Studying Living Organisms

Estimation of Population Growth by Counting Offspring: Seed Multiplication

Judith K. Wood
Woodrow Wilson Biology Institute

Target age or
ability group:
Biology I-any level.
Class time
15-30 minutes.
Materials and equipment: Picture of a multi-seeded fruit such as tomato, pepper, squash, watermelon, etc., or the fresh fruit.
Summary of activity: This activity allows students to discover or reason through a portion of Darwin's theory of natural selection. Students will be able to hypothesize/calculate the reproductive rate of a population, realize that such large populations do not exist and conclude that organisms face a constant struggle to survive.
Prior knowledge, concepts or vocabulary necessary to complete activity: Multiplication and graphing skills

Teacher Instructions

A copy of a drawing of a multi-seeded fruit is necessary for each student or group. Alternatively, have each group remove all the seeds from a fresh fruit. Each group follows the directions on the student page(s). The ultimate goal is to have students "discover" Darwin's two observations and, from that, draw the conclusion that offspring must struggle for existence.

Estimation of Population Growth by
Counting Offspring: Seed Multiplication

1. Count and record the number of seeds in the above fruit.

2. If every seed in this fruit germinated, how many plants would there be in this second generation?

3. If each of these plants produced ten fruits, how many new fruits would be produced?

4. If each of the second generation plants produced 100 seeds, how many new plants would be produced in this third generation?

5. The original fruit would have produced how many new plants by the third generation?

Draw a graph of the generation number (on the x-axis) vs. the number of offspring (on the y-axis) for the first three generations.

6. What general conclusion can be drawn from this in terms of parents and offspring?

7. Do all of the seeds germinate and all of the plants produce the expected number of fruits?

8. What is the usual relationship between number of parents from generation to generation?

Darwin made two observations:

  • a.the number of offspring produced exceeds the number of parents
  • b.the number of parents remains constant from generation to generation

9. How many parents were present in the first generation?

10. Based on Darwin's observations, how many parents would be present in the second generation? Does this agree with your calculations?

11. Assuming Darwin's observations are more correct (in the real world) than your calculations, what are some possible reasons for the difference?

12. Using Darwin's two observations, what conclusion can you draw?

  • Darwin came to the conclusion that the offspring must struggle for existence.

13. How does your conclusion compare to Darwin's?

Darwin was influenced by the following:

Through the animal and vegetable kingdoms, nature has scattered the seeds of life abroad with the most profuse and liberal hand. She has been comparatively sparing in the room, and the nourishment necessary to rear them. The germs of existence contained in this spot of earth, with ample food, and ample room to expand in would fill millions of worlds in the course of a few thousand years. Necessity, that impervious all-pervading law of nature, restrains them within the prescribed bounds. The race of plants, and the race of animals shrink under this great restrictive law. And the race of man cannot, by any efforts of reason, escape from it. Among plants and animals its effects are waste of seed, sickness, and premature death. Among mankind, misery and vice.

-Thomas Malthus (1798)

On to Mimicry: An Example Of Adaptation
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