Animating Pupil Size

Is there a way to animate the pupil size without making the different parts of an eye? The eye I’m using right now is just a simple UV sphere. It’s basically a bead (hole through UV sphere), with areas different colored, the pupil black, the rest white. I tried shape keys and stuff, but that ended up morphing the eye shape since the vertices change places. Any ideas?

When you setup the new shape key, scale all of the verts that make up the pupil then scale several of the edge loops in the white of the eye around the pupil. Finally select all and click To sphere. This will get the eye back in to a sphere shape and the pupil should still be smaller than before. I did a quick test and the distortions where minimal with this method. Assumeing the eye is in a head, I don’t think anyone will be able to tell.