I've read on several occasions that the last name "Evans" really is just a coincidence. JKR was asked whether or not he was related to Lily and Petunia (and, therefore, Harry) and she said no.
Of course, I do believe her if she said that, but it's hard to believe just because JKR doesn't usually DO coincidences. Remember Mrs. Figg? Took over half the series before we found out who she REALLY was.