Every culture and language that spreads out disintegrates into subcultures and dialects, which later diverge. You can often tell how ethnic groups spread out and fractured by the time depth of their linguistic features. Often you can get cladistic analyses of DNA, cultural artefacts from archeology and linguistic innovations to line up neatly in historical progression.
You don't get two cultures and languages coexisting in the same space for very long before one goes away. Usually by assimilation, leaving traces behind. Sometimes by genocide or out-migration. In rare cases you get things like female/home language versus public language, I think I've read of a case in N. Africa and another in some small ethnic community in the extreme S. of China. In those cases the two languages become specialized by situation. Gal wrote a bit about how Hungarian was giving way to German. ("Gal" is the researcher's actual surname, and is not pronounced as "gal" in "guys and gals".)
It's a commonplace to say that it's really time and space that triggers dialect and cultural divisions. That's usually the case, but we've documented other instances.
One linguist documented a linguistic divergence in a Mayan settlement. Two villages were very close together, but something happened to make them antagonistic to each other. The first research was done in, IIRC, the 1930s, and they differed only slightly. By the 1980s the dialects had diverged quite a bit as each village was an anti-language to the neighboring village, in effect. They innovated "insider" words, pronunciation, and grammar to exclude the other village's speakers. (There's still a debate over the extent to which this is occurring in current American culture. Those most opposed to assimilation are the one's arguing that it's not happening, because they're often advocates for that subculture and that kind of exclusionary thinking is contrary to their basic views. It's not formal linguistics, is sociolinguistics and that's often about as rigorous in handling evidence critically and dispassionately as education or anthropology. If you make this a formal discussion, then you have to somehow operationalize and quantify "similarity" and "divergence". When such topics dropped off my radar perhaps 15 years ago nobody had a clear sense as to how to do that.)
Czechoslovak and Serbo-Croatian are good examples, as well. Nobody thinks Czechoslovak exists, but given a bit of effort and good will and they're mutually intelligible. For a brief while, they emphasized using the words that they had in common, even if they weren't always the most common word for a thing in Czech or Slovak. After the breakup, they emphasized differences. Still, when in Bratislava and I needed to communicate I spoke Czech, they spoke Slovak, and things went smoothly enough.
It's the same kind of situation for Serbian and Croatian (and the further fractured Montenegrin and Bosnian). The Stokavian dialects are easily mutually intelligible, whether Croat or Serb, but choose the right words and you can impede communication. Croatian includes Cakavian, which is harder for a Croatian speaker to get than Serbian. Serbian includes Torlak, which is harder for a Serbian to understand than Croatian. And yet they insist that the language boundary is the political boundary. (As for Montenegrin, I still sneeze in it's general direction. And Bosnian just includes some Turkish words from the time of Ottoman colonialization and apartheid. Yes, I've taken to routinely calling some of those situations "apartheid" in a completely anachronistic manner as a reaction to those who insist other places with less 'separateness' are obligatorily under apartheid.)
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