Where can I find a good artificial life and evolution simulator?

I am looking for a free Alife simulator that does a good job of demonstrating evolution and natural selection. There should be complex behavior, with no “optimal life form” that a human could create easily. I would prefer for it to have some graphical component, so I can see the life moving around.

Note: Conway’s game of life does not count. It is a cellular automaton, and does not demonstrate natural selection.