Explain Darwin's theory of natural selection. Give industrial melanism as an example.

1. 1 · State the idea in one line. Natural selection is the process by which individuals with heritable variations suited to the environment survive and leave more offspring, so the population changes over generations.
2. 2 · List the observations.
   • Populations can grow exponentially.
   • Resources are limited; population size stays stable.
   • So there is competition.
   • Individuals vary, and most variation is inherited.
3. 3 · Explain the conclusion. Variants that use resources better survive and leave more progeny (reproductive fitness); over generations new forms arise. Branching descent links all forms.
4. 4 · Give the example. Before industrialisation, white lichens hid white moths. After soot darkened trunks, birds ate white moths and dark moths became common.
5. 5 · Link to today. Misuse of antibiotics and pesticides selects resistant bacteria and insects within months or years.
6. 6 · Close in one line. So nature selects; it does not create the variation.

Scoring tip: Use Darwin's words: variation, struggle or competition, fitness, natural selection. Say clearly that the moths did not change colour.