Classification, part 2- Dichotomous Keys

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Objective

Students will practice observation and classification of unfamiliar jelly beans in order to learn how to use a dichotomous key.

Big Idea

A dichotomous key is a useful tool for identifying unknown organisms.

Warm-Up

5 minutes

Warm-Up: How is the plant kingdom different from the fungi kingdom? 

This question serves as a quick formative assessment of students’ knowledge from the previous lesson, Classification, part 1. Look for students to identify that plants are different from fungi because plants are autotrophs and fungi are heterotrophs. If students are not able to convey an understanding of the differences, perform a quick review of the 6 kingdoms and the characteristics of each kingdom. Note: It’s not uncommon for students to not be able to provide a correct and complete response after learning new material.  Repetition of content in different ways is often a necessary and useful instructional practice for most learners.

Introduce New Material

20 minutes

Inform students of the learning targets:

  • I can explain how dichotomous keys are used to identify organisms.
  • I can use a dichotomous key to identify unknown organisms.

Begin the lesson with a video clip that displays animal diversity. Consider using an insect video clip because their is such a great diversity of insects. 

Viewing exotic organisms creates prior knowledge for students who are not as familiar with the idea that the planet is full of exotic and diverse life forms. After the clip, allow students to share their thoughts about what was viewed. Prepare a few questions to ask to initiate the discussion:

  • Were you were surprised by anything you saw? Why?
  • Do any of you think that we’ve seen and know all of the life forms that exist?” 

Instruct students to turn and talk with a neighbor for 1 minute. Ask for 1-2 students to share their thoughts with the class. End the discussion by clarifying that the earth is full of a variety of organisms and we have not likely seen all the various life forms that exist. Pause and allow students to consider this statement before asking them to respond to this statement with why they think this is so. Look for students to identify that man does not have the ability to travel to all parts of the earth so we don’t know what life exists in places man has not yet gone.

Introduce the vocabulary associated with the lesson: dichotomous key, organism, and couplet. Using an LCD projector, provide instruction on dichotomous keys. Distribute guided notes or use a note-taking format that has been taught.  For this particular lesson, Cornell notes is an appropriate note-taking format instead of guided notes since there is not a lot of information to be recorded.

Begin by explicitly teaching the term, dichotomous key. Remind students to add this term to their Vocabulary Map. Ask students to identify any prefixes or suffixes in the term. If needed, prompt their thinking by asking students to think of other words that use this same prefix, di-. Look for students to identify words like dihybrid or dioxide (as in carbon dioxide). If they cannot think of these terms, identify them for the class. Identifying these words should help them make the connection that the prefix di- means two. 

Once it’s established that di- means two, explain that a dichotomous key is a tool that allows the user to determine the identity of items in the natural world, such as trees, wildflowers, mammals, reptiles, rocks, shells, and fish.  

Emphasize the language connection that dichotomous means "divided into two parts“ or couplets. Therefore, dichotomous keys always give two choices in each step. Ask students to identify what word they see in the term, couplet. They should identify that the word, couple is in the term, couplet.  This should help them remember that each part of a dichotomous key is made of a couple of choices, either this choice or that choice.

Expect that the use of a dichotomous key will not be clear for most students until the process is modeled during guided practice.

Guided Practice

10 minutes

Use a LCD projector to display a simple dichotomous key.

Note: There are many dichotomous keys to choose from so conduct an internet search to select the level and complexity of the dichotomous key you feel is best to model for your students.  

Model how to use a dichotomous key  with a “think aloud” so that students will be able to observe the thinking and mental processing behind how both choices are read and compared with the specimen to be identified.

Using a visual aid, emphasize that once a decision is made, the selected choice directs you to another couplet (either the next in order or one further on in the key), and this process is repeated until a conclusion (successful identification) is reached.

Inform students of a few things to keep in mind when completing a dichotomous key:

  • Always read and consider both choices, even if the first one seems to be appropriate.
  • Jumping to conclusions may lead to the wrong classification of the item.
  • Always understand the meaning of the words used in each choice. If you not sure of the meaning of a term, look it up in a dictionary or use your word decoding strategies to identify its meaning. Never guess, as this could also lead to the wrong classification of the item.

Independent Practice

20 minutes

Display the Jelly Bean Dichotomous Key Lab. Note: There are many versions of this lab on the internet from various sourcesThe type of jellybean used does not have to be a particular brand. Just make sure that you are able to locate or create a dichotomous key for the brand of jellybeans used. 

Distribute copies of the Jelly Bean Dichotomous Key Lab Procedures, the dichotomous key, lab write-up form and markers or colored pencils. Provide a summary of the lab procedures.

Review the lab materials, procedures, lab sheet and analysis questions. I find it useful to preview the lab analysis questions with the class before releasing students to work because it gives them an idea of what they should be looking to observe, consider or learn while conducting the lab procedures.

Discuss the Data Table found on the lab sheet. Take time to discuss how each of the 5 sections in the Data Table should be completed:

Brief Description of each jelly bean- Use observation skills to provide a written description using descriptors like color, markings, smell etc..

Sequence followed to determine the type- What sequence of numbered couplets did you follow to determine the jellybean type?

Drawing of the jellybean- Draw what each jellybean looks like.

Common Name and scientific name (student created)- Write the name of the jellybean after you identify it then create a scientific name, using binomial nomenclature (genus species)

Does the bean taste like the description name?- Answer yes or no to the question.

Assign students to work in groups of two to complete the lab. Distribute the jellybeans and release students to work in small groups on the lab. To create an sense of urgency around the assignment, it might be helpful to use and project a digital timer to help students manage their time efficiently to complete the tasks on time. Periodically, call out the remaining time as a monitoring tool that will help groups work productively.

Walk around the room to monitor the groups as they work and ensure that all students are actively engaged in the completion of the data table. Both student work 1  and student work 2 show what to expect when students follow the instructions and complete the data table as directed. The genus species names that the students created for the jellybeans shows that they possesses an understanding of binomial nomenclature.

Make sure that students complete the analysis questions found on the back of the lab sheet. Student work 1 exhibits responses that reflect a understanding of taxonomic ranking and classification, while the responses to questions 2-3 on student work 2  indicate that the student is not clear about family, genus, and species taxonomic ranking.

Close

5 minutes

Engage students in a “Silent Discussion”. This strategy involves asking a question, then have students pass notes at the tables discussing the question.

Ask students, “Was it easy for you to identify the flavor of the jellybeans? Why or why not?  How do you think the ease or difficulty of classifying the jellybeans applies to the classification of organisms?

Circulate the room and read some of their notes to assess their understanding. Randomly select students to share their responses with the class. Look for students to identify that the classification process was aided by their ability to use their senses (sight, touch, smell) to help them have an idea of the flavor of the jellybeans. before it was confirmed by tasting the jellybeans. Listen to their responses to see if they are able to make the connection that use of our senses is also be helpful when classifying organisms. However, confirmation of classification of organisms requires a lot a more investigation and DNA testing.