Natural selection is the process where heritable traits that make it more likely for an organism to survive long enough to reproduce become more common over successive generations of a population. It is a key mechanism of evolution.

The natural variation within a population of animals, plants, bacteria, etc. means that some individuals will survive better than others in their current environment. For example, the peppered moth exists in both light and dark colors in the United Kingdom, but during the industrial revolution many of the trees on which the moths rested became blackened by soot, giving the dark-colored moths an advantage in hiding from predators. This gave dark-colored moths a better chance of surviving to produce dark-colored offspring, and in just a few generations the majority of the moths were dark.