They begin by visiting an African lake that teems with strange fish. They're called cichlids. The cichlids in the lake come in 200 different species. They're all pretty similar. Only their lips, jaws, and teeth have all evolved differently.
Some cichlids eat snails.
Each has evolved into a tiny niche of the ecology. That kind of subdividing is pretty common. Humankind is only one species. One young hunter can chase down a rabbit. Another can spear a fish.