I'm struggling a bit in my worldbuilding on justifying the creation of different cultures on my land, when all originated from the same ancestors.
So on my continent I have a group of humans that migrated thousands of years ago, and let's say they had a culture similar to ancient Greeks or Romans. This group expanded on the land, and killed or exiled any natives they found who were more primitive. They created the first Kingdoms and also had their own religion, of a pantheon of deities.
So, many years after, I'd like to have a few groups migrating from them, towards various regions further to the west, and establish different cultures; more French let's say or others maybe more Celtic. And some of them keep their original religion.
Another case is that I want a kingdom which was established by the first group to change into a different culture after many years. Keep the religion and the kingdom's name, but change their namings and cultural ways.
As I have no clue how such things work, is any of the above possible and in what ways and circumstances I can have them justified? If not, then maybe any suggestions on a different approach?
Thanking you all in advance!