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## NetLogo User Community Models

## WHAT IS IT?

This model is an example of genetic drift. It shows what happens when competing breeds of (asexual) organisms ("turtles"), each reproduce with equal likelihood on each turn. That is, neither breed has a selective advantage.

## HOW IT WORKS

The model starts with a random distribution of colored turtles. They move by wiggling randomly across the world. Each turn, a turtle produces between 0 and 4 offspring. If the total number of turtles is greater than the original number, then turtles are randomly killed until the original number is restored.

## HOW TO USE IT

The "setup" button initializes the model.

The "go" button runs the model.

Use the "colors" slider to select the number of competing colors.

The "number" slider sets the initial number of turtles.

## THINGS TO DO

Set the initial number of colors to “2”, and set the initial number of turtles to “100”. What do you observe? How many generations did it take for one color to go extinct?

Run the model under the same conditions at least 5 times. Record which breed went to fixation and how many time steps it took to get to fixation.

Is there any evidence to assume that one color is less “fit” than another? Did one color go to fixation more frequently than another?

Now adjust the model to begin with a population of 200. Run it at least 5 times again, recording the color that went to fixation and the time step when it reached fixation.

How long, on average did a color take to reach fixation with a population of 200?

Now set the population to 1,000 and run the model. Look at the size of the jumps in allele frequency from one generation (tick) to the next. How does it compare to the size of the jumps when there were only 100 organisms? (go back and run it with 100 if you need to)

Now, adjust the model to have 10 colors, with a beginning population of 200. What trends do you notice as you run the model? How many colors survived to 50 generations? 100? 200?

How many generations did it take for one color to reach fixation in this run, as compared to the run with only 2 colors? Why is this?

If you were to run the model again, would you suspect that the same color would reach fixation? Why?

## THINGS TO NOTICE

Notice that often colors can get to quite a high population but still fail to win the race.

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 a series of dominant colors before one color finally wins. Equally important is the fact that a color can never come back once it dies out.

The idea, explained in more detail in Dennett's "Darwin's Dangerous Idea", is that trait drifts can occur without any particular purpose or 'selecting pressure'.

## EXTENDING THE MODEL

Carlo Maley tweaked the code to make the organisms move more slowly.

The grim reaper in the procedure `death` does a random harvesting of the population to keep it roughly constant. This might be somewhat like a natural environment with a limited food supply. Can you think of other ways to write this procedure? Are the results affected?

## RELATED MODELS

* GenDrift P global
* GenDrift P local
* GenDrift T interact

## HOW TO CITE

If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.

For the model itself:

* Wilensky, U. (1997). NetLogo GenDrift T reproduce model. http://ccl.northwestern.edu/netlogo/models/GenDriftTreproduce. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

Please cite the NetLogo software as:

* Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

![CC BY-NC-SA 3.0](http://ccl.northwestern.edu/images/creativecommons/byncsa.png)