dotted original curve, solid new curve, arrows for selection pressure]

Scoring tip: Label both axes on every graph and show the old curve as a dotted line. Unlabelled curves lose marks.