I think it suffices to show that selection can operate at the level of the group. Even if all of the traits involved provide some advantage to individuals, if they also provide an advantage to the group, then group-level selection needs to be considered.
It is more interesting if you can show that a trait that does not confer an advantage to an individual, has an effect on group selection. But it is an unreasonable bias to demand that group selection requires traits that do not provide any advantage to an individual, and yet at the same time not insist that the theory of individual selection requires traits that do not provide any advantage to the group.
I should clarify - "group selection" connotes what Tim is describing: Selection for altruistic traits in individuals, by selection at the group level. That's because, historically, group selection has been invoked only to explain things that individual selection can't.
However, this has led to people excluding selection at the group level from models and simulations, because "group selection bad".
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.