Tuesday's Lesson: Teaching Evolution to Fourth Graders
By Paul Horwitz
The year 2009 marks the 150th anniversary of the publication of Charles Darwin’s The Origin of Species in which he famously described for the first time his theory of evolution by natural selection. Perhaps to mark the occasion, the National Science Foundation recently awarded the Concord Consortium a grant to design curriculum for introducing this difficult concept to elementary students. Try our three Evolution Readiness computer-based activities with your fourth graders and watch plants evolve.
Activity One: The Virtual Greenhouse
The first activity focuses on the differences between various plants and animals and introduces the concept of a species—a group of organisms that can interbreed. Students see several examples of such groups, some familiar (dogs and cats), some not (exotic plants). Students then use a computer model that introduces adaptation—the fact that every organism possesses certain traits that enable it to live in a particular environment.
Figure 1. Students are challenged to figure out which plants grow best in which light level. A healthy plant produces a flower. Unhealthy plants wither before producing a flower.
Students work with three varieties of plants, distinguished by different leaf types and adapted to different light levels. They plant seeds of each variety and watch them grow in five flower boxes that are illuminated by different amounts of light. The challenge is to figure out which plants grow best in which light level (Figure 1).
You can evaluate student inquiry skills as they attempt this challenge. Roam around the room and watch as your students interact with the model. Do they proceed systematically? Do they try every combination of seed and flower box at least once? Encourage students to record their work by taking snapshots, which are stored in a lab book.
Activity Two: The Virtual Field
While the first activity begs the question of how adaptations like the leaves-and-light correlation come about, this next activity provides a partial answer by introducing birth and death. Plants that are maladapted to their environment die; those that survive produce seeds that grow into offspring. Some of the offspring differ slightly from the parent plant.
Students are presented with an outdoor field that varies in light level from top to bottom and told to plant the three varieties of seeds where they think they will grow best. If students have planted their seeds judiciously, they grow and produce a flower. But then there’s a surprise. Winter arrives and the plants die. If any of the plants have produced seeds, there is a good chance that some of these will grow up to be plants capable of producing their own seeds. The bars on a graph move up and down each year showing the number of healthy plants of each type.
Initially, all the offspring plants look exactly like the parent plant. As each of these plants scatters its seeds randomly, some fall outside the plant’s “comfort zone.” These seeds will grow, but the plants they produce wither and die prematurely without producing seeds.
Next, students learn about variation— members of the same species can differ, like puppies from the same litter. When students return to the model of the field, the plant offspring differ, too. (Behind the scenes, we have modified the model to allow for this.) And the different offspring have different optimal conditions for growth.
Students are now restricted to planting only “medium” plants—the ones that grow best halfway up the field because they are adapted to medium light levels. This time, however, because of random variation in the seeds, that single plant type evolves and after many generations produces a full spectrum of plant varieties capable of living everywhere in the field.
Activity 3: Changes in the Environment
Evolution is driven by changes in the environment—changes that usually take place over very long stretches of time. For a typical elementary student, this is a difficult concept.
Figure 2. Students watch mountains grow and affect the environment. Will the plants evolve and adapt or will they go extinct?
The third activity starts with a uniform field where the light level is intermediate between the very sunny and very dark light levels that previously characterized the top and bottom strips of the field. Medium plants thrive anywhere in this field and although their offspring may include a small number of mutations, these rapidly die out, since the environment is not suitable for them. The bar graph, therefore, shows only one bar.
Students can change this peaceful but static environment using radio buttons to control in seconds a process that normally takes place over millennia—the growth of mountains (Figure 2). The mountains grow in a vertical chain in the middle of the screen. As they do, they have a powerful effect on the environments on either side. To the left of the mountains the environment grows steadily darker; on the right, it gets lighter. If this process takes place too rapidly, it results in an ecological disaster, a mass extinction of life on both sides as the plants cannot adapt fast enough. But if the mountains grow more gradually, the plant population on the right slowly evolves toward plants with thin leaves that are adapted to the increased amount of light, while those on the left adapt to the encroaching darkness by evolving toward large leaves.
The process, while seemingly purposeful and almost miraculous, is a natural— indeed inevitable!—consequence of the two properties of the model that the students investigated earlier, adaptation and inheritance with variability. Any model with these properties will behave the same way. But populations of organisms can only evolve in response to environmental changes that take place very slowly, allowing the random variability from one generation to the next to “keep up” with the change. Abrupt changes tend to have disastrous effects on most species. Forcing students to slow the growth of the mountains so as not to kill all their plants provides them with a reference point for understanding how the evolutionary mechanism works.
Elementary students may not be able to appreciate the vastness of evolutionary time or to conceive of how a series of tiny changes can cumulatively produce the dramatic effects we observe in the fossil record. But that’s okay. Our goal is “evolution readiness.” By giving young children a basic understanding of how natural selection works, we hope to prepare them to take the next step on an exciting intellectual journey.