Natural selection: how populations change — Biology, 11–13 years
Individuals vary, and some differences help survival or reproduction in a particular environment. Across generations, those inherited differences can become more common.
Populations change over generations
Natural selection is a pattern in which inherited differences affect who survives or leaves offspring in a particular environment. Helpful traits tend to become more common in the population over many generations. The individuals do not change because they need to; the population changes because some inherited versions are passed on more often.
Why populations adapt
Environments do not offer unlimited food, shelter or safe places, so organisms compete and many fail to reproduce. This creates a problem for a population: which inherited differences help when conditions change? Natural selection explains adaptation without planning or a goal; the environment filters existing variation, generation after generation.
Moths on dark trees
Suppose a moth population has 60 light moths and 40 dark moths. On pale tree bark, birds spot dark moths more easily, so light moths leave more offspring: imagine 24 light and 8 dark young. If the bark later becomes dark, the advantage can reverse. The population's proportions change; no moth chooses a colour because it knows the future.
The common mix-up
A reasonable mistake is to say that an animal changes a trait because it wants or needs it, then passes that new trait on. This feels natural because we describe actions with purpose. Selection works differently: random inherited variation already exists, the environment affects success, and successful versions become more common across descendants.
Where it shows up
Natural selection helps explain why bacteria can become resistant to an antibiotic: resistant variants survive treatment and reproduce, while others die. It also helps us understand camouflage, beak shapes and changes in wild populations. The lesson is about populations over generations, not about a person choosing to evolve during a lifetime.
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