Hi Spidey and Jag,
You can't believe how incredibly complicated and sad the story of some of these tribes are. After the mid 1800s when most of the open warfare with the Indians was over, there were two more big, nation wide land grabs that effected almost all Indians but effected different tribes in different ways. The first one was in the late 1800s when the federal government attempted to break up the reservations by dividing the land. which had been held in common, among individual families. First of all the total amount of land divided was less than the original reservation and the remainder was often sold by the government to white farmers. Second- individual familes sometimes lost their lands for various reasons, not the least of which was they had no concept of individual land ownership. Or land ownership was a very recent concept imposed by the Europeans.
Next-In 1956 a Bill was passed in congress to try to assimilate Indians into the general population. If a tribe was very sucessful financially they could be considered ready to stop being a tribe-whether they wanted to or not. This happened to the Klamath Indians here in Oregon and it took them until just a few years ago to sucessfully go through the courts to regain their tribal status. Other tribes lost their tribal status because they had too few members or no longer had a tribal government. I did a job for the Klamath Tribe and their treatment by the US government was totally unbelievable. How different tribes responded to these kinds of events- I think- has had a big impact on how different tribes determine who is a member and why some tribes don't require much native blood, while others do. The individual tribes are still sovereign nations and that status is hugely important to them for a whole gamet of reasons-not the least of which is that as sovereign nations they can still hold their treaty rights. That is why ending tribal status for some tribes was such a total farce and tragedy-it opened the door for the government and unscrulous individiuals to trample over both individuals and whole groups of people. This is really complicated stuff and I am probably making mistakes here, but there are reasons for the colleges to be a little careful about who they consider to be an Indian. The different tribes have very different histories and that is reflected in how they determine membership. I think you are still considered an Indian, even without membership if you have 1/4 Indian heritage. There is something about this on the BIA website.
AS far as being Irish or Italian or African-American. I consider myself Italian but my Italian cousins consider me American. I have an African American friend who spends a lot of time in Ghana and they consider him American. Obviously these parts of our heritage are tremendously important to us and Spidey is right about claiming that, but the folks who are still in the old country or who never drifted from the tribe sometimes don't see it the very same way as we do.