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Teaching resources · Evolution

Natural selection bug model

Birds hunt a population of bugs. Turn heritability, variation, and selection on and off to test what natural selection needs to work.

Modified by Jessie Ebie from the NetLogo Bug Hunt Coevolution model by Michael Novak and Uri Wilensky (Northwestern University).

About the activity

Students use this model of natural selection acting on a population of bugs to test the three requirements for natural selection (heritable variation, variation in the population, and selection) by switching each one on and off and watching the population change over generations. Using NetLogo also introduces students to open-source modeling software that biologists use.

Natural selection needs three things at once: individuals differ in a trait, offspring inherit that trait from their parents, and the trait affects which individuals survive and reproduce. Take away any one of them and the population stops changing in a consistent direction. Instead of memorizing that list, students find it themselves by breaking the process one piece at a time.

The model also helps with a common misconception: that individuals change because they need to. A bug in this model never changes color during its life. Only the mix of colors in the population changes, because some bugs live long enough to reproduce and others don’t. That makes it a natural way to compare Darwin’s and Lamarck’s explanations.

Learning objectives

  • Describe Darwin’s theories and explain how heritable variation and limits on reproductive success lead to differential reproduction (natural selection)
  • Propose explanations for the rise of adaptations that are consistent with evolution by natural selection
  • Compare and contrast Lamarck’s hypothesis of evolution by inheritance of acquired characteristics with Darwin’s theory of evolution by natural selection

Below the model: how it works, what each control does, and things to try.

Try the model

An interactive NetLogo Web model. Press setup, then go. It takes a few seconds to load. On a phone, open the model in its own tab for more room.

How the model works

Each bug is one of five shades of green. Bugs wander, run from birds they can see, reproduce on their own (asexually), and eventually die of old age. The population can grow only so large before it runs out of room.

Birds chase the closest bug they can see and eat it when they catch it. An X marks the spot where a bug was eaten. The histogram shows how many bugs of each shade are alive right now, so you can watch the population change as generations go by.

Choose your settings, press setup to start a new population, then press go.

The controls

number-bugs and number-birds
How many bugs and birds there are at the start. Birds start at 0, so add some to bring in predators.
bug-color-variable?
On: bugs come in five shades of green. Off: every bug is the same shade.
bug-color-heritable?
On: offspring are the same shade as their parent. Off: each offspring gets a random shade.
survival-is-selective?
On: birds spot some shades more easily than others. Off: every bug is equally easy to see.
show-vision-cone? and wiggle?
Change only how the model looks: whether you can see what each animal sees, and how smoothly they move. Neither changes the outcome.

Things to try

  1. Add a few birds and turn all three switches on. Predict what will happen to the histogram, then run the model for a few hundred ticks. Which shade is most common at the end?
  2. Turn off one switch at a time and run the model again. Does the population still change in the same direction? What does that tell you about each requirement?
  3. Set the birds back to 0. What happens to the mix of colors when nothing is hunting the bugs?
  4. Pick one bug and follow it. Does its color ever change? How, then, does the population change?

Run each setting more than once. Chance alone can nudge the histogram a little, so a real effect is one that shows up run after run.

What students should see

With all three switches on, birds find the darker bugs more easily, so the population shifts toward the lightest shades within a few hundred ticks. Turning off any one switch stops that steady shift, and with no birds the mix of colors changes only by chance.

For instructors

Modified by Jessie Ebie from the NetLogo Bug Hunt Coevolution model: Novak, M. and Wilensky, U. (2007), Center for Connected Learning and Computer-Based Modeling, Northwestern University. Licensed CC BY-NC-SA 3.0.