What process causes heritable traits that improve survival to become more common in populations?

Natural selection causes heritable traits that improve survival to become more common in populations.

The process begins with variation among individuals. If some differences are inherited and affect survival or reproduction, individuals carrying advantageous variants tend to leave more descendants. Those variants can therefore increase in frequency across generations.

Natural selection does not mean that organisms consciously choose useful traits. It also does not guarantee perfection: a trait is favored only in a particular environment, and conditions can change. Selection acts on individuals, while evolutionary change is measured in populations.

Natural selection differs from genetic drift, which changes variant frequencies through chance, especially in small populations. It also differs from artificial selection, in which humans choose which organisms reproduce. Mutation creates new genetic variants, but selection determines whether some variants spread more successfully than others.