Animals adapt to new environments. New species develop from survivors. There is variation among individuals within a species. Insecticides may cause genetic mutations.
I gave my students this multiple choice question: Five hundred flies of one species were sprayed with a new insecticide. Twenty-four hours later, nearly all the flies were dead. However, a few survived. Which principle best explains why some flies survived?

Only one answer is correct among these 4 choices:
- Animals adapt to new environments.
- New species develop from survivors.
- There is variation among individuals within a species.
- Insecticides may cause genetic mutations.

Could you tell me which one is correct?

There is variation among individuals within a species.