What is the opposite to artificial? Natural!
What are 'natural' borders?
Mountains, rivers, deserts, oceans. Oh, and magnificent walls
Are there "natural borders" between peoples, different from those just listed?
Peoples are never divided 'naturally' by anything other than the features just listed. And even those can't account for much. Frankfurt/Oder, Görlitz? Texas along the Rio Grande urban agglomerations?
If you look at any ethnic map, regardless from where-ever, when-ever, you will never see anything like "all X live East, all Y live West". It is always a mix. Absolutely no matter when or where.
Where did Han Chinese vs Uigurs, Mongols, Manchu, live in 1400? Where did Germ-Germans live at the same time? Easy? Try again with Slavics and Germanics, or Manchu and Han.
Look at any map of ethnicity or even race for mid-eastern Europe during the whole 19th century (like here, example below). Then tell me where you would draw any 'natural boundary' between ethnicities. Try Germany/Denmark, Germany/France, Belgium, Germany/(Poland)/Russia, Germany/(Czechia/Bohemia)/Austria, Switzerland.
Anything South America emerging from the Spanish Colonial Empire, the US/Canadian border?
Is in this map a single 'natural border' drawn between ethnicities corresponding to state borders?
There is none.
You might produce a kind of heat-map, or go by some 'majority' for certain areas. For the vast majority of places or lines it does not make any sense at all. Or if it does 'make any sense' (for those peddling it), it is a grave injustice for anyone else within that vicinity.
Outside powermongering hate-seeders, nationalism does not make any sense at all.
Artificial borders in the piece to analyse is a language problem. In such cases it means: "recent border" — people adapt to anything, even state borders. But that means transaction costs for adoption processes. And time. Impose a border, any border, and people left or right from that will slowly start to diverge.