I've always just used the term "homeland effect" for this, but websearches suggest that that is not actually its name and probably something I made up at some point.
What I'm talking about is definitely a recognised effect that actually happens--I'm certain I learned about it from a reputable source some years ago, just can't recall what that source was or what they called it. It's the reason that dialects vary so wildly in the British Isles compared to in the US, Canada, Australia, and New Zealand, where the English language was only introduced comparatively recently. I've also heard it said that this applies to other languages as well, like European French and Canadian French or European Spanish and Latin American Spanish, but since I don't speak any of those languages I can't comment on its accuracy.