Survival of the Fittest: Natural Selection in the Wild

Introduction

In nature, life is a constant competition for survival. Whether it’s a moth blending into the bark of a tree or a lizard hiding among gray rocks, an organism’s traits can mean the difference between life and death. When an environment changes, which individuals are most likely to survive? How do these “advantages” change the entire population over time? In this investigation, you will use a simulation to track the evolution of a population over many generations and use statistics to explain the patterns you observe.

Part 1: Engage (Anchoring Phenomenon)

Consider the famous case of the Peppered Moth in industrial England.

1. Before the Industrial Revolution: Tree bark was light-colored, and light-gray moths were common.
2. During the Industrial Revolution: Soot from factories turned tree bark black.
3. The Shift: Within a few decades, almost all the moths in the area were black.
4. The Question: Did the moths “decide” to change color to hide better, or was something else happening to the population?

Part 2: Explore (Simulation Investigation)

Open the Natural Selection Simulation. You will test how environmental pressure (predation) affects the distribution of traits (color) in a population.

Your Challenge:

Observe how a population’s average color changes over 20 generations in a “Dark” environment vs. a “Light” environment.

Procedural Steps:

1. Trial A (Light Environment): Set the Background Color (Environment) to 200 (Light Gray). Set Mutation Rate to 10% and Predation Level to 50%. Run the simulation for 20 generations.
2. Trial B (Dark Environment): Reset the simulation. Set the Background Color to 50 (Dark Gray). Keep other settings the same. Run for 20 generations.
3. Trial C (Environmental Shift): Start with Background Color at 200. Run for 10 generations, then suddenly shift the background to 50 while the simulation is running. Observe the “Average Population Color” graph.

Data Collection:

Record the Average Population Color at specific intervals.

Part 3: Explain (Sensemaking)

Analyze your data and the graphs generated by the simulation.

1. Identify Patterns: In Trial A, did the average population color increase (get lighter) or decrease (get darker) over time? How does this compare to the Environment Color?
2. Probability & Statistics: If an organism’s color is $150$ and the environment is $50$, is it more or less likely to be eaten by a predator than an organism with color $60$? Explain using the concept of probability.
3. The Mechanism: Does the simulation show individuals changing their own color, or does it show certain individuals being “filtered out” of the population?
4. Evidence-Based Claim: Use your data to support the following statement: “Organisms with an advantageous heritable trait tend to increase in proportion to organisms lacking this trait.”

Part 4: Elaborate/Evaluate (Refining the Model)

Critical Thinking

1. Mutation Rate: If you set the Mutation Rate to 0%, would the population be able to adapt to a new environment? Why or why not?
2. Predation Pressure: What happens to the speed of evolution if you lower the Predation Level to 10%? Explain the relationship between “selection pressure” and the rate of change in a population.
3. Human Impact: Imagine a city builds a new reflective glass skyscraper next to a forest. How might this “abiotic” change in the environment act as a selective pressure on local bird populations?

Part 5: Summary

Write a 3-sentence summary of the “Recipe for Evolution” based on your investigation. Include the following terms: Variation, Inheritance, and Selection.