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# GenDrift T reproduce

 If you download the NetLogo application, this model is included. You can also Try running it in NetLogo Web

## WHAT IS IT?

This model is an example of genetic drift. It shows that competing breeds of turtles, each reproducing with equal likelihood on each turn, will ultimately converge on one breed without any selection pressure forcing this convergence. 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'.

## 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. 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.

## 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 NOTICE

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

## EXTENDING THE MODEL

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.

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