This is a simple model of population genetics. There are two populations, the REDS and the YELLOWS. Each has settable birth rates. The reds and yellows move around and reproduce according to their birth rates. When the carrying capacity of the terrain is exceeded, they die to maintain a relatively constant population. The model allows you to explore how differential birth rates affect the ratio of reds to yellows.
For detailed instructions on using the program, take a look at the info window from the Simple Birth-Rates model.
Click on one of the pictures to see a quicktime movie of the model: