Coevolution Case Study

Created by Laura Conner, Susan Hester, Anne-Marie Hoskinson, Mary Beth Leigh, Andy Martin ,and Tom Powershis, and contributed by Yale University's Center for Scientific Teaching, this case study lesson reinforces the concept of coevolution and gives students practice with the analysis and interpretation of data.

The case study is designed for a single 50-minute class period after students have completed a brief pre-class reading assignment introducing coevolution.  Evidience for interactions among red squirrels (Tamiasciurus hudsonicus), red crossbills (Loxia curvirostra), and lodgepole pines (Pinus contorta v. latifolia) are analyzed.  The case describing the interactions among these species invites students to answer three questions:  1) What is required for demonstrating coevolution?  2) What specific evidence supports the conclusion that that red squirrels, red crossbills, and lodgepole pines are coevolving (or not)?  3) Why does the evidence support coevolution?

Learning outcomes: Interpret and draw graphs;  Evaluate evidence about whether two species are coevolving; Make testable predictions based on the hypothesis that two species are coevolving. (text from the Center for Scientific Teaching at Yale's Teachable Tidbits.)

Below is a sample sequencing of this lesson:

Time (min)

Learning Outcome(s)

Activity/assessment

Explanation, notes, suggestions, tips

Preclass

 Define coevolution

Brief reading on coevolution

 

Introduction

10 minutes

 

State conditions under which coevolution occurs

Lecture/class discussion

The instructor should review two necessary conditions for coevolution to occur: geographical overlap between species, and reciprocal change in traits. The instructor should briefly elucidate this definition, then invite students to view a video of a cheetah chasing a gazelle. Instructors can ask students to consider how co-evolution is shown in the video, and to consider what happens when cheetahs get faster. The instructor should ask students for their ideas.  The fundamental concepts that students should notice are that speed is under strong directional selection in each species, and that the players themselves (populations of cheetahs and gazelles) are the selective agents for changes in this trait. Instructors should make sure to emphasize the population-level interaction between cheetahs and gazelles in this and future coevolutionary interactions.

Introducing the case study

7 minutes

Lecture/class discussion

The instructor should invite students to consider a more complex system of 3 species: crossbills, squirrels, and lodgepole pine cones.  After briefly describing the natural experiment that exists in the Rocky Mountains, the instructor should step through the plot/map handout and the relationship map handout with the entire class, clarifying the instructions about how the students should draw the arrows between species based on their evidence.

Group evaluation of evidence

10-15 minutes

Interpret and draw graphs.

Evaluate evidence about whether two species are coevolving.

Case study: small group data analysis

Students should divide into small groups of 3-5. Each group should get one of the three handouts (Exhibits 1-3) containing data supporting a directional interaction between 2 of the 3 species. A minimum of three groups is required so that each group can consider one data set. In a large class, multiple groups can receive the same data set.

Students are asked to work with the data and consider what the evidence shows. They will work with the relationship map and decide whether the data set they have been given (Exhibits 1, 2, and 3) indicates a relationship between any of the species. Each group will draw a directional arrow connecting the two species represented in their Exhibit, indicating which species is impacting the traits of the other.

During this time, the instructor and TAs/learning assistants should circulate among groups, asking and answering questions and gathering formative feedback about how the process is going.

The instructor should draw the relationship map on the board without the arrows while the students are working on the task.

Class discussion (10 minutes)

Interpret and draw graphs

Evaluate evidence about whether two species are coevolving

Interactive discussion of group work and synthesis

When the groups have finished examining their Exhibit and determining the relationship and direction of interaction, the instructor should invite a member of each group to draw an arrow on the map on the board, indicating which relationship their evidence supported. While the instructor displays each Exhibit in the accompanying PowerPoint slide, a member of each group should describe the relationship and what impact was found (e.g., “in areas where squirrels occurred, pine cones became…”). The instructor should guide the conversation as necessary, focusing on what the evidence shows, rather than on what could be assumed or inferred.

When the map is complete, there will be bidirectional arrows between pinecone and crossbill, but only a uni-directional arrow between squirrel and pinecone.

Clicker question

3 minutes

Evaluate evidence about whether two species are coevolving

Explain how data supports conclusions

Clicker question

Class discussion

Pair-wise discussion

The instructor should ask the students to consider the completed relationship map and present the clicker question asking students to conclude which species have a coevolutionary relationship according to the data presented.  If there is not agreement about the answer (b) after polling the students initially, the instructor can ask the students to turn to a neighbor and try to reach a consensus. The instructor should re-poll the students, then ask a volunteer to explain his or her reasoning for the answer (co-evolution requires reciprocal change in traits).

 

Class discussion

5 mins

Identify types of evidence that would help determine whether two species are currently in a coevolutionary relationship

Whole class interactive discussion

The instructor then asks whether or not students can say for sure if the squirrels and the pinecones have a co-evolutionary relationship (no—there is no evidence to evaluate for whether the pinecones are impacting squirrel traits). The instructor can then lead students in a discussion about what kinds of evidence would be necessary to support a co-evolutionary relationship between squirrels and pinecones.

Optional extension

10 minutes

Make testable predictions based on the hypothesis that two species are coevolving

Individual graph generation-prediction

Class discussion

The instructor can end here, or as an extension, could ask students to generate a graphical prediction based on the hypothesis that the evolution of squirrel jaw musculature has been affected by pinecones. After giving students a chance to generate their predictions, the instructor can call on some students to describe their predictions or draw them on the whiteboard and explain their reasoning. The instructor should then present evidence that squirrel jaw musculature has, in fact, been impacted by pinecones (Exhibit 4).

Additional teaching resources are uploaded below.  

Handouts and examples of student work are available here.

This activity was contributed by Yale University.

coevolution_case_study_and_teaching_notes.doc57 KB
coevolution_case_study_framework.docx44 KB
coevolution_in-class_presentation.ppt857 KB