Mapping the Tree of Life: Phylogenetic Reconstruction (Part 3/3)
Lesson 3 of 5
Objective: Students will explain how scientists develop phylogenetic trees.
It's time for students to hone their scientific thinking by examining two opposing explanations of evolutionary relationships. Here is what they will learn in today's lesson.
Open class by playing the first minute of the Natural History museum video.
Remind students that this is the third and final day of the Build a Museum Challenges. Today they are going to compare their cladograms with their classmates and with a cladogram that a "scientist" has developed.
Have student groups or individual students share the cladograms they created in Challenge 3. Question students about their thinking process in determining which phyla had a more closely related common ancestor. Encourage students to question each other where their cladograms differ and where they are the same.
Possible questions to help students explain their thinking include:
- What types of characteristics are shared by some of the animals, but not all of them. (Vertebrae, 4 limbs or appendages, land movement, amniotic egg, flight, fur/hair, production of milk)
- Why did they pick characteristics they did?
Note: It is important for both teacher and students to understand that scientists have approximately three phylogenetic trees that are currently accepted as possible explanations of common ancestry for mammals. The purpose of these lessons is to have students explore them. Students will naturally come up with one of the three phylogenetic trees. It is a good exercise for them to practice their Claim, Evidence, and Reasoning skills. The teacher should act as the facilitator for the discussion and let student naturally explore the possible cladograms that the students developed.
Show students the cladogram on Challenge 4. As a class, create a class chart that explains how this cladogram was developed. Consider all shared, derived characters.
This activity is important to help students understand how phyla branch from the ancestral line. Students often misunderstand Darwin's concept of descent with modifications. Allowing students the chance to practice by comparing models allows them to see that the modification is a derived character that branches from the ancestral line. A common misconception is that one phylum morphs into another phylum.
Once the class chart is made, give them several minutes to compare their cladogram with it. Have them answer the questions on the Challenge 4 worksheet.
Using their worksheet responses, encourage students to discuss where their cladograms differed from the sample cladogram shown on Challenge 4. Most students' cladograms will differ from the sample cladogram. In fact, some scientists agree that mammals should be the phyla that are shown last on the cladogram, while others are convinced of a polyphyletic origin of mammals and birds.
Further refine student understanding of how to interpret branches of cladogram by discussing how birds are more closely related to mammals than mammals are related to reptiles. Help students see that the location of the labels is not an indication of relatedness. More related living things are shown by a more recent fork. The closer the fork in the branch, the closer the relationship. Watch my description of how to do this with students!
Have students sketch which cladogram that shows relationships among phyla they currently accept. They should write a response in their lab notebook that explains their reasoning. They should support their argument with three evidentiary statements. Students should turn in their lab notebooks for evaluation at the end of the hour.
For homework, students will view a flipped lecture about systemics and phylogeny. They will answer the following question on a Google form.
The diagram shown in the lecture; recognizes Taxa 1, 2, & 3 as examples of "monophyletic", "polyphyletic", and "paraphyletic" groups, respectively.
- In the diagram, identify two additional examples each of groups that would be considered "holophyletic," "polyphyletic," and "paraphyletic".
- Identify all possible "holophyletic" taxa.