When a gene changes within a population over time, it is referred to as
evolution
mutation
transformation
natural selection

When a gene changes within a population over time, it is referred to as evolution. This refers to a long but steady rate of mutations that occur in genes of organisms within the environment. A lot of factors affect a population's evolution (e.g. climate, reproduction, food source) and it is a determinant whether a species will survive for the next generations.