Building a cladogram (Part 2/3)
Lesson 2 of 5
Objective: Students will create a cladogram and describe methods used to reveal phylogeny.
Now that students are beginning to support their ideas with scientific evidence. Here is what they will learn in today's lesson.
Discuss the main idea from yesterday's lesson depicted in the bulls-eye graphic made from the data that was collected using the results of the one sentence summary CAT. Students should understand that Aristotle classified living things by habitat, while Linnaeus classified living things by anatomy, habitat, and behavior.
Begin discussing how modern taxonomists think today by describing a tools that taxonomists specifically the cladogram. First show Paul Anderson's video about Cladograms.
(Note: Typically I show a clip from 0:00-3:20. This video demonstrates the skills students will practice in the student activity. Later in the video, Anderson explains some of the guidelines taxonomists use when constructing cladograms.)
Then help the students determine the four skills that must be used to construct a cladogram. They are as follows:
1) Choose an outgroup.
2) Determine the characteristics to use.
3) Determine the polarity (inclusiveness or importance).
4) Divide animals into groups by characteristics.
Hand out Challenge 3: Building a cladogram to the students. Explain that each of the tasks correspond with the skills that were listed above. Give the students the specimen cards from yesterday and allow them a minute to determine the outgroup. Students should write their answers on top of today's Challenge (i.e.: OUTGROUP=Salmon or Fish).
Once the outgroup has been selected, allow the students to move into the lab groups. Within their groups, have the students look at the remaining specimens and brainstorm the characteristics that these living things have in common. Next, have students rank the items in order of importance (known as polarity to taxonomists). For example, which of the characteristics do most of specimens have in common. Move around the room and help student refine their thinking. If students get stuck or confused in their thinking, then ask students the following questions:
- What characteristics do all but two have in common?
- What characteristics do all but three have in common?
Allow the students to continue this line of questioning until all but one living thing is sorted. Then allow student groups the time to draw their cladogram on their paper.
(Note: Students need to know have to create cladograms to help them understand the thought process of taxonomists and evolutionary biologists. Creating a cladogram of their own helps students determine phylogenetic relationships and ultimately common ancestry.)
Once students have a sample cladogram constructed, bring students back together to discuss their ideas. Have them share the categories by which they determined their characteristics and have them explain what thought process they used to determine the polarity of their characteristics.
(Note: Some characteristics that students might mention are four limbs, lungs, feathers, fur, production of milk, flight, warm-bloodedness. When students determine polarity of the characteristics, they will determine which of the characteristics all the living things share and then determine the order of exclusiveness. For example, four appendages, lungs, amniotic egg, warm bloodedness, production of milk.)
Complete the blank cladogram that is on the board. Discuss with students the characteristics or characteristics that will be used to separate each group (clade). At the point, you might define the word clade for them as being a group of living things that share the same common ancestor (more will be added to this definition tomorrow). The next clade would be the crocodile (reptiles), followed by birds as a group, and mammals as a group.
Next, ask the students if the birds could be separated into smaller clades. Typically, a student or students will suggest to separate them into flightless (i.e. ostrich and penguin) and flighted birds (seagull and turkey buzzard). However, suggest it yourself if the topic does not come up. Branch the bird clade into flightless and flighted birds. Group all of the mammals together as one clade. This clade will be discussed more tomorrow.
Using their completed challenge sheet, have students compare their cladogram with the teacher/class constructed cladogram. Encourage them to list three similarities and three differences between the two cladograms.
(Note: Examples of student work are shown to show possible ways to develop a cladogram. There are several ways a student could design their cladogram. Their development of ideas is more important than whether or not the construct a "correct" cladogram.)
Have the student watch the rest of Cladograms.
Start the video at 3:20 and play until 5:39.
Stop the video and talk a little about parsimony. Have the students define it in their lab notebooks using the Frayer method. Students should include the definition, an example, a picture. and a sentence correctly using the word. Explain to students the importance of setting parsimony as a criteria in constructing cladograms.
Start the video again and let it play until it ends. Next, have the students talk about how the criteria used in the exercise apply to the construction of the cladogram of the primates. Fill in any knowledge gaps students might have. (Note: You may have some students that find this part of the lesson controversial. If you feel you need help in addressing this issue, check out my teaching reflection.)
Desired student answers might include: four kinds of teeth, moveable head and front-facing eyes, large brain, size of eye socket, central eye area for more acute vision, type of teeth, wet vs. dry nose, tail vs. no tail, omivorous diet, non-opposable vs. opposable thumbs, claws vs. nails, grasping vs non-grasping hand.
As an extension, have students reverse engineer the primate cladogram by constructing a table of characteristics. Have students place a + sign for living things that have that characteristic and a - sign for living things that do not have that characteristic.
Have the students write a one-sentence response in their lab notebooks to the following scenario.
Organize the following derived characters on a cladogram in order of ascending complexity: multicellularity, hair, backbone, unicellular, four appendages.
(Note: Students should answer this question as follows--unicellular, multicellularity, backbone, four appendages, and hair)
Students should hand in their lab notebooks for evaluation. Rank the response in three categories: on-target if they got the order completely correct, close if they got the order somewhat correct, or off-target if they did not get the order right at all. Put the data collected in a bulls-eye graphic and share it with the students the following day.