Lesson Plan: Candy and Cichlids: Exploring Dichotomous Keys

This lesson allows students to take on the role of real scientists, as they identify species from one another. Dr. Reade Robert’s lab research focuses especially on Lake Malawi Cichlids, which are some of the most colorful of the Family Cichlidae. There are more than 1,600 described species of cichlids, worldwide, and they comprise one of the largest vertebrae families on Earth. In Lake Malawi alone, there are at least 700 species, with more being discovered and estimates of as many as 1,000 species in the lake. It is the ninth largest lake in the world and has more species of fish than any other lake.

Author: Linda Dion

View the lesson and supporting materials here.

Introduction

This lesson plan is designed to expose students to something familiar (candy), and then test the same skill on something unknown (cichlids). There are two options for the cichlid portion of the activity, depending on the student population and time constraints: student-directed inquiry or student practice, bolded in a later section.

A dichotomous key allows a user to determine the identity of objects or items by a series of choices. Each statement has only two options, such as:

1a. The object is red….go to 2

1b. The object is blue…go to 3

The statements are mutually exclusive, and prompt the user to move to successive statements in order to identify the object. 

In this lesson, students will be guided through the practice of reading dichotomous keys through the Power Point provided, and then be challenged to create their own dichotomous key using various types of candy. A short list is provided, but the teacher may modify the key and the candy depending on supplies available, student allergies, etc. 

After students complete their candy dichotomous key, the teacher may ask students to complete a background reading before completing the cichlid dichotomous key activity. The reading provides context and relevance for the use of cichlids in this classification activity. Teachers may also assign the reading as homework, or as an extension activity. There are two options for the cichlid dichotomous key:

  • Student-Directed Inquiry
    1. In this option, students will create a textual dichotomous key from the provided cichlid species sheet. They will then trade their key with a partner to assess if their key was usable in identifying the various species.
  • Student Practice
    1. In this option, students will identify the cichlid species from the provided textual key. The teacher may then check answers, or students may trade and grade.

These cichlids are current and past species used in the Dr. Reade Roberts lab at North Carolina State University. There are over 700 species in Lake Malawi alone (where these species are sourced) and about 1,600 known species of cichlids worldwide, with more being discovered often. Lake Malawi is one of the world’s largest lakes with an immense amount of biodiversity. After students have identified the cichlids, discussion can be opened to how all of these fish are in the same family, as well as how adaptive radiation has played a role in their physical characteristics and species proliferation. 

A short Exit Ticket assessment is included on the Power Point for closure of the lesson. Teachers may choose to have students complete this assessment on an index card.

Extension: This short extension is included on the Power Point for discussion after the cichlids have been identified. This extension shows the vast array of cichlids and a diagram of their mouth shape, which are evolved for different food items. The teacher may use this diagram to illustrate or introduce terms like evolution, natural selection, adaptation, etc. Discussion generated from this image can be used as a segue into the Evolution unit.

Curriculum Alignment

NC Essential Standards for Biology

Bio.3.4 Explain the theory of evolution by natural selection as a mechanism for how species change over time.

Bio.3.4.1 Explain how fossil, biochemical, and anatomical evidence support the theory of evolution. 

Bio.3.4.2 Explain how natural selection influences the changes in species over time. 

* Depending on how teachers structure their curriculum, teachers may use this to review natural selection and how species characteristics can become differentiated to serve specific niches, or as a pre-cursor to natural selection 

Bio.3.5 Analyze how classification systems are developed upon speciation. 

Bio.3.5.1 Explain the historical development and changing nature of classification systems. 

Bio.3.5.2 Analyze the classification of organisms according to their evolutionary relationships

Objectives

  • Students will be able to use a dichotomous key to identify organisms accurately
  • Students will be able to create a textual or pictorial dichotomous key that accurate identifies the genus and species of nine different Lake Malawi Cichlids

Time & Location

This lesson requires one 90-minute class period, or two 45-minute class periods and can be completed indoors. 

This lesson may be modified as needed to suit a particular time constraint. Some suggestions for modification include:

  • Students completing the candy dichotomous key as homework
  • Students completing the cichlid dichotomous key as homework
  • Using the reading as an optional assignment and creating questions to fit into the pacing of the evolution unit
  • Extension: Students may be assigned to create a dichotomous key for everyday objects

Teacher Materials

  • Candy and Cichlids: Dichotomous Key Activity – Guided Inquiry Power Point 
  • Candy and Cichlids: Dichotomous Key Activity – Student Handout
    • Recommended: Class set
  • Environmental change drove diversity in Lake Malawi Cichlids (Brown University) Reading
    • Recommended:  Available to all students
  • Cichlid Species Sheet
    • Recommended: Class set
  • Various types of candy*

Student Materials

  • Candy and Cichlids Student Handout
  • Pencil
  • Blue or Black Ink Pen 
  • Various types of candy*

* The teacher may buy the necessary variety of candy, or ask students to bring in candy for the candy activity.

Safety

This lesson involves a component that uses candy. Be aware of any food allergies students may have.

Student Prior Knowledge

To be successful, students should be familiar with taxonomy, specifically binominal nomenclature, genus and species. Students should have basic knowledge of animal landmarks (tail fin, dorsal fin, etc.) but a key is also provided to aid in identification of these anatomical features.

Teacher Preparations

Students may work individually or in pairs, depending on the number and abilities of the student population. Students should be strategically placed into partner groups (if not already selected) based on ability level and social relationships. It may be helpful to have a timer to keep students on track. 

To aid with cost of candy activity, the teacher may ask students or parents to bring in a variety of candy to classify and identify. Be sure to have the candy at least two days in advance to ensure that you have an ample supply for the activity. 

It is recommended that the cichlid species sheet and the reading be printed front and back on the same sheet, then placed in a sheet protector or laminated to aid in set up from class to class and to decrease the amount of copying needed. 

Activities

As stated earlier, there are two variations to this lesson. Both involve a candy classification, and will differ where the step is in bold. 

  1. (10 minutes) Anticipatory Set
  2. (15-25 minutes) Candy Classification Lab Activity
  3. (30 minutes) Cichlid Classification (Student Guided Inquiry and/or Practice)
  4. (10-15 minutes) Cichlid Reading 
  5. (5-10 minutes) Exit Ticket
  6. Optional Extension

——-

  1. Anticipatory Set
    1. The teacher will introduce the agenda to the students so that the expectations are set in order to keep the lesson on pace.
    2. Reading a Dichotomous Key
      1. Using a dichotomous key is like a choose-your-own-adventure story: you pick which trait or characteristic is representative of the species you are trying to identify
      2. (Slide 4) This is a textual representation of a dichotomous key. Teacher will tell students that dichotomous keys have two choices per characteristic (ex: beak is long and slender OR beak is stout and heavy). 
        1. Steps for use:
          1. Select organism to identify.
          2. Go to start (the first characteristic) 
          3. Use choices to arrive at the most specific characteristic
          4. Identify organism
        2. Teacher will walk students through the identification of Bird W. 
          1. Is the beak long and slender, or stout and heavy? It appears stout and heavy. Let’s go to 2. 
          2. Is the bottom surface of the lower beak flat and straight or curved? It appears flat and straight so Bird W is in the genus Geospiza. 
        3. Give students a few minutes to think-pair-share to see if they can correctly identify each bird. Teacher will ask students to share answers to check. Answers are:
          1. Bird W: Geospiza 
          2. Bird X: Platyspiza
          3. Bird Y: Certhidea
          4. Bird Z: Camarhynchus 
      2. (Slide 5) This is a pictorial representation of a dichotomous key. Point out to students how each choice is mutually exclusive.
        The following questions may be posed, or additional variations may be asked by the teacher: 

        1. Ask students what characteristics angiosperms have (flower, seeds, roots, and stems)
        2. Ask students if gymnosperms have seeds (yes)
        3. Ask students what organism does not have roots, stems, or leaves (algae)
  2. Candy Classification Lab Activity
    1. (Slide 7) The teacher will give the students context on their objective. The class will attempt to classify and identify different types of cichlids found in Lake Malawi. There are over 700 species of cichlids in Lake Malawi alone, with over 1,600 species scientifically described worldwide 
      1. To put it into perspective, ask students how many species of bear they think exist. Ask three to four students for a guess. 
      2. (Slide 8) There are just eight species of bear
        1. Recommendation: The teacher may wish to gather background information about traits that differ between bear types to offer a richer discussion to students. In the notes section, there are also some facts that may help. 
    2. The teacher should state that the goal of this lesson is to be able to classify known species, but that candy will be used as practice. 
    3. Students will model use of a dichotomous key by classifying candy. Below is a list of the candy with “scientific names” that students will classify.
      1. Gummi bears (Gummus ursus)
      2. Hershey’s Kiss (Kissus milkus)
      3. Starburst (Stellus chewus)
      4. Jolly Rancher (Jollus rancheria) 
      5. Skittles (Shellus rainbowia) 
      6. Sour Gummi Worms (Gummus vermisourus) 
      7. M&Ms (Shellus mmnomeltia)
      8. Tootsie Rolls (Tootsi rollus)
    4. Students are tasked with creating both a textual and a pictorial dichotomous key. The teacher should remind students that any dichotomous key should have statements that are mutually exclusive to separate the objects of interest into two distinct groups.
    5. Hand out the candy to each student. Students will need one to two pieces of each type of candy.
      1. Modification option: If your students need more practice working in pairs, they may be paired up at this point. 
    6. Explain to students that they should focus on one dichotomous key first and then translate it into the other form (textual first then pictorial, or vice versa).
    7. There is a practice question on Power Point included so that students can see a model.
    8. The teacher should give students between 15-25 minutes to work on this activity. Students may be done early. If this is the case, prompt students onto the cichlid identification sheet.

Option 1: Student-Directed Inquiry

  1. Cichlid Classification
    1. Students are tasked with creating a textual dichotomous key and then having a partner evaluate it by seeing if they can use it to correctly identify each species. The teacher should remind students that any dichotomous key should have statements that are mutually exclusive to separate the objects of interest into two distinct groups.
    2. The teacher should give between 10-15 minutes for students to create the dichotomous key based on the time it took them to complete the dichotomous key for the candy.
    3. Once the teacher observes that all students are done, students should trade and try to complete their identification based on their partner’s dichotomous key (roughly 5-10 minutes). 
    4. Students will sign each other’s paper, and should be given about 5 minutes to discuss if there were challenges or difficulties in identifying the cichlid based on the dichotomous key written. 

 

Option 2: Student Practice

Cichlid Classification

    1. The teacher should give between 10-15 minutes for students to use the dichotomous key to identify all cichlid species on their species sheet. 
    2. Once the teacher observes that all students are done, teachers can ask students to trade and grade each other’s work. 

Regardless of the option chosen, students should be given between 5-10 minutes to complete a short analysis of the questions provided, using the reading as guidance. 

  1. Cichlid Reading
    Recommendation: Copy a class set of the cichlid reading and the cichlid species sheet on one sheet of paper (front and back) and place in a plastic cover sheet (or laminated). This will enable the teacher to reuse the reading and species sheet from class to class.

    1. This reading is from Brown University and gives students a brief overview of the environment of the cichlids and one possible reason as to their adaptive radiation. 
    2. If they are having trouble answering the last question, prompt students to read the reading for guidance.

Depending on time remaining, the teacher may discuss the analysis questions, or collect students’ work to be graded. Teachers may also choose to have the class complete an Exit Ticket. It is recommended that teachers point out the last question to tie the discussion into evolution of specific characteristics. Some guiding questions for discussion are:

    1. What characteristics did you include in your cichlid dichotomous key?
    2. What would that characteristic be advantageous for?
    3. Do you think these cichlids eat different things? Why or why not?
    4. Why is there a range of appearances? 
    5. Why do you think there are so many different types of cichlids?

 

  1. If completing an Exit Ticket: Exit Tickets are short assessments used for closure of a class and can be completed and collected on an index card. If there is time that remains in the class, the teacher may pass out index cards so that students can complete the 3 question Exit Ticket Assessment provided on the Power Point. These questions are designed to mimic standardized test questions. This Exit Ticket may also be used as a Warm Up for the following day, if applicable. 
    1. If time allows, students may trade their Exit Tickets with one another. The teacher may then choose to review the answers with the whole class. If there is limited time, students may hand in the Exit Ticket for grading by the teacher.
  2. Extension: The teacher may wish to link this lesson to Darwin’s finches with a discussion about adaptive radiation. 

Assessment

The quick assessment is the Exit Ticket that is included at the end of the Power Point. It is a modification from the question shown here on the right, which is a question from the 2011 Regents Living Environment Exam. 

Students should be able to identify Fish A, Fish B, and Fish C given the fish fin diagram and the dichotomous key. 

Fish A is C. ephippium
Fish B is C. kleinii
Fish C is C. auriga

Post-Lab Assessment

The following are questions that may be added to cumulative assessments:

Use the dichotomous key below to answer questions 1 and 2.

1a. leaves needle-like………………………Pinus spp.
1b. leaves not needle-like………………….Go to 2
2a. leaves broad…………………………….Go to 3
2b. leaves not broad (scaly)……………….Juniperus spp.
3a. leaves with rough edges……………….Quercus spp.
3b. leaves with smooth edges……………..Gleditsia spp.
  1. What is the name of the genus that has green scaly leaves?
  2. What is the name of the genus that has green, broad leaves with rough edges?
  3. An illustration and a dichotomous key are given. What is the correct classification for this organism?
    1. Class Insecta
    2. Order Amblygygi
    3. Order Araneae
    4. Order Scorpiones

  4. A table of arachnid and insect characteristics is shown. Which characteristic could be used to distinguish between arachnids and insects?
    1. Jointed legs
    2. Body segments
    3. Presence of eyes
    4. Presence of antennae
  5. A botanist discovers a new plant in jungle. Which of the following observations of the plant’s characteristics would be useful in determining its identity with a dichotomous key?
    1. It was discovered in the summertime
    2. It is wet from a recent rain storm 
    3. Its leaves curl up on the edges
    4. It is eaten by native jungle ants

  1. A dichotomous key for trees is provided. Hikers found a tree that has large brown cones and single sharp needles. What species of tree have the hikers found? 
    1. Juniper
    2. Pine
    3. Spruce
    4. Fir

Answers:

  1. What is the name of the genus that has green scaly leaves? Juniperus spp.
  2. What is the name of the genus that has green, broad leaves with rough edges? Quercus spp. 
  3. C, Order Araneae
  4. D, Presence of antennae
  5. C, Its leaves curl up on the edges
  6. C, Spruce 

Critical Vocabulary

Taxonomy – the study of general principles of scientific classification 

Genus – a taxonomic category that includes related groups; these are more closely related than a family, but less related than species

Species – a taxonomic category that includes similar organism that are capable of interbreeding

Binominal nomenclature – the system of nomenclature in which two terms are used to describe an organism, with the first word indicating genus and the second word indicating species

Dichotomous Key – a written device constructed from a series of alternative statements that allows for the identification of an object or organism

Author Information

Credits: This work was supported by the Kenan Fellowship Program and written by Linda Dion, mentored by Dr. Reade Roberts, Dr. Ashley Elias, and other researchers at the Dr. Reade Roberts Lab at North Carolina State University. 

Linda Dion is a teacher at Holly Springs High School, located in southwestern Wake County. She teaches students from 9th to 12th grade in Physical Science, Biology, AP Biology and Anatomy/ Physiology and is entering her sixth year as a high school science teacher. 

Dr. Roberts is an Assistant Professor of Biological Sciences at NC State. Research in the Roberts laboratory examines the genomic basis of adaptation and speciation, with focus on dietary adaptation, host-microbiota interactions, sex determination and sex differences, and complex behavior.

Dr. Elias is a National Science Foundation Postdoctoral Research Fellow working in the lab of Dr. Reade Roberts at NC State. She completed her doctorate in the Department of Biological Sciences at Purdue University. Currently, Dr. Elias is using genomic approaches to explore sex determination and differences in cichlid fish.

Reviewed By: Rebecca Brown, Will Edwards, Anna Glasgow, Bill Worley; July 2017