WHAT IS IT? ----------- This model is an example of random selection.  It shows
that patches that randomly exchange colors converge on a single color.  The model
starts with a random distribution of colors.  It  then randomly picks an
x-coordinate and y-coordinate to copy a color from, and randomly picks an
x-coordinate and y-coordinate to change to the copied color.  The coordinates are
global, so it could happen that the same patch is selected for both roles.  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 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.

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 turtle
agents who move, and exchange colors on contact.

RELATED MODELS: --------------- The other models in this set have slightly
different mechanisms but also show genetic drift:

GD-Local Patches GD-Random-walking Turtles GD-Reproducing turtles