Red hair may have evolved in Neanderthals and then was introduced into human populations via interbreeding. If this is true, this would be an example of

gene flow

Explanation:

Population genetics depends on mathematical models which allows predictions to be made. Geneticists build abstract mathematical models that incorporate the effects of selection, genetic drift, gene flow and mutation on gene frequency dynamics. From these models, they draw conclusions about the possible patterns of genetic variability in real populations, subsequently their conclusions are tested with empirical data.

Natural selection, mutation and gene flow are deterministic microevolutive processes: estimating the value of certain parameters it is possible to generate mathematical models that allow predicting the behavior of the process through generations

One of the processes that causes a population's genetic heritage to change is the flow of genes from neighboring populations.

The genetic content of all populations, with the exception of those that live totally isolated, is strongly affected by the immigration and emigration of individuals from and to other groups of the same species. This exchange of individuals between neighboring populations contributes to genetic variability.

An example is what happens in chimpanzee communities that live in natural reserves. When the females reach the age of procreation, they migrate to the neighboring community, where they are welcomed by the males.

The effect is that the genetic pool of both communities changes.