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MUSE | Natural Selection | Unit Overview and Materials | Section 4: Extending the Natural Selection Model... | 4A: Monarch/Viceroy Case | Instructional Notes

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INSTRUCTIONAL NOTES


Learning Outcomes

Natural Selection:

  • Populations are made up of individual organisms whose traits (structural, behavioral, biochemical) exhibit variation from one individual to the next

  • Many trait variations are inherited

  • Some trait variations may be advantageous and others not advantageous

  • Whether or not a variation is advantageous depends on the environment within which a population lives

  • The frequency of particular trait variations in a population may change over time due to differential survival and reproduction

  • Evolutionary changes occur in populations or species, not individuals

  • Offspring are not exactly like their parents, but the variations of their traits are likely to be similar to those of their parents

  • Variability in a population can be represented graphically

  • Organisms produce more offspring than can survive

  • There is competition between organisms for resources

Modeling:

  • Models are ideas that scientists use to explain patterns they see in the world
  • A model is an idea that has predictive and explanatory power and is consistent with other ideas about how the world works
  • Knowledge claims are based on data, inferences, and prior knowledge and beliefs

General:

  • Use of classroom norms (basic interpersonal skills)
  • Make observations
  • Organize data

Supplies
  • Monarch/Viceroy case materials
  • File folders to hold student work
  • Student handouts (readings) on warning coloration and mimicry
  • Discussion questions on warning coloration and mimicry

Time frame and sequence

Five 45 minute class periods.

The monarch/viceroy case is the second of three case studies that are part of this unit. Like the seed case (case one), students should work in groups to develop their explanations. The total time needed for this student activity will vary depending on the amount of time students are given to work in class. Typically, we allow 5 days. The first three days are primarily work days and the last two are used for discussion.

Day One

Begin by explaining to students that they will be working on a second case that will be similar in some ways to the seed case. Pass out the page called Extending the Use of the Darwinian Model to the whole class. This page introduces the task for the Monarch/Viceroy case. Go over this page with your students pointing out that while the task is similar to the Seed case in that they will be using the Darwinian model to explain phenomena, the product they develop will be different. The final product for this case will be a written paper that has two parts. The first is a Darwinian explanation for the similarity in color between the two groups of butterflies. The second part of the paper will be a discussion of the data that was used in the construction of the explanation. Warn them that the amount of data in this case is much less than they had available to them for the seed case. Connecting data to specific claims in their explanation may be difficult for students. Remind them of the work they did with scientific arguments at the beginning of the unit and ask them to tease apart the argument they will be making (their Darwinian explanation) into the data, inferences, and prior knowledge and beliefs that support it.

A second way that this case differs from the seed case is that the adaptive advantage of bright coloration may not be immediately obvious to students. It is also complicated by the fact that they are asked to explain the similarity in color between two different organisms. We consider application of the model to this scenario to be an extension because at first look it appears to actually go against the ideas of Darwin. Students recognizing that even something as seemingly maladaptive as bright coloration can arise as the result of natural selection is an important goal of this case.

Once students have been introduced to the task, hand out the Case Materials and have students get into their groups to begin work. They should have a significant portion of the class period to look over the case materials and develop a work plan. We only make one copy of the case materials per group because of the expense of the color copies. We pass the materials out in a file folder and designate a place in the class where groups should keep their case folders and the work in progress. This prevents the problem of having one student take home the work of the whole group and forget to bring it back the following day. Generally, students will spend the first 10 to 15 minutes looking over materials. We have seen several different strategies employed by different groups to become familiar with the materials. Some may choose to read each page aloud while other groups spend the time reading silently. You may want to point out that the pages can easily be read in any order. Alternatively, make black and white copies of the case for each student and pass them out for students to look over the night before as a homework assignment.

Days Two and Three

These two days are basically work days for student groups. Begin each day with an update on when you will be concluding the work on this case. It may be useful to provide guidelines to students in terms of their progress. For example, you may tell them that they should have a draft of both parts by the end of day two and use day three to edit.

While the students are working, walk around and listen in on their conversations. It is important that you interact with the groups as a co-collaborator and not in the more traditional role of a teacher making sure they are coming up with the "right" answer. This is a challenging task and one that takes practice. It is possible that groups will develop explanations that are not the typical warning coloration and mimicry ideas you may expect to see. As long as the groups are applying the natural selection model in ways that are true to the model components and the underlying assumptions of the model this should not be a problem.

As you walk around and speak to groups ask them questions that will help them write the second part of their written paper. When they tell you what they are thinking of as the explanation, ask for the evidence. Remember that students will be struggling and may become frustrated. Give them encouragement when they need it, but try not to do much telling. The purpose of these case studies is for students to have practice using the powerful ideas of Darwin and not that they learn the particulars of the monarch/viceroy interaction (although this is a likely by product).

Days Four and Five

These two days should be used for whole class discussion of the case and the explanations that the students developed. The class periods can be structured as a roundtable discussion. Each group is asked to bring enough copies of their two-part paper to share with another group and one to turn in. At the beginning of the class period, pair each group with another group and have them trade explanations. Collect the remaining copy. Allow each group of students to spend approximately fifteen minutes reading and discussing the written material of their partner group.

Call the whole class together to begin the discussion. At this point, each student will be familiar with two explanations: his or her own and that of one other group. Begin the discussion by having students suggest a particularly clear explanation to be read aloud to the group. This will help get the conversation started and provide a focus for the discussion. Once one explanation has been read, ask the students to compare it to their own and the one that they read at the beginning of the hour. Specifically, direct their attention to the components of the Darwinian model and ask if there were any differences in the way they applied components of the model to this scenario. The most common difference will most likely focus on the adaptive advantage of the bright coloration. Because they are required to discuss the similarity in color between two species of butterfly, they may differ from group to group in terms of which they think evolved first or how bright coloration is advantageous for either the monarch or the viceroy. Once this discussion has run its course, direct their attention to the data, or evidence, used by each group to back up their explanations. We have found this portion of the discussion to be the most lively as students rarely place the same importance on all of the data available.

This conversation may take anywhere from one to two class periods. Once the discussion seems to be complete, have the students read the Articles that describe warning coloration and mimicry and the controversies surrounding these ideas in evolutionary biology. There are Discussion Questions that accompany this reading for the students to consider individually or in groups.


Teaching Strategies and Student Ideas

Unlike the seed case, this case is based on real organisms, therefore, the amount of data available in the case materials is limited. Students will notice this right away when they see that the Monarch/Viceroy case is significantly less data-rich (and much shorter) than the seed case. For example, there are no descriptions of ancestral organisms and there are no reports on experiments or research. Another way in which this case differs from the seed case is that it is asking students to apply the natural selection model to a situation which may not seem to be consistent with the assumptions of the Darwinian model. Whereas the advantage of the different seed characteristics was intended to be relatively clear, the bright coloration of these two butterflies may seem at first to be a disadvantage. The challenge will be for students to develop an explanation for the similarity in color between these two species of butterflies. It should become apparent to them that they must define a starting point for their explanation. If they propose that the similarity in color is a consequence of the interaction between these two groups, then they will have to decide which evolved first and provide an explanation for the initial change. If some students don't know where to start, it may be necessary to ask them to consider which organism they think evolved the bright coloration first.

At times it may be difficult to monitor the progress of all the groups in one class. One strategy to help with this could be to tell students to leave their drafts in the classroom for comments from you. It may be necessary to define progress points if students tend to waste classroom work time. A reasonable timeline is for them to have an outline of their Darwinian Explanation after day one, drafts of both parts at the end of day two, and fairly polished responses at the end of day three. The feedback students receive (both written and oral) will be important in shaping their final product. You may also want to schedule time for peer review and critique.


Because this is an example of warning coloration and mimicry that is frequently used in textbooks, some students may be familiar with it. This should not be a problem as the task for students is to apply the natural selection model and create an explanation that follows the narrative structure developed earlier in the course. They also need to bring specific pieces of data from the case materials to bear on their argument and therefore, even if they are familiar with this example they will still be required to think about it carefully.

Typically, during this case, most if not all student groups develop explanations that are consistent with the ideas of warning coloration and mimicry. One area where there is significant variation in the explanations, however, concerns the starting point. Some groups may develop an explanation that begins with the monarch evolving the ability to ingest milkweed, followed by the evolution of monarch and then viceroy coloration, while other explanations may begin with the assumption that the monarch was already brightly colored and simply discuss the evolution of the viceroy coloration. The task description is intentionally non-prescriptive on this point. Some very interesting class discussions may revolve around defining a starting point when discussing evolutionary events. These discussions will serve to deepen student understanding of the ongoing process of change shaped by natural selection, therefore it is desirable to have some variation in this regard from group to group.

The most difficult portion of the assignment is for students to write explicitly about the evidence they have used in support of their argument. Emphasize the importance of using the Darwinian Model Handout as a tool for this task. Advise students who are struggling to return to each portion of their Darwinian explanation and describe the data from the case that supports the statements made. There may be some cases where they have made an assumption and it is not supported by any one piece of information in the case. For example, they may have mentioned variation in the population even though there is no information on variability in the case. In this instance you can point out to students that they have based their assertion about variation on their prior knowledge about variation in living populations. The important thing is to help students examine their own arguments so that they can think explicitly about the claims they have made and the evidence for those claims. These skills will strengthen their own arguments and also help them develop the ability to be critical of the arguments of others.

 

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