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When your Y-DNA test predicts that you share a "common ancestor" with people of other surnames, is it only referring to a Common Y ancestor, as in, you all had the same Y-line daddy at a certain point before people started taking on surnames or changing surnames? Or does it simply mean that you all had a similar grandfather at some point in the chain, among the millions of other grandfathers you descended from so many generations back? I would assume it would be the former since Y-info needs to be passed unbroken from son to son, but I just want to verify.
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You are correct, Y DNA deals only with relationships by a shared patrilineal descent. However, mutations of the STR's and/or SNP's that are usually tested occur very infrequently (and that's why they work), with the result that the prediction of a "relationship" may not be particularly helpful in terms of genealogy. The relationship may be way, way too far back to be useful. Still, knowing that there are other people having your surname that also share a Y DNA profile with you is a clue.
Another kind of clue that can come out of Y DNA tests is that particular families having your surname may turn out NOT to share a common Y DNA profile, in which case you know they're not related to you (by patrilineal descent). That result was particularly helpful for me, because it means I can safely ignore all of the other McCoy families that have been tested!
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