It's been a long time since I studied running stats, but my recollection is that milers were Europeans, 10K were Kenyans, and ultramarathons were Europeans again. (By "European" and "Kenyan" I include people descended from same.)
5K to marathon, it's about as I said. The really, really top events are won by people from a tiny area in East Africa. Minor exceptions exist, but the success of a tiny, tiny percentage of the world's population in this endeavor is a so-far largely unexamined genetic miracle.