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L06 — The Road

Module: M07 — The Arms Race Periods: 1 NGSS: HS-LS4-2, HS-LS4-5 Lesson Type: Elaborate / Explain

Learning Objective

  • By the end of this lesson, students will be able to analyze data from cliff swallow populations near roads to explain how human-created environmental changes drive rapid natural selection, and evaluate whether the observed changes represent evolution or behavioral adaptation.

Phenomenon / Hook

“Since cars appeared on American roads roughly 100 years ago, millions of cliff swallows have been killed by vehicles. But something strange is happening: the swallows that live near busy highways are evolving shorter, more maneuverable wings. The birds that get hit have longer wings. Evolution is happening on the side of the road — right now, in our lifetimes.”

Show a photograph of cliff swallows on a highway overpass and a graph of wing length data over 30 years.

Materials

  • Chromebooks (one per pair) with spreadsheet access
  • Cliff swallow data set: [[handouts/L06-cliff-swallow-data]] (wing length, road mortality, population counts from 1982–2012)
  • Projector for phenomenon images and data summary
  • Graph paper (backup if Chromebooks unavailable)

Agenda

Opening (7 min)

  • Show cliff swallow highway images + wing length data graph (3 min)
  • Think-Pair-Share: “A bird that can dodge a car needs to turn fast. What kind of wings would help with that? What kind of wings would be bad?” (2 min)
  • Predict: “Before we look at the data, write down: do you think swallows near highways have longer or shorter wings than swallows far from roads?” (2 min)

Explore (18 min)

  • Cliff Swallow Data Analysis (pairs on Chromebooks)
    • Students receive a spreadsheet with 30 years of data from researcher Charles Brown’s study:
      • Column A: Year (1982–2012)
      • Column B: Average wing length of live swallows near roads (mm)
      • Column C: Average wing length of swallows killed on roads (mm)
      • Column D: Number of road-killed swallows found per year
      • Column E: Total swallow population near roads
    • Tasks:
      1. Graph the data: Line graph comparing wing length of live vs. road-killed swallows over time
      2. Identify the trend: Is wing length in live birds increasing or decreasing? What about road-killed birds?
      3. Apply the four conditions of natural selection:
        • Variation: (wing length varies in the population)
        • Inheritance: (wing length is genetic)
        • Selection pressure: (cars on roads — long wings = less maneuverable = more likely to be hit)
        • Differential reproduction: (shorter-winged birds survive to reproduce more)
      4. Calculate: What is the approximate rate of change in wing length per decade?
      5. Critical evaluation: Is this evolution or just behavioral change (birds learning to avoid cars)? What evidence would help you decide?

Explain (10 min)

  • Teacher-led synthesis:
    • Show the key finding: Road-killed swallows consistently had longer wings than survivors. Wing length in the live population decreased significantly over 30 years.
    • Brown’s conclusion (2013): This is natural selection — shorter wings allow quicker takeoff and maneuverability, which helps avoid cars. The selection pressure is cars, not predators.
    • Is it “real” evolution?
      • Yes: the genetic composition of the population changed (shorter wings became more common)
      • This is microevolution — change within a population over a relatively short time
      • It could lead to macroevolution if isolated long enough, but that’s speculative
    • Human impact as selection pressure: Humans create novel environments that species must adapt to — roads, cities, pollution, light, noise. This is a major theme in modern evolutionary biology.
    • Connect to L01–L05: “We’ve now seen natural selection driven by: camouflage (moths), food availability (finches), predation (speed simulation), antibiotics (MRSA), and human infrastructure (swallows). The mechanism is always the same — variation, inheritance, selection pressure, differential reproduction.”

Elaborate (8 min)

  • Human Selection Pressure Brainstorm: Groups brainstorm 3 more examples of human-created selection pressures:
    • Examples to prompt if needed: light pollution affecting moth behavior, fishing regulations selecting for smaller fish, city noise affecting bird song, pesticide resistance in insects
    • Each group picks their most interesting example and writes a 3-sentence explanation using the four conditions
    • Gallery walk: groups post their examples, class circulates and adds a star to the most surprising one

Closing (5 min)

  • Exit Ticket: [[exit-tickets/L06-exit-ticket]]
    1. Explain how cars on highways create a selection pressure on cliff swallows. Use the four conditions in your answer.
    2. A classmate says, “The swallows just learned to dodge cars — it’s not evolution.” How would you respond? What evidence supports your argument?

Differentiation

SupportExtension
Provide a pre-built graph with live bird data already plotted — students add the road-killed data and describe the trendChallenge: Research Charles Brown’s full study. What other traits besides wing length changed? (e.g., swallow social behavior, nesting near roads). Are these genetic or learned?
Give sentence starters: “The live swallows have _____ wings because _____” / “This is evidence of natural selection because _____“Evaluate: Is human-created selection pressure “natural”? Argue both sides. Does it matter whether we call it natural or artificial?

Assessment

  • Formative: Check data analysis sheets — are students correctly identifying the trend and applying the four conditions?
  • Exit Ticket: Selection pressure explanation + evolution vs. behavior argument

Teacher Notes

  • This lesson is intentionally the synthesis point. By now students should be able to independently apply the four conditions to a new example. If they can’t, spend extra time on the explanation phase.
  • The “is it evolution?” debate is valuable. Students often think evolution requires millions of years. This example shows it can happen in decades.
  • The Charles Brown study is real, published, and well-documented. Use the actual citation to model how science communicates: Brown, C. R. & Brown, M. L. (2013). “Where has all the road kill gone?” Current Biology, 23(6).
  • Bridge to L07: “We’ve built the mechanism across 6 lessons. Tomorrow, you prove you can use it — in any context, on any organism. The big picture.”