This simulation was written by Thomas Ray, a biologist at the University of Delaware.
Visually it's not that impressive, changes over time can only be seen as bars on the screen, but the results are extremely complex. Different bar widths and colours represent various creature's sizes and roles (eg parasite or host ).
The creatures exist in an electronic ecosystem called Tierra ( Spanish for Earth). The program breeds and evolves many 'species' of organisms. A creature can both reproduce ( be dividing itself ) and be killed ( by a Reaper which kills on the Darwian basis of survival of the fittest ).
Any of this remind you of the human race?