I came across this article the other day. At the bottom there's a family tree of English dialects, both extant and extinct ones. It makes it out that southern English dialects came from Wessax English, Standard English came from Mercian and Kentish English, etc.
I wonder how they came to this? Did they take a look at features of Standard English and conclude that "yes, such and such feature is obviously a Mercian innovation", or did they say "Received Pronunciation came from East Midlands; East Midlands was in Mercia; therefore, Received Pronunciation descended from medieval English of Mercia"?
What are some examples of Old English dialectal features that survived in modern English variants which allow people to make a family tree like that?