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## NetLogo User Community Models

## WHAT IS IT?

This is a simple model of how variation in the population fuels natural selection. This is what we would call an ecological model, because there are no new mutations. It is assumed that the time scale is too short for new mutations to arise.

Every time step (tick) is a generation, where each organism reproduces asexually, and its offspring inherit its fertility. If the population rises above the terrain's carrying capacity, organisms randomly die (starve) until the population is reduced below the terrain's carrying capacity.

## HOW TO USE IT

Always use the SETUP button at the start of these models to initialize the population of organisms.

The CARRYING-CAPACITY slider sets the carrying capacity of the terrain. The model is initialized to have a total population of CARRYING-CAPACITY.

The STARTING-VARIANCE slider sets how much variation there is among the fertilities of the organisms at the start.

The GO button runs the model. Clicking it again stops the model.

The POPULATION plot displays the number of organisms currently in the population.

The FERTILITY plot shows a histogram of the fertilities in the population.

## THINGS TO DO

Set the starting variance to an intermediate value and the carrying capacity to 500. Run the model and watch the distribution of fertilities in the population. Does it change? How does it change? Does it stabilize? Why does it stabilize?

Now set the starting variance to its maximum. Does the population stabilize at a higher or lower fertility than before? Why?

Now set the starting variance to 0. What happens? Why? What necessary and sufficient condition of natural selection has been violated?

## EXTENDING THE MODEL

If you want to try messing with the code, download Netlogo and the model. You are welcome to modify it.

Try adding a mutation rate, so that, occasionally, the offspring's fertility is slightly different from its parent.

## HOW TO CITE

This model is based on Wilensky's Simple Birth Rates modelm, and was extended by Carlo C. Maley.

For the model itself:

* Wilensky, U. (1997). NetLogo Simple Birth Rates model. http://ccl.northwestern.edu/netlogo/models/SimpleBirthRates. 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)