Variation: there is variation in the physical and behavioural characteristics of a set of organisms. This variation is caused by a combination of genetic and environmental factors.

Inheritance: there is a mechanism by which physical and behavioural characteristics may be passed between generations. Some traits may be more heritable (e.g. eye colour), whilst others (e.g. weight) may be more affected by environmental factors.

Selection pressures: in most cases, organisms produce more offspring than resources can sustain. This creates competition for resources (e.g. food, light, or nest space). These selection pressures mean that some individuals will die, as they do not have enough resources to survive.

Differences in survival and reproduction: because there is variation in different characteristics, and there is competition, those organisms which have the most suitable traits for their current environment will be more likely to survive to be able to reproduce. Conversely, those organisms with unsuitable traits are more likely to die. Therefore, some organisms will produce more offspring than others, and their genes will survive to the next generation.