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The Weeds That Came Back: Selection on the Soccer Field

Grade 8 natural selection practice: compute percents from five years of weed killer tests and explain why resistant weeds became more common.

Grade 8ScienceInteractive · Printable

Aligned toMS-LS4-6

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What’s included

Single worksheet
Auto-graded online
1 NGSS standard
Printable answer key
11 questions
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Details

Format: Interactive · Printable
Grade: Grade 8
Subject: Science
Question types: Fill in the blank, Multiple choice, Sorting, Short answer

About this worksheet

What students practice

Students calculate the percent of a population with a trait from counts over several generations.
Students use data to explain how natural selection increased a trait in one population but not another.

How to use

Read the passage and compare the two fields as a class before students calculate; have calculators ready for the percent questions.

Curriculum

NGSS standard covered

MS-LS4-6

Use math and data to explain how natural selection changes the share of a trait in a population over time.

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The Weeds That Came Back: Selection on the Soccer Field

Use five years of weed counts to explain how a trait spread through a population.

Part 1: Five years of counts

Read the passage and the table, then answer the questions.

Why the Weeds Came Back

1Westbrook Middle School has two grass fields: a game field and a practice field. Both are planted with the same turfgrass, and both have a low, tough weed called goosegrass. Goosegrass is an annual plant. Each plant sprouts in spring, makes seeds in late summer and dies in the fall, so each summer's plants are a new generation that grew from seeds of earlier plants. A single goosegrass plant can make thousands of seeds, and some seeds stay in the soil for a few years before they sprout.

2Starting in June 2019, the groundskeeper sprayed the game field with the same weed killer at the same strength every June. The practice field, on the far side of the school, was mowed but not sprayed. The groundskeeper, Mr. Reuben Castellano, checked that the sprayer was set the same way each year. By 2022, soccer coach Alma Rydberg noticed that more goosegrass was still standing on the game field a week after the spray, and she asked the eighth grade science class to look at the school's records.

3The records came from a test the school had run every May since 2019, before that year's spray. The groundskeeper dug up 200 young goosegrass plants from random spots on each field, put each in its own pot with the same soil, and sprayed all of them once with the weed killer in a fenced work area. Two weeks later, students wearing gloves counted the plants that were still alive and green. A plant that survives this test is called resistant.

4The class, led by Mr. Desmond Achebe, ran two more tests in 2023. First, they collected seeds from game-field plants that had survived the pot test and seeds from practice-field plants that had never been sprayed. They grew 100 seedlings from each group in identical trays, and the groundskeeper sprayed them all the same way. The graph shows the results.

5Second, they checked whether a plant becomes resistant after it survives a spray. The groundskeeper sprayed 50 practice-field plants with a weak dose, half the normal strength, which did not kill them. A month later, he sprayed the same 50 plants at full strength. Three of the 50 plants survived.