"All the time in the back of mind is the question of machine autonomy. The problem I would face if I ever figured out how to do it, in giving Aaron as it were its own head, is I might hate what it does...If the program did a drawing in August that it couldn't have done when I stopped programming it in January, then I'll consider it creative."
It wouldn't be hard to achieve what he is asking for in a trivial way-- the program could take in new imagery from the web to learn new leaf shapes, for example. But I suspect more than this would be needed for Cohen to feel satisfied. When he says, "I might hate what it does" that might be okay if other people liked it. The question is if we can set a program free and yet give it enough of a human sense of aesthetics that any people will like what it produces.
Another good article is from a course page at MIT about genetics and culture. This describes a little of how the code works, and the design process behind the program.