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L05 — The Chase

Module: M07 — The Arms Race Periods: 1 NGSS: HS-LS4-2, HS-LS4-4 Lesson Type: Elaborate / Explore

Learning Objective

  • By the end of this lesson, students will be able to explain how predator-prey dynamics create reciprocal selection pressures by simulating a chase scenario and analyzing how speed evolves in both populations.

Phenomenon / Hook

“Cheetahs can run 70 mph. Gazelles can run 60 mph. Neither species existed 10 million years ago. So who got fast first — the cheetah or the gazelle? And why does it matter?”

Pose the question: “If you were a slow gazelle, what happened to you? If you were a slow cheetah, what happened to you? This is an arms race — and it’s been going on for millions of years.”

Materials

  • Open space (gym, hallway, or outdoor area)
  • Cones or markers (to mark boundaries)
  • Stopwatches (one per group)
  • Measuring tape
  • Data recording sheet: [[handouts/L05-chase-data]]
  • Colored pinnies or bandanas (predators vs. prey)

Agenda

Opening (5 min)

  • Cheetah-gazelle phenomenon (2 min)
  • Think: “Is being the fastest always the best? What are the costs of being fast?” (2 min)
  • Quick framing: “In nature, speed is expensive. It takes energy. It requires bigger muscles. Sometimes being sneaky or having good camouflage is better than being fast. Today we’re going to simulate what happens when speed is the trait that matters.” (1 min)

Explore (20 min)

  • Predator-Prey Speed Simulation (outdoor or gym)
    • Split class into predators (1/3) and prey (2/3)
    • Setup: Mark a 20x20 meter square. Prey start at one end, predators at the other.
    • Round 1 — Baseline:
      • On “go,” predators chase prey. Tagged prey are “caught” and sit out.
      • After 30 seconds, count surviving prey.
      • ALL survivors reproduce (stay in). ALL caught prey are replaced by new “offspring” — but here’s the twist:
      • Prey offspring inherit speed: Caught prey’s replacements are SLOWER (they walk, not run). Surviving prey’s replacements are FAST (they run full speed).
      • Predator offspring inherit speed: Predators who caught nothing get SLOW replacements. Predators who caught prey get FAST replacements.
    • Round 2–5: Repeat with inherited speeds. Watch what happens.
    • Data collection: After each round, students record:
      • Number of prey caught
      • Number of predator-successful catches
      • Observations about speed changes
    • After 5 rounds, stop and debrief in groups (3 min):
      1. What happened to prey speed over 5 rounds?
      2. What happened to predator speed over 5 rounds?
      3. Did one side “win”? Why or why not?
      4. What are the costs of being fast? (Energy, injury risk, etc.)

Explain (10 min)

  • Teacher-led synthesis:
    • Reciprocal selection: Prey get faster → predators must get faster → prey must get even faster. This is an arms race — the title of our module.
    • Trade-offs: Speed costs energy. A cheetah that sprints and misses may starve. A gazelle that sprints constantly has less energy for reproduction. Natural selection doesn’t optimize one trait — it balances many.
    • Equilibrium: In nature, arms races reach a stalemate. Neither side “wins” permanently. This is why cheetahs are fast but not infinitely fast.
    • Other strategies besides speed: camouflage, burrowing, herding, venom, armor — evolution doesn’t always mean “faster.”
  • Students add to their data sheets: “Arms race = reciprocal selection pressure between predator and prey”

Elaborate (8 min)

  • Trade-Off Analysis: Groups receive a card describing an organism and must identify the trade-off:
    • “A poison dart frog is brightly colored and extremely toxic. What’s the trade-off?” (Warning coloration: visible to predators but deadly to eat)
    • “A porcupine has 30,000 quills. What’s the trade-off?” (Defense but slow and clumsy)
    • “A cheetah is the fastest land animal. What’s the trade-off?” (Speed but low stamina, high energy cost)
    • Groups present their trade-off in 30 seconds. Class discusses: “Why doesn’t evolution just make everything perfect?”

Closing (5 min)

  • Exit Ticket: [[exit-tickets/L05-exit-ticket]]
    1. Explain what “arms race” means in evolutionary terms. Use an example from today’s simulation.
    2. Why can’t cheetahs just keep getting faster and faster? What limits them?

Differentiation

SupportExtension
Assign students specific roles: data recorder, timer, counter — so they focus on one task during the fast-paced simulationChallenge: Research one real predator-prey arms race (e.g., rough-skinned newts vs. garter snakes — toxin resistance). What trait evolved in each species?
Provide a pre-made data table with round numbers and column headersAsk: What if the environment changed — say, the prey’s food became scarce and they got weaker? How would that affect the arms race?

Assessment

  • Formative: Group debriefs after simulation — listen for understanding of reciprocal selection and trade-offs
  • Exit Ticket: Arms race explanation + trade-off reasoning

Teacher Notes

  • Physical space matters. If you can’t go outside, push desks to the walls and use the classroom. It’s tight but works.
  • The “inheritance” mechanic in the simulation is simplified but effective. The key is that students feel the arms race happening — each round gets faster.
  • Safety: no tripping hazards. Remove backpacks. Mark clear boundaries.
  • The trade-off discussion is where the deep thinking happens. Push students past “evolution makes things better” to “evolution makes things work given the constraints.”
  • Bridge to L06: “We’ve seen selection in the wild, in labs, in simulations. But what about evolution happening on the road — literally? Next lesson: birds evolving to dodge traffic.”