In 1989, A.K. Dewdney wrote an article in Scientific American entitled "Simulated evolution: wherein bugs learn to hunt bacteria" as a part of the "Computer Recreations" column (May, pp. 138--141). The ideas in that article were included in his book Turing Omnibus (1989).
The idea described in these works is a very simple artificial life experiment. A tauroidal landscape houses moving agents (which we will call "bugs") and immobile food elements ("bacteria") for the agents. The bugs are incapable of sensing their environment, but they do make a kind of "choice" regarding the direction they move. This choice is made by a simple distribution across six different discrete turning choices, defined by a set of genes. Bugs gain energy when they eat bacteria and burn energy when they move; however, a bug that runs out of energy will die (be removed from the simulation), and a bug that has sufficient energy and age will divide into two nearly identical copies. At the start, the bugs "jitter" around, turning randomly; however, they will often eventually evolve to glide around the world, scooping up bacteria in their path.
The applet below is a MASON implementation of the bugs described in the article. [Sean writes:] Note that the applet is a 200x200 image on the Mac, which causes Java to garbage collect quite a bit, resulting in occasional pauses; even so, using the image is much faster than drawing each rectangle separately.
Source code as a zip file.