Choose a habitat and a predator, then use the animals you survey to work out how each pressure reshapes the population over hundreds of generations.
Data format
A run is surveyed ten times, evenly spaced, and each survey records up to 50 living animals as one row each, so a run yields about 500 rows whatever its length. Each row carries Animal ID, Generation, Coat Darkness, Body Size, Ground Darkness, Trial, Habitat, and Predator. The three measurements all run from 0 to 100: for a coat or for the ground, 0 is palest and 100 is darkest; for a body, 0 is smallest and 100 is largest. A finished run exports itself and adds to the table, so trials stack up side by side.
Model details
Each animal is an individual carrying its own coat darkness and body size, and passes both to its young with a little variation. A population therefore starts as a spread of traits rather than a single type, and nothing is nudged toward an answer: a predator simply finds conspicuous animals more easily, and cannot handle prey above a certain size.
Numbers are held down by food rather than by a cap. Plants are grazed where the animals stand and regrow slowly, so a crowded population goes hungry and a thinned one recovers. Six predators hunt in every run, because fewer applies too little pressure for selection to show and more drives the population extinct.
Traits are reported as those 0 to 100 measurements rather than as genes, and no allele frequencies appear anywhere. Surveys sample 50 animals rather than counting every one, as a field biologist would, so runs stay comparable. A run can end in extinction, which is a real outcome rather than an error.
Standards
HS-LS4-3 Apply concepts of statistics and probability to support explanations that organisms with an advantageous heritable trait tend to increase in proportion to organisms lacking this trait.
HS-LS4-2 Construct an explanation based on evidence that the process of evolution primarily results from four factors: (1) the potential for a species to increase in number, (2) the heritable genetic variation of individuals in a species due to mutation and sexual reproduction, (3) competition for limited resources, and (4) the proliferation of those organisms that are better able to survive and reproduce in the environment.
Design intent
- Nothing in the sim states what happens. Students find it in the data, by plotting a trait against Generation and colouring by whatever they changed between trials.
- The two traits answer to different pressures. A coat is judged against the ground it is seen on; body size matters because a predator can handle prey only up to a certain size.
- Six predators hunt in every run, fixed rather than offered as a control, and the two habitats differ in both ground colour and tree cover, so a class comparing habitats is changing two things at once.
- All four of the factors named above are in the model: breeding supplies the potential to increase in number; offspring inherit both traits with small variations; grazing a finite, regrowing food supply creates competition for limited resources; and the predator removes the animals it finds easiest to catch, so the survivors proliferate.
Discussion prompts
- The same starting population ends up matching one habitat and not the other. Where did that difference come from?
- Compare body size across two runs that differ only in the predator. Does it settle in the same place both times?
- Look at the spread of a trait late in a run, not just its average. Why does variation matter for what happens next?
- Which animals are missing from a late survey, and what does their absence tell you that the survivors cannot?
How do I explore?
- Pick a habitat and a predator, then press Play in the top bar to watch the animals move, or Fast forward to race through the generations.
- When the run reaches the end it stops and sends its animals to the table on its own. Collect in the top bar grabs a run early, if you do not want to wait.
- Change one thing, run it again, and the new trial joins the same table so you can compare the two.
What should I do/notice?
- Watch each animal's colour against the ground it is standing on. Which ones are hard to spot, and which stand out?
- Every animal is a little different. Does the whole population change together, or do particular animals simply disappear?
- Run the same habitat again with a different hunter. Does that change which animals survive?
What about the data?
Put Generation along the bottom and Coat Darkness or Body Size up the side. That shows how the trait changed across one run, but one run on its own cannot tell you what caused the change.
So collect at least two trials, change only one thing between them, and colour the points by whatever you changed, Habitat or Predator. If the two trials end up in different places, that one change is doing the work. Ground Darkness records what each animal was standing on, and Trial keeps runs apart even when their settings match, so you can repeat a setup and check you get the same answer twice.