Final answer:
The concept that language variation appears gradually over distance, allowing mutual intelligibility among neighboring groups, is known as a dialect continuum. Dialects may eventually evolve into separate languages, influenced by political factors. Within a language, there are distinctive speech communities defined by unique language features.
Step-by-step explanation:
The term for the idea that variation in languages appears gradually over distance, allowing groups of people who live near one another to speak in a way that is mutually intelligible, is dialect continuum. A dialect continuum occurs when a large number of contiguous dialects exist in such a way that the dialects closest to each other at any point in the continuum are mutually intelligible, but the differences accumulate over distance so that dialects at the opposite ends become less mutually intelligible. This reflects a gradual shift in language features such as vocabulary, pronunciation, and syntax.
Dialects can develop into separate languages over time if they accumulate enough unique linguistic features, though the distinction between a dialect and a language can be political. For instance, nation-states may consider regional differences as dialects to maintain linguistic unity. Alternatively, regions seeking independence might advocate for their dialect to be recognized as a separate language.
Within every language, there are speech communities where groups of speakers use the language in distinctive ways that demonstrate membership in their community. These communities are characterized by unique vocabularies, grammatical forms, and intonation patterns.