I have no straight answer with ABDE as of your lack there-of. But here.
Gene flow is the introduction of genes/alleles between populations. An example is a flower having it´s pollen carried by a bee. The bee then introduces it (intentional or not.) to other flowers.
Another example is a pack of gray wolves suddenly having a different wolf introduced. *As long as it can breed, that is* They will slowly incorporate the new alleles/genes into the pack.
