NetLogo User Community Models

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## WHAT IS IT?

This simulation models coevolution of a predator-prey sytem. It is based on the board game **SIMULATING NATURAL SELECTION** by Robert P. Gendron from Indiana University of Pennsylvania. The model incorporates the following elements of a natural system: Variability, heritability, competition, predation, carrying capacity and differential reproduction. Users can set the intial values for adaptation scores, variability of adaptation scores and population size. The simulation produces behaviors that demonstrate directional selection, the coevolution of traits, genetic drift and predator-prey population cycles.

## HOW IT WORKS

The two populations in the model are hawks and mice. Hawks have a vision score and mice have a camouflage score. Populations are intialized with user specified number, mean score and score variation. Individual mice and hawks are assigned a random genotype on setup. Each population is then randomly distributed in the world.

Each tick in the model represents a discrete set of events that occur in the following order:

1. **PREDATION:**
If a hawk and mouse are on the same patch and the hawk's vision score is greater than the mouse's camouflage score then the hawk eats the mouse, reproduces and dies (See 2 below). If there is a tie in the scores a coin flip determines if the mouse is eaten or survives. If there are many mice on a patch, the mouse with the minimum camouflage score is eaten.
2. **HAWK REPRODUCTION:**
Each hawk that successfully eats produces two offspring as follows:
**> If hawks-evolve? is ON:**
Hawk vision **is** affected by mice camouflage. Each hawk reproduces with another random hawk. Each child's genotype is determined by their parents genotypes based on simple Mendelian probabilites.
**If vision-dominant? is ON:** Homozygous dominant and hetrozygous children are assigned a vision score equal to the parent score _**plus**_ a random value between 0 and 1. Homozygous recessive children are assigned a vision score equal to the parent score _**minus**_ a random value between 0 and 1.
**If vision-dominant? is OFF:** Homozygous recessive children are assigned a vision score equal to the parent score _**plus**_ a random value between 0 and 1. Homozygous dominant and hetrozygous children children are assigned a vision score equal to the parent score _**minus**_ a random value between 0 and 1.
**> If hawks-evolve? is OFF:**
Hawk vision **is not** affected by mice camouflage. Each hawk reproduces with another random hawk. Each child's genotype is determined by their parents genotypes based on simple Mendelian probabilites. Both children are assigned vision scores drawn randomly from the normal distribution defined by the init-mean-vision and init-SD-vision sliders.
**>Both Conditions:**
Each new hawk is assigned an age equal to the tick count when it is hatched.
The parent hawk dies immediately after reproducing.
If there are still mice on the patch where the new hawks are hatched, the newly hatched hawks get a chance to hunt and reproduce until all mice on a given patch with camouflage scores lower than hawk vision scores are eaten.
3. **HAWK STARVATION:**
If a hawk does not eat within 2 ticks of being hatched it dies.
4. **MOUSE REPRODUCTION:**
Mice that survive and are alone on a patch (i.e., no competition for resources from other mice) reproduce two offpring as follows:
**> If mice-evolve? is ON:**
Mouse camouflage **is** affected by hawk vision. Each mouse reproduces with another random mouse. Each child's genotype is determined by their parents genotypes based on simple Mendelian probabilites.
**If camo-dominant? is ON:** Homozygous dominant and hetrozygous children are assigned a camouflage score equal to the parent score _**plus**_ a random value between 0 and 1. Homozygous recessive children are assigned a camouflage score equal to the parent score _**minus**_ a random value between 0 and 1.
**If camo-dominant? is OFF:** Homozygous recessive children are assigned a camouflage score equal to the parent score _**plus**_ a random value between 0 and 1. Homozygous dominant and hetrozygous children children are assigned a camouflage score equal to the parent score _**minus**_ a random value between 0 and 1.
**> If mice-evolve? is OFF:**
Mouse camouflage **is not** affected by hawk vision. Each mouse reproduces with another random mouse. Each child's genotype is determined by their parents genotypes based on simple Mendelian probabilites. Both children are assigned camouflage scores drawn randomly from the normal distribution defined by the init-mean-camo and init-SD-camo sliders.
**>Both Conditions:**
The parent mouse dies immediately after reproducing.
5. **RANDOM DISTRIBUTION:**
All surviving hawks and mice are randomly redistributed in the world and the the above steps are repeated.

## HOW TO USE IT

1. The **mice-evolve?** and **hawks-evolve?** switches determine if the mouse and hawk populations evlove in response to environmental pressures (On) or randomly (Off)
2. The **vision-dominant?** and **camo-dominant?** switches determine if improvements in each trait are dominant (On) or recessive (Off)
2. The **init**, **mean** and **SD** sliders set the initial population size (mice or hawks), the intial mean value of the adpatations (camouflage or vision) and the variation within the population for each adaptation (camouflage or vision), respectively.
3. The **Reset** button returns all settings to their default values.
4. Use the **SETUP** button to initalize the simulation to the values you set using the sliders and switches described above. Press the **GO** button to run the simulation continuously or use the **5 TICKS** button to advance the simulation 5 ticks at a time.

## THINGS TO TRY

Use the model to experiment. Start by making changes to only one setting at a time. Run each experiment several times as the results may differ. Make note of these different outcomes and the frequency with which they occur. For each experiment be sure to explain what you think might be causing the results you obseve.

Some suggested experiments are listed below
(**NOTE:** All settings are assumed to be default EXCEPT those indicated):

1. Run the model using the default settings.
2. Set the init-hawks slider to zero.
3. Set both hawk and mice evolve? switches to off.
4. Turn off mice evolution only.
5. Turn off hawk evolution only.
6. Change the mean value of the mice camouflage value.
7. Change the variation (SD) value of the mice camouflage value.
8. Change the mean value of the hawk vision value.
9. Change the variation (SD) of the hawk vision value.
10. Change vision, camouflage or both to recessive.
11. Increase or decrease the mouse, hawk or both populations.
12. Try to get both populations to survive most of the time when mice-evolve? is off.

## THINGS TO NOTICE

1. What happens to the vision and camouflage scores under different settings?
2. How do these scores change relative to one another?
3. What happens to the vision and camouflage variation under different settings?
4. How do the populations change relative to one another?
5. Does either population go extinct? reach a carrying capacity?
6. Are there any notable trends in the allele frequencies?
7. Do any alleles become fixed / extinct?

## EXTENSIONS

Play the board game (see link below) and explore the code to see how it works.

## RELATED MODELS

In the NetLogo Library:

1. Bug Hunt Camouflage
2. Bug Hunt Coevolution
3. Bug Hunt Drift
4. Red Queen
5. Wolf Sheep Predation
6. Wolf Sheep Stride Inheritance

## CREDITS AND REFERENCES

Dr Robert Gendron's Faculty Page at IUP:
http://www.iup.edu/page.aspx?id=85437

A copy of the original board game directions:
http://biology4teachers.com/Evolution/natural%20selection%20game.doc

## COPYRIGHT AND LICENSE

Copyright 2013 Michael Zito.


This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.