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Selection Modes Visualizer

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About This MicroSim

This animated visualization shows how three types of natural selection — stabilizing, directional, and disruptive — change a population's phenotype distribution over 30 generations. A green fitness zone shows which phenotype values are favored under each mode. Students select a mode, click Run, and watch the bar chart distribution shift in real time. An overlay toggle shows the original distribution (dashed line) for comparison, making the shape change dramatic and clear.

How to Use

  1. Select a mode — click Stabilizing, Directional, or Disruptive.
  2. Click "Run" to animate the distribution changing over 30 generations.
  3. Adjust the speed slider to control how fast generations advance.
  4. Click "Show Original" to overlay the starting distribution as a dashed line for comparison.
  5. Click "Reset" to return to generation 0 and try a different mode.
  6. Hover over bars to see exact frequency values.

Lesson Plan

Grade Level

9-12 (college placement Biology)

Duration

10-15 minutes

Prerequisites

  • Understanding of phenotype variation within populations
  • Knowledge of natural selection as differential reproductive success
  • Familiarity with normal (bell curve) distributions

Activities

  1. Exploration (5 min): Run all three selection modes with the overlay turned on. For each, describe how the distribution shape changed: Did it narrow? Shift? Split? Sketch the final shape for each mode.
  2. Guided Practice (5 min): For each mode, identify a real organism example: (a) stabilizing — human birth weight, (b) directional — antibiotic resistance in bacteria, (c) disruptive — beak size in African seed crackers. Explain why each example matches the pattern you observed.
  3. Assessment (5 min): Given a before-and-after distribution that has shifted to the right, identify the selection mode. Repeat for a distribution that has narrowed, and one that has become bimodal. Explain your reasoning.

Assessment

  • Can students match each selection mode to the resulting distribution shape (narrowed, shifted, bimodal)?
  • Can students identify real-world examples of each selection type?
  • Can students explain why the fitness zone shape determines the distribution change?
  • Can students predict the outcome after 30 generations given a fitness zone diagram?

References

  1. Stabilizing selection - Wikipedia
  2. Directional selection - Wikipedia
  3. Disruptive selection - Wikipedia