Darwin's theory is based on four observations:
- Members of a population of the same species vary in their traits.
- Traits can be inherited, or passed from parents to offspring.
- Populations are capable of producing more offspring than the environment can support.
- Due to a lack of food or other resources, many of the offspring do not survive.
Natural selection will only occur if all these points can be observed and followed.
According to Darwin, organisms that are better adapted to the environment have greater chances of survival than those less adapted, leaving a greater number of descendants. The best adapted organisms are therefore selected for that environment.