WHAT IS IT? ----------- This model is an example of genetic drift. It shows that competing breeds of turtles, each reproducing with the same parameters each turn, will ultimately converge on one breed without any selection pressure forcing this convergence. The model starts with a random distribution of colored agents. Turtels move by wiggling randomly across the screen. Each turn, a turtle produces between 0 and 4 offspring of its color. If the total number of turtles is greater than the original number, then turtles are randomly killed until the original number is restored. After enough turns, a color will gain a slight dominance; by statistical advantage, a dominant color becomes more likely to win the entire grid. However, because the process is random, there will usually be many series of dominant colors before one color finally wins. The idea, explained in more detail in Dennett's "Darwin's Dangerous Idea", is that trait drifts can occur without any particular purpose or 'selection pressure'. HOW TO USE IT: -------------- The Setup Button intializes the model. The Go Button starts it, and runs continuously. Use the Colors Slider to select the number of colors competing. The Number Slider sets the total number of turtles in the model. Note that the two plots -- one for colors 1-5 and the other for colors 6-10 -- may have different vertical scales. THINGS TO NOTICE: ---------------- Notice the Graphs of the colors. Often colors that start with a higher initial number fail to win the grid. EXTENDING THE MODEL: -------------------- Try writing the model with a 'randomness' slider that increases the randomness in the turtles' movements. How does this affect the rate at which one color dominates the space? Introduce a slight 'selecting pressure' along with the random process shown in the model. RELATED MODELS: --------------- The other models in this set have slightly different mechanisms but also show genetic drift: GD-Local Patches GD-Global Patches GD-Random-walking Turtles