Darwin's Model of Natural Selection:
Learning outcomes related to modeling:
Time Frame and Sequence
This activity can be completed in one class period if students do the first part as a homework assignment.
The primary goal of this discussion is to involve students in generating the criteria for an acceptable explanation using the Darwinian model. Earlier in the unit they should have read an excerpt from the Origin of Species and discussed the mechanism of natural selection (Material 2D). This activity will provide an opportunity for them to use the mechanism as laid out in the reading to explain a particular adaptation.
Prior to Day One
One or two days before this activity, hand out the assignment called Camouflage Case. This assignment asks students to develop a Darwinian explanation for the cryptic coloration of a particular species of lizard. There are minimal guidelines for students on this task because a major part of the activity is for students to develop the criteria for a good explanation on their own.
Begin the period by asking students to take out their lizard explanations. At this point, direct students to work in groups to compare their explanations. Because they worked on these individually and without much direction, their answers will probably vary quite a bit. Tell students that there are two goals for them to consider as they work. The first goal is to work together to come up with a complete explanation for the scenario. To do this they should draw on the strengths of all the explanations presented by members of their group. The second goal is for them to generate a list of criteria for an acceptable explanation. This list should be worded more generally and it should provide guidelines for writing an acceptable explanation. As the students work, walk around and listen to their discussions.
After about 15-20 minutes of discussion, students should have a fairly complete list and an explanation they are happy with. Call the class back together as a whole group so that they can share their ideas. Begin by having one or two groups read their composite lizard explanations. Ask students to comment on these explanations by either complementing important aspects, or pointing out areas that are weak or absent.
Next, ask the students to help compile a class list of criteria for an acceptable explanation. One way to organize this discussion in order to have all groups involved is to have each group share one thing on their list. Once you have gone around the room once, ask if there are ideas that groups had that are not on the list. In this way a fairly complete list should be generated.
Teaching Strategies and Student Ideas
This activity begins to set the stage for using the Darwinian model in later parts of the unit. We have found that it is important to involve students in thinking about how the model should guide the development of explanations rather than simply providing guidelines for them to follow. Now that students have a clear understanding of the argument made by Darwin and how that compares to those made by Paley and Lamarck they are ready to begin to examine the Darwinian model more completely. The lizard scenario provides a concrete example that will be helpful in discussing the more abstract components of the Darwinian model. One way to frame this task is to ask students to try to provide guidelines for using the Darwinian model for someone without the background they have had.
Let the Darwinian model handout guide you as you look for key student ideas to highlight. If the students are struggling, you may want to give them a few examples to start with. For instance, based on their earlier reading of Darwin, they should know that variation is an important part of his argument. Therefore, a complete Darwinian explanation needs to include reference to the variability of traits. Once students have a few such examples of criteria in hand they may find it easier to come up with some of their own.