Random Walking Turtles
This model is an example of genetic drift by random selection. Differently-colored turtles wiggle across the screen. When two turtles bump into each other, one turtle loses its color and adopts the color of the other one. By statistical advantage, a dominant color becomes more likely to win the entire grid. However, because the process is random, there will usually be several dominant colors before one color finally wins. The idea is that trait drifts can occur without any particular purpose or 'selection pressure'.
For detailed instructions on using the program, take a look at the info window from the Random Walking Turtles model.
Click on one of the pictures to see a quicktime movie of the model: