Gorilla groups: monkeying around?

There are two species of gorillas, mountain and lowland, and they seem to have diverged around 80,000 years ago. The lowlanders split into fairly well defined subspecies, eastern and western. However, these authors report there is a "zone of introgression between eastern gorillas and migrants from the west," meaning that they are hybridizing. The point, without getting too deep into taxonomic waters that I'm really not qualified to wade into, is that gene flow works in all sorts of directions, and gorillas turn out to be a very complex group. It's a good lesson when puzzling out relationships of any creatures.