If the 13 colonies were founded by the British, where in the world did the Southern accent come from? Or, for that matter, other dictinct east coast accents? I can see some being the result of the melting pot, but the Southern accent is a long-lived tradition!
The accents must have been pretty well entrenched by the time of the American Revolution. Massachusetts soldier Joseph Plumb Martin made this observation:
The Differences Between Colonists
They put me in this regiment, half New Englanders and half Pennsylvanians. Folks as different as night and day. Myself, I’d rather be fighting with a tribe of Indians than with these Southerners. I mean they’re foreigners, can’t hardly speak English. They don’t like me either. They call me that ‘damn Yankee.’ That’s about the nicest thing they say.
You might as well ask where the mid-west accent, or Texas accent, or Maine accent, or Minnesota accent comes from. It’s not like there is a universal accent and the South are the odd man out. We all have accents, even you.
I’m a native of Georgia. Most of my ancestor’s names are of Scottish origin (although my last name is probably English). My guess is that most of the white settlers who came south had Scots ancestry with varying dashes of English, Irish, & German.
The north probably had a higher concentration of English with stronger influences from Dutch & German.
That’s my wild-ass guess.
Right, but in this quote, “the southerners” he’s talking about are Pennsylvanians, and he says they’re mostly foreigners. Here’s the actual quote:
A quick wiki check shows that a lot of Pennsylvania colonists were of Scots-Irish background.
I have no accent! …Just everyone else does! 
Variations of this question come up pretty often here and lead to some interesting threads. “Did George Washington Speak with a Southern Accent” was one of the first but there are many others. The Southern accent has been around for a very long time and very old recordings of people who were very old when they were made suggest that the basic accent was similar to the current one in some places since before the Civil War and probably well before that.
The book that would answer this beautifully is the magnum opus, “The Atlas of North American English” by W. Labov, S. Ash and C. Boberg. Let me see if I can get online access to it, or whether his other presentations have the relevant information… but if you have online access to a university library system you will probably be able to look for yourself.
Define “north.” Pennsylvania and NY had a lot of germans during the colonial era, but farther north in New England there weren’t many germans nor dutch to speak of. Instead the north east had (and still has) a large precentage of English and Irish folks, plus a fair number of people of Scottish and French-Canadian decent too.
I couldn’t get access to many of the papers and books that would have been useful, but here is a discussion of the data point that I did find.
First, language change is a continual process, and what we think of as an “accent” (or dialect) is the accumulation of many changes which took place over some period of time. If you want to trace an accent, the first place to start is to look at individual linguistic features which set that dialect apart from others.
One important dialect feature of Southern English is the Southern Shift, a sound change in most of the vowels changed their place of pronunciation-- sort of a vowel version of “musical chairs.” You can see a chart here,, though I am afraid that it is hard to understand if you don’t have a background in linguistics.
The first step in the Southern Shift was the monophthongization of /ay/ (think about the change from “Hi!” to “Haaaa!”). Guy Bailey (1997) apparently looked at data from recordings made in Texas, and found that monophthongized /ay/ was absent before 1875, variable between 1875 and 1945, and consistently present after 1945. This would imply that the Southern Shift, at least in Texas, took place long after the state had been settled by English speakers, even after the Civil War.
So then, your answer for where the Southern accent came from, for at least one of the major phonological changes, seems to be that it was a language change that took place once the South had been settled. In fact, various stages in the Southern Shift are still language changes in progress in certain areas.
Admittedly it would be important to see data from other parts of the South and look at other dialectal features, but this is the best I can come up with at the moment.
Southern American English is no further or no closer to the various dialects spoken by the British settlers in the 16th through 18th century than the various other current American dialects. For that matter, it’s no further or closer to the dialects of those settlers than modern British dialects are. You need to get out of your head the idea that there are dialects that don’t change over time. Languages are always changing. Dialects are always changing. There are no people today speaking a dialect that is the same as one spoken hundreds of years ago. That’s just how language works. There’s no need for an external influence for a language or a dialect to change. There’s no more reason to explain the fact that Southern American dialects are different from others than there is to explain that other American or Canadian or British or Australian, etc. accents are different from others.