Is an example of how

gene flow could occur.

When animals immigrate into another breeding population they may bring with then alleles that are rare or unknown in the population they breed into.

Explanation:

Gene flow is the movement of genes from one population to another population. Examples of this include a bee carrying pollen from one flower population to another, or a caribou from one herd mating with members of another herd. Genes can come in different forms called alleles.