The dialect is regional, not isolated to the island, and they probably find ways to exchange genetic material with other people in the region. Rumor has it that they go at it with enthusiasm. Just a bit down the coast they call this accent an "Ocracoke Brogue" or simply a "Hoi Toide" (high tide) accent.

Dialect changes over time in ways that are not predictable, but which follow consistent patterns, simply because we have to make words sound different from each other. The rhoticity of this dialect and fragments left over from the tail end of the great vowel shift are how linguists know this dialect has seen relatively little change. But it is a mistake to think that everyone talked this way in the 1700s. Accents in England are very diverse based on region and social class, and they were even more so before things like public education, railroads, and mass media. This Shakespeare dialog is a pretty well sourced performance of late 1600s London English, but people from other parts of the country would have sounded different, and colonists would have developed idiosyncratic regional dialects.