Types of natural selection
Selection can favour the average (stabilising), one extreme (directional) or both extremes (disruptive) of a trait, changing the shape of its bell curve.
Selection acts on a range of values
Many traits, such as height, weight or beak size, vary over a range. If you plot number of individuals against character value, you get a bell-shaped curve: most are near the mean (average), and a few are at the two ends. Natural selection can change this curve in three ways, depending on which values are favoured by the surroundings. Your textbook shows the three types as graphs but names no example for each; the examples below are extra, to help you understand.
The three types compared
| Type | Which values are favoured | Effect on the curve | Extra example (not in textbook) |
|---|---|---|---|
| Stabilising | The mean value; both extremes are removed | Peak stays in place; curve becomes narrower and taller | Human birth weight: very light and very heavy babies have lower survival |
| Directional | One extreme | Peak shifts towards that end; mean changes | Dark moths in industrial England; DDT-resistant mosquitoes |
| Disruptive | Both extremes; the mean is removed | Curve splits into two peaks | Birds with small and large beaks survive where only small and large seeds are found |
Directional vs disruptive: a common mix-up
How to draw the three graphs
- Draw three small graphs side by side. On each, label the x-axis phenotype (character value) and the y-axis number of individuals.
- In each graph, first draw the original bell curve with a dotted line and mark its mean with a short vertical line.
- Stabilising: draw a solid, taller and narrower curve on the same mean. Draw arrows from both ends pointing inwards.
- Directional: draw a solid curve of the same shape shifted to one side. Draw one arrow showing the direction of the shift.
- Disruptive: draw a solid curve with two peaks near the two ends and a dip at the old mean. Draw arrows pointing outwards.
- Label each graph with its type, and shade or tick the region that is favoured.
In India, regular spraying of the same insecticide against mosquitoes has favoured resistant mosquitoes: a case of directional selection. That is why health teams rotate insecticides and also remove stagnant water.
3 marks: “With the help of graphs, explain stabilising, directional and disruptive selection.” Three graphs of number of individuals vs character value. Stabilising: mean favoured, curve narrows. Directional: one extreme favoured, peak shifts. Disruptive: both extremes favoured, two peaks.
2 marks: “Which type of selection can lead to the formation of two species from one population? Why?” Disruptive selection. It favours both extremes and removes the middle, so the population splits into two groups that can become separate species.
Stabilising Squeezes, Directional Drifts one way, Disruptive Divides in two.