Question

Explain the three different ways in which natural selection operates. Explain with the help of graphs.

Hint:

Name stabilising, directional and disruptive selection and describe the bell curve for each: narrower, shifted, or split into two peaks.

Answer 1

Step 1: Understanding the Concept:
In a population a trait such as body size follows a bell-shaped curve. The x-axis shows the trait value and the y-axis shows the number of individuals. Natural selection can act on this curve in three ways.

Step 2: Stabilising selection:
The average individuals are favoured and both extremes are removed.
On the graph, the peak becomes taller and narrower at the same position. The variation shrinks.

Step 3: Directional selection:
Individuals at one extreme are favoured, for example the larger ones.
On the graph, the whole curve moves towards that extreme, so the mean value shifts. The peak moves to a new position. Example: industrial melanism in peppered moths.

Step 4: Disruptive selection:
Both extremes are favoured and the average individuals are selected against.
On the graph, the single peak splits into two peaks, with a dip in the middle. This can lead to two new forms in the population.

Step 5: Sketch tip:
Draw three bell curves with a dotted original curve and a solid new curve, as described above, with labelled axes.

Final Answer:
Stabilising selection narrows the curve, directional selection shifts it to one side, and disruptive selection splits it into two peaks.