• Approximately 2,000 small, uniformly shaped objects (kernels of corn, dried beans, wooden markers, plastic beads)

  • 10 paper cups or small beakers

  • A 250-ml or 400-ml beaker  

Instructional Strategy

Engage Initiate a discussion on human population with such questions as: How long have humans been on the earth? How do you think the early rate of human population growth compares with the population growth rate today? Why did this rate change?

Tell students that this investigation represents a model of population growth rates.

Explore Have student groups complete the following activities.

  • Place the glass beakers on their desks. Begin by placing two objects (e.g., corn or plastic beads) in it. The beaker represents the limits of an ecosystem or ultimately the earth.

  • Place 10 cups in a row on their desk. In the first cup, place two objects. In the second cup, place twice as many objects as the first cup (four). Have students record the number of objects on the outside of the cup. Continue this procedure by placing twice as many objects as in the former cup, or doubling the number, in cups 3 through 10. Be sure students record the numbers on the cups.

  • Take the beaker and determine its height. Have students indicate the approximate percentage of volume that is without objects. Record this on the table as 0 time.

  • At timed intervals of 30 seconds, add the contents of cups 1 through 10. Students should record the total population and approximate percentage of volume in the beaker that is without objects.

  • Students should complete the procedure and graph their results as total population versus results.  

Students may question the need for the 30-second intervals. The length of the time interval is arbitrary. Any time interval will do. Preparation of the graph can be assigned as homework.

Range of Results

The mathematics involved in answering the questions may challenge some students. Assist students when necessary to enable them to accomplish the objectives of the investigation. Table 1 shows the population and the percent of the beaker's volume without objects. A typical student graph is shown in Figure 1.

Explain Ask the students to explain the relationship between population growth and biological evolution in populations of microorganisms, plants, and animals. Through questions and discussion, help them develop the connections stated in the learning outcome for the activity. Evolution results from an interaction of factors related to the potential for species to increase in numbers, the genetic variability in a population, the supply of essential resources, and environmental pressures for selection of those offspring that are able to survive and reproduce.

Elaborate Begin by having students explain the results of their activity. During the discussion of the graph, have the students consider some of the following: Are there any limitations to the number of people the earth will support? Which factor might limit population growth first? How does this factor relate to human evolution? Are

Table 1 Population growth

Time Internal

Population

Percentage of empty volume (400-ml beaker)

0

2

99%

1

4

99%

2

8

99%

3

16

98%

4

32

97%

5

64

95%

6

128

93%

7

256

88%

8

512

80%

9

1024

70%

10

2048

50%

11

4096

0%



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