An example I'm using in my dissertation is a modification of Donald Davidson's Swampman example, which is a standard enough example in philosophy to have its own Wikipedia entry.
The Wikipedia description of the Swampman example is as follows:
Suppose Davidson goes hiking in the swamp and is struck and killed by a lightning bolt. At the same time, nearby in the swamp another lightning bolt spontaneously rearranges a bunch of molecules such that, entirely by coincidence, they take on exactly the same form that Davidson's body had at the moment of his untimely death. This being, whom Davidson terms 'Swampman', has, of course, a brain which is structurally identical to that which Davidson had, and will thus, presumably, behave exactly as Davidson would have. He will walk out of the swamp, return to Davidson's office at Berkeley, and write the same essays he would have written; he will interact like an amicable person with all of Davidson's friends and family, and so forth.My modification targets the view that being a member of a certain race requires having an ancestor of that race. Besides having an infinite regress problem (since races have to come into existence at some point), that view is at odds with what I think our intuitions would be with a Swampman-like case. Suppose an exact duplicate of Chris Rock were to appear out of nowhere, with no causal history and certainly no ancestry, never mind black ancestry. I think most people, even knowing this origin of the Chris Rock duplicate, would take the duplicate to be as black as Chris Rock. I've discussed this case with a lot of people, and almost everyone takes that to be the implication.
If that's right, then there can't be an ancestry requirement for race-membership, since the duplicate is black, and he's got no ancestors.
Incidently, my dissertation supervisor, in a parenthetical remark in the middle of an objection to this example, indicated that she thought my name for this example -- Swamp Rock -- was slightly offensive. I haven't had a chance to ask her about that, and I might not. I'm happy enough to change the name of the example or just not give it one. But I'm a little curious what led her to find it slightly offensive, unless it's something she sees offensive in the original name Davidson used. Is it that the name is all right until it gets applied to a black person, and then it's slightly offensive? If it had been someone named Dave Rock, who was white, and I was using it to show that the duplicate is white despite having no ancestors, would it be equally (i.e. still slightly) offensive?