I was thinking a bit on the Picts and Scots and so many other ethnic peoples.

Is there any cultural development concept that predicts how groups form, diverge, come together, etc?

I believe the word 'Celt' is often used to describe a lot of peoples in northern Europe at one point, but a few centuries pass and you have Franks, Anglo, Scot, etc.

Then we have the Greeks that were a few groups in a region (eg. Athens, Sparta, etc.) whose identity merged over time and still exist today as an identifiable group/religion.

The Franks turned into the French.

Of course all of the above is only my understanding and could be wrong. The point is that when I make a region and its history, I would very much like to put some realistic processes to work for peoples I choose to put there.