Knowing the evolutionary relationships between various organism, I would predict that "my protein"--that is, the one I have studied for the last few years--would be most similar between rats and mice, less similar between mice and humans, very much less similar between mice and fish, and quite different between mice and fruit flies. Experimental evidence confirms that this is, indeed, the case. "My protein" has certain areas necessary for its function, and other areas important for its structure. Since structure is more flexible than function, I would predict that most of the differences seen across species are located within structurally important areas, not functionally important areas. Again, experimentation has confirmed that this is the case.
I dated a girl who had just finished her biology studies and was about to go to work for Max Planck. She was so proud of "her protein."