We actually have flys in our lab that have been selected for living in low oxygen environments (4% O2 vs ~21% O2 in room air) over the course of 32 generations. While that by itself is not particularly impressive, the really interesting part is that these flies can be put back in room air for several generations, and then re-introduced to the 4% O2 environment and still survive, which wild type flys cannot do.

Here's a link to the paper: http://www.plosone.org/article/info:doi/10.1371/journal.pone...

The relevant info is in the Results and Discussion section.