The modern science of genetic genealogy is revealing that more people with a certain surname share common ancestry than used to be thought. I recommend you to read Bryan Sykes's book "Adam's Curse. A Future Without Men". Sykes is a professor of genetics at Oxford University and the founder of the DNA-testing agency Oxford Ancestors.
In the abovenamed book Sykes describes his research into some Scottish clans. He tested representatives of the chiefly lines, and also as many "ordinary" bearers of the name as he could find. His McDonald research was particularly interesting. He identified a common pattern of Y DNA (male, from the father's father's father ...) which he believes is the patrilineal strain going back to the founder of Clan Donald, Somerled, Lord of the Isles. The point about chiefs like that is that they had a lot more "wives" and concubines than the average male, so their genetic inheritance is pretty widespread.
Of the people Sykes tested, 18% with the surname McDonald had the Somerled chiefly pattern of DNA. But this rose to an astounding 40% of testees with the name McAlister. Traditionally, the McAlisters claim to be descended from Somerled too and to be kin to the McDonalds. Sykes's theory is that when surnames became more widespread, or even compulsory, a lot of people would have taken on the surname of a powerful local clan, like McDonald, but there was no pressing reason to take on a common or garden name like McAlister, so a lot of McDonalds at the present day probably have non-McDonald origins, but most McAlisters are the "real McCoy", to coin a phrase!
I read the other day that, I think, about 15% of Stewarts are descended from the royal lot, and on the Scottish DNA Project blog I read that the Clan Chisholm DNA project has identified their chiefly line as being a classic Y DNA I1 type, which means Norse, and seems to confirm that the founder of the clan Chisholm was an Anglo-Norman, whose ancestors would have gone back beyond Normandy to Norway. Of Chisholms who have been roped into the Chisholm DNA project, between 30% and 40% have tested as being of the chiefly line. Vikings under the skin!
Harry