Genetic Drift Simulator
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About This MicroSim
This stochastic simulation models genetic drift by sampling alleles each generation using binomial probability. Students adjust population size (N=10 to 10,000 with logarithmic slider), initial allele frequency (\(p\)), number of generations, and number of independent trials. Each trial produces a line on the chart showing how \(p\) changes randomly over time. Small populations show wide variation with frequent fixation or loss; large populations show tight clustering around the starting frequency. A "Bottleneck" button simulates a population crash (N=10 for 5 generations) followed by recovery, demonstrating the lasting genetic effects.
How to Use
- Adjust sliders — set population size, initial \(p\), generations, and number of trials.
- Click "Run" to watch lines grow generation by generation (animated).
- Click "Instant" to compute all generations immediately.
- Compare results — the summary box shows fixations (\(p\)=1), losses (\(p\)=0), and still-drifting trials.
- Click "Bottleneck" after a run completes to simulate a population crash and observe the lasting effect.
- Click "Clear" to reset and try different parameters.
Lesson Plan
Grade Level
9-12 (college placement Biology)
Duration
10-15 minutes
Prerequisites
- Understanding of allele frequency and Hardy-Weinberg equilibrium
- Knowledge of the five conditions for HWE (especially large population size)
- Familiarity with probability concepts
Activities
- Exploration (5 min): Run 10 trials with N=20 and N=5000. Compare the spread of lines. How many trials reach fixation or loss in the small population? In the large one?
- Guided Practice (5 min): Set N=100, \(p\)=0.5, 200 generations, 10 trials. Run the simulation. Then click "Bottleneck." Observe how the bottleneck permanently shifts allele frequencies even after the population recovers. Why does this happen?
- Assessment (5 min): Predict what will happen with N=10, \(p\)=0.1, 100 generations. Will the allele likely be lost? Run the simulation to test. Explain why rare alleles in small populations are especially vulnerable to drift.
Assessment
- Can students explain why drift has a stronger effect in small populations?
- Can students predict whether an allele is more likely to be fixed or lost based on its initial frequency?
- Can students describe how a population bottleneck causes a lasting reduction in genetic diversity?
- Can students distinguish genetic drift from natural selection as a mechanism of evolution?