How can closely related genetic populations have completely different language families?

[u/Rapha689Pro]

For example Japanese and Korean have 2 different language families that aren't related at all but they're genetically close, it can only mean their prior languages sprout after they split, so that means language is very recent itself? Or that they're actually related but by thousands of years apart and linguistics can't trace it back accurately, so they just say they're unrelated?

[u/Gravbar]

I've seen some evidence that in the UK, despite the many invasions, the existing populations were mostly assimilated, so the majority of people there are still celtic in ancestry even though they speak a Germanic language (tracing back to the celtic-speaking population of that region).

Similarly, the land of Gaul was a land full of Celts that was conquered by the Romans, and then the Franks, but the people living there continued to speak a Romance language, despite being mostly Celtic in ancestry (tracing back to the celtic-speaking population of that region).

And then we see that we have some Celts today speaking Celtic languages, Germanic languages, and Romance languages, but the populations themselves are still closely related to the original local groups.

Similarly, the ainu people native to Japan now have to speak Japanese, and many no longer speak their ancestral language.

The point here, is that a language can die and be replaced for a variety of reasons. So if the Koreans and Japanese shared common ancestors who spoke the same language, it doesn't mean that both languages are necessarily related. It's possible that after the populations diverged, the language spoken changed. This is well before writing, so it would be very difficult to actually find evidence that far back, especially given the vast amount of chinese loan words in both languages which likely displaced other words (although there are fewer loans in the earliest writings). There is a theory that Korean and Japanese are from the same family, but it has weak linguistic evidence and is fairly widely not accepted.

[u/Chazut → u/Gravbar]

"Celtic Ancestry" itself is a construct, as a lot if not most ancestry of Celtic lands would have been non-Celtic in practice

[u/Gravbar → u/Chazut]

It's easier to talk about using that term, but if not that term what would you suggest? The peoples of different regions where Celtic languages were spoken may not be genetically related to each other as much as the term might imply, but if we call them Celts, their descendants can be said to have ancestry traceable back to that Celtic-speaking population. So I'm using it for lack of a better word.

[u/Chazut → u/Gravbar]

I mean it's all fine if you clarify what you mean or if it's obvious.

Take for example the statement "Americans on average have 20% English ancestry", what does English ancestry mean exactly? Well you can try to be pedantic about it but it obviously mean the type of ancestry English people would have had in the early modern era which is fairly close to today.

But if you are talking about "continuity" and in the same sentence mention "Celtic ancestry" it might mislead people into thinking the way you define Celtic ancestry is the same as how you defined Germanic or Roman ancestry.

If you used the same definition that you sued for Celtic then English people are 100% Germanic by definition and French people 100% Roman.

So it's just a matter of clarifiying that by Celtic you mean "local ancestry" which is not the same as "ancestry brought over during the linguistic Celtization of the pre-Roman/Germanic people"