Biology Lesson
Survival of the Fittest: How Natural Selection Shapes Populations Over Time
Shared by A STEM Educator
10th Grade5E Model60 min
Learning Objective
Students will be able to explain how heritable trait variation within a population, combined with environmental selective pressures, leads to differential survival and reproduction, and will be able to predict directional changes in allele frequency across multiple generations using simulated and real-world population data.
Lesson Overview
Students investigate how trait variation within a population interacts with environmental pressures to produce differential survival and reproduction, ultimately shifting allele frequencies across generations. Through a hands-on simulation and data analysis, students build a mechanistic understanding of natural selection as the engine of evolutionary change.
Materials
- Colored paper squares (red, green, brown — 50 of each color per group)
- Green felt or green construction paper (1 large sheet per group, ~60x60 cm, simulating grass habitat)
- Brown felt or brown construction paper (1 large sheet per group, simulating leaf-litter habitat)
- Plastic cups or small containers (1 per student, acting as 'beak')
- Stopwatch or phone timer
- Student lab worksheets (printed)
- Graph paper or access to Google Sheets / Desmos
- Whiteboard and markers for class data compilation
- Colored pencils (red, green, brown)
- Projector and slides for Engage and Explain phases
- Printed 'Population Data Cards' for Elaborate phase (one set per pair)
- Exit slip cards (printed or index cards)
Scaffolded Task Progression
1Task 1
Look at the two peppered moth photographs displayed on the board. In your notebook, answer the following:
(a) Describe ONE visible difference between the two moths.(b) Before industrialization, pale moths were common and dark moths were rare. After decades of soot covering trees, dark moths became common and pale moths became rare. The moths did not change color during their lifetime. Write a hypothesis: how could the POPULATION shift from mostly pale to mostly dark without any individual moth changing color? [ Interactive graph / diagram, available in the full lesson ]
(a) Describe ONE visible difference between the two moths.(b) Before industrialization, pale moths were common and dark moths were rare. After decades of soot covering trees, dark moths became common and pale moths became rare. The moths did not change color during their lifetime. Write a hypothesis: how could the POPULATION shift from mostly pale to mostly dark without any individual moth changing color? [ Interactive graph / diagram, available in the full lesson ]
2Task 2
SIMULATION DATA RECORDING TASK — Complete the data table below as you run each round of the Colorful Prey Simulation on the GREEN habitat. After Round 3, calculate the PERCENTAGE of each color among survivors.
After completing the table, answer:
(a) Calculate what percentage of total survivors in Generation 3 are GREEN-colored.(b) Compare your group's Generation 3 percentages with two other groups. Are the results identical? Why or why not?(c) Despite variation between groups, what GENERAL TREND do all groups share regarding green survivors on the green habitat?
| Generation | Start: Red | Start: Green | Start: Brown | Captured: Red | Captured: Green | Captured: Brown | Survivors: Red | Survivors: Green | Survivors: Brown | Total Survivors |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 15 | 15 | 15 | ? | ? | ? | ? | ? | ? | ? |
| 2 | (from prev.) | ? | ? | ? | ? | ? | ? | ? | ||
| 3 | (from prev.) | ? | ? | ? | ? | ? | ? | ? |
After completing the table, answer:
(a) Calculate what percentage of total survivors in Generation 3 are GREEN-colored.(b) Compare your group's Generation 3 percentages with two other groups. Are the results identical? Why or why not?(c) Despite variation between groups, what GENERAL TREND do all groups share regarding green survivors on the green habitat?
3Task 3
HABITAT COMPARISON ANALYSIS — After completing both the GREEN and BROWN habitat simulations, use your Generation 3 percentages to complete the comparison table below. Then create a hand-drawn double bar graph on graph paper or in a spreadsheet. Use one bar color for the GREEN habitat and a different bar color for the BROWN habitat. Include a title, axis labels, and a legend.
Analysis Questions:
(a) Which color dominated in the green habitat after 3 generations? Which dominated in the brown habitat?(b) Is the brown color "better" than green in an absolute sense? Explain using evidence from your data.(c) What would happen to the brown-dominated population if the habitat suddenly changed to green? Describe what would happen over 3 generations.(d) What is the SELECTIVE PRESSURE in this simulation? What real-world factor does it represent?
| Color | Green habitat: % of Generation 3 survivors | Brown habitat: % of Generation 3 survivors |
|---|---|---|
| Green | ___ | ___ |
| Red | ___ | ___ |
| Brown | ___ | ___ |
Analysis Questions:
(a) Which color dominated in the green habitat after 3 generations? Which dominated in the brown habitat?(b) Is the brown color "better" than green in an absolute sense? Explain using evidence from your data.(c) What would happen to the brown-dominated population if the habitat suddenly changed to green? Describe what would happen over 3 generations.(d) What is the SELECTIVE PRESSURE in this simulation? What real-world factor does it represent?
4Task 4
FORMALIZING THE MECHANISM — Using the class simulation data compiled on the whiteboard, fill in the blanks in the Natural Selection Mechanism Flowchart below. Then, in your own words, write a 4-sentence explanation of how natural selection works, using ALL FIVE bolded vocabulary terms correctly.
Vocabulary: trait variation, selective pressure, differential survival, heritability, allele frequency
TEXT FLOWCHART VERSION:
[1] A population contains individuals with different ___(1)___
e.g., different colors of beetles
↓
[2] A ___(2)___ exists in the environment
e.g., a predator that hunts by sight
↓
[3] ___(3)___ occurs:
Individuals with advantageous traits survive and reproduce MORE
↓
[4] Traits are ___(4)___:
Surviving individuals pass their alleles to offspring
↓
[5] Over generations, the ___(5)___ of advantageous alleles INCREASES
in the population
↓
[6] Population has CHANGED — Natural Selection has occurred
After filling in the flowchart, write your 4-sentence explanation below.
Vocabulary: trait variation, selective pressure, differential survival, heritability, allele frequency
TEXT FLOWCHART VERSION:
[1] A population contains individuals with different ___(1)___
e.g., different colors of beetles
↓
[2] A ___(2)___ exists in the environment
e.g., a predator that hunts by sight
↓
[3] ___(3)___ occurs:
Individuals with advantageous traits survive and reproduce MORE
↓
[4] Traits are ___(4)___:
Surviving individuals pass their alleles to offspring
↓
[5] Over generations, the ___(5)___ of advantageous alleles INCREASES
in the population
↓
[6] Population has CHANGED — Natural Selection has occurred
After filling in the flowchart, write your 4-sentence explanation below.
5Task 5
REAL-WORLD DATA ANALYSIS — The Galápagos Finch Drought Study
In 1977, a severe drought struck Daphne Major Island in the Galápagos. Small, soft seeds became scarce, leaving mainly large, hard seeds. The medium ground finch (Geospiza fortis) population was studied before and after the drought. Use the data below to answer the analysis questions.
Additional information: Beak depth is a heritable trait. Finches with deeper beaks can crack large, hard seeds more effectively. Finches with shallower beaks could not access the remaining food source and died at higher rates.
(a) Calculate the percentage of the population that survived the 1977 drought. Show your work.(b) Identify the selective pressure in this scenario and explain how it caused differential survival.(c) The average beak depth increased from 9.42 mm to 9.96 mm in one generation. Calculate the change in average beak depth. Is this change large in absolute terms? Why does even a small change in average beak depth matter at the population level?(d) Predict: if rainfall returned to normal in 1979 and small soft seeds became abundant again, what would you expect to happen to average beak depth over the next 5 generations? Explain your reasoning using the mechanism of natural selection.
In 1977, a severe drought struck Daphne Major Island in the Galápagos. Small, soft seeds became scarce, leaving mainly large, hard seeds. The medium ground finch (Geospiza fortis) population was studied before and after the drought. Use the data below to answer the analysis questions.
| Year | Average Beak Depth (mm) | Population Size |
|---|---|---|
| 1976 (pre-drought) | 9.42 mm | 1,200 |
| 1977 (during drought) | — | 180 (survivors) |
| 1978 (post-drought, next generation) | 9.96 mm | 890 |
Additional information: Beak depth is a heritable trait. Finches with deeper beaks can crack large, hard seeds more effectively. Finches with shallower beaks could not access the remaining food source and died at higher rates.
(a) Calculate the percentage of the population that survived the 1977 drought. Show your work.(b) Identify the selective pressure in this scenario and explain how it caused differential survival.(c) The average beak depth increased from 9.42 mm to 9.96 mm in one generation. Calculate the change in average beak depth. Is this change large in absolute terms? Why does even a small change in average beak depth matter at the population level?(d) Predict: if rainfall returned to normal in 1979 and small soft seeds became abundant again, what would you expect to happen to average beak depth over the next 5 generations? Explain your reasoning using the mechanism of natural selection.
6Task 6
ALLELE FREQUENCY MODELING — Antibiotic Resistance as Natural Selection
A population of 1,000 bacteria contains two allele variants for a cell-wall protein gene:
- Allele R (resistant): present in 2% of the population initially
- Allele S (sensitive): present in 98% of the population initially
An antibiotic is introduced that kills all S-allele bacteria. R-allele bacteria survive and reproduce, doubling every generation. Assume the population is capped at 1,000 individuals each generation due to resource limits.
(a) Complete the allele frequency table for 4 generations after antibiotic introduction:
(b) Plot the % R allele frequency over generations 0–4 on the graph below: [ Interactive graph / diagram, available in the full lesson ](c) This is an extreme example — the antibiotic kills 100% of sensitive bacteria. In reality, some antibiotics kill 80% of sensitive bacteria per generation. How would the allele frequency graph look different if only 80% of S bacteria were killed each generation? Describe the shape of the curve (do not calculate — reason qualitatively).(d) A hospital uses the same antibiotic for 10 years. Explain, using natural selection vocabulary, why the antibiotic becomes less effective over time. What should doctors do to slow this process, and why does your natural selection knowledge support that recommendation?
A population of 1,000 bacteria contains two allele variants for a cell-wall protein gene:
- Allele R (resistant): present in 2% of the population initially
- Allele S (sensitive): present in 98% of the population initially
An antibiotic is introduced that kills all S-allele bacteria. R-allele bacteria survive and reproduce, doubling every generation. Assume the population is capped at 1,000 individuals each generation due to resource limits.
(a) Complete the allele frequency table for 4 generations after antibiotic introduction:
| Generation | # R bacteria | # S bacteria | % R allele | % S allele |
|---|---|---|---|---|
| 0 (before antibiotic) | 20 | 980 | 2% | 98% |
| 1 | ? | ? | ? | ? |
| 2 | ? | ? | ? | ? |
| 3 | ? | ? | ? | ? |
| 4 | ? | ? | ? | ? |
(b) Plot the % R allele frequency over generations 0–4 on the graph below: [ Interactive graph / diagram, available in the full lesson ](c) This is an extreme example — the antibiotic kills 100% of sensitive bacteria. In reality, some antibiotics kill 80% of sensitive bacteria per generation. How would the allele frequency graph look different if only 80% of S bacteria were killed each generation? Describe the shape of the curve (do not calculate — reason qualitatively).(d) A hospital uses the same antibiotic for 10 years. Explain, using natural selection vocabulary, why the antibiotic becomes less effective over time. What should doctors do to slow this process, and why does your natural selection knowledge support that recommendation?
7Task 7
EXIT SLIP — Novel Scenario Application
Rock pocket mice (Chaetodipus intermedius) live in the Sonoran Desert. Most populations live on light-colored sandy soil and have light-colored fur. However, in areas where ancient lava flows created dark black rock fields, populations of dark-furred rock pocket mice are common. Owls are the primary predator — they hunt by sight at night.
[ Interactive graph / diagram, available in the full lesson ]
Question 1: A new lava flow covers a previously sandy habitat, turning the ground dark. The mouse population living there is currently 90% light-furred and 10% dark-furred. Using the four steps of natural selection, explain what will happen to this population over the next several generations. Use the terms: trait variation, selective pressure, differential survival, heritability, allele frequency.
Question 2: A scientist claims: 'The dark fur mutation appeared BECAUSE the lava flow happened — the environment caused the new trait.' Using your knowledge of natural selection, explain why this scientist's claim is INCORRECT and provide the accurate explanation.
Rock pocket mice (Chaetodipus intermedius) live in the Sonoran Desert. Most populations live on light-colored sandy soil and have light-colored fur. However, in areas where ancient lava flows created dark black rock fields, populations of dark-furred rock pocket mice are common. Owls are the primary predator — they hunt by sight at night.
[ Interactive graph / diagram, available in the full lesson ]
Question 1: A new lava flow covers a previously sandy habitat, turning the ground dark. The mouse population living there is currently 90% light-furred and 10% dark-furred. Using the four steps of natural selection, explain what will happen to this population over the next several generations. Use the terms: trait variation, selective pressure, differential survival, heritability, allele frequency.
Question 2: A scientist claims: 'The dark fur mutation appeared BECAUSE the lava flow happened — the environment caused the new trait.' Using your knowledge of natural selection, explain why this scientist's claim is INCORRECT and provide the accurate explanation.
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