How to make a dichotomous key

How to make a dichotomous key

Classification is very important to the field of biology. As we continue to discover new species, learn better techniques for analyzing relationships between species (i.e. DNA analysis) and share information internationally it is important to have systems in place to identify and classify organisms. A dichotomous key is a tool that helps to identify an unknown organism. A dichotomous key is a series statements consisting of 2 choices that describe characteristics of the unidentified organism. The user has to make a choice of which of the two statements best describes the unknown organism, then based on that choice moves to the next set of statements, ultimately ending in the identity of the unknown. Dichotomous keys are often used in field guides to help users accurately identify a plant or animal, but can be developed for virtually any object. They are particularly helpful when two species are very similar to one another.

This project is about learning how to use a dichotomous key to identify plants or animals. Through the process of creating their own dichotomous key and field guide the student will sharpen their observation and classification skills, both of which are necessary for success in upper level science courses. The student also builds an appreciation for nature through extended periods of observation.

Problem:

Learn how to make a dichotomous key. How can we use a dichotomous key to identify plants or animals?

Materials:

  • Materials are available at the library, office supply store or from home
  • Posterboard
  • Tree, bird, fungus, amphibian, wildflower, etc. field guides
  • Notebook
  • Camera

Procedure:

  • Study examples of dichotomous keys, like the ones below or others you might find in a field guide. There are a variety of ways you can design a dichotomous key, and they can be used to identify pretty much anything. Figure 1 below is a dichotomous key for types of potato chips, and Figure 2 is one for identifying organisms.
  • Practice making a dichotomous key with everyday items or people. Start with the most obvious features of the item and move to more specific statements. Remember, each statement must have 2 choices. For example you might start by creating a dichotomous key to identify students in your class. Begin with very general statements: Is the student male or female? Does the student have blue eyes or brown eyes? Does the student wear glasses? Etc. You can set up your key as a flow chart, or as a grid.

Here is an example of a partial dichotomous key for identifying classmates:

The students will explore the benefits of creating dichotomous keys as a means of identifying an organism or object.

Lesson Objective

The student will create a dichotomous key to identify fossil oyster shells.

Background Information

A dichotomous key is a tool that allows the user to determine the identity of items and organisms in the natural world. It is the most widely used form of classification in the biological sciences because it offers the user a quick and easy way of identifying unknown organisms. Keys consist of a series of choices that lead the user to the correct name of a given item. “Dichotomous” means “divided into two parts.” That is why dichotomous keys always give two choices in each step. In each step, the user is presented with two statements based on characteristics of the organism. If the user makes the correct choice every time, the name of the organism will be revealed at the end.

There are two kinds of descriptions that might be presented to the user of a dichotomous key: qualitative and quantitative descriptions. Qualitative descriptions concern the physical attributes, or qualities, of the item being classified. Examples of qualitative descriptions are such phrases as “contains green striations on top surface” or “feels slick on bottom surface.” Quantitative descriptions concern values that correspond with the item being classified. Examples of quantitative descriptions are such phrases as “has 10 striations on top surface,” “has 8 legs,” or “weighs 5 grams”. Knowing the difference between these two types of descriptions can be immensely beneficial for creators and users of dichotomous keys.

There are two ways to set up a dichotomous key. One way is to present the two choices together, and the other way is to group by relationships. When the dichotomous key is set up by presenting the two choices together, it is easy to distinguish between them. However, relationships between various characteristics are not emphasized. When the dichotomous key is grouped by relationships, the choices are separated, yet it is easy to see the relationships between them. While this method may prove to be more difficult to construct, many users prefer it because it gives them more information.

Teacher Preparation

In this activity, the teacher will need or collect five to six oyster shells for each student or student group. If oyster shells are not readily available, download the pictures of the oyster shells from the website. Enclosing the pictures in sheet protectors may be a good idea. From these shells or pictures, the students can now create their own dichotomous keys.

Student Preparation

You will create a dichotomous key in order to identify fossil oyster shells (using the images provided in Blackline Master 4) or a variety of shells provided to you by your teacher. Before writing your dichotomous key, read the background information on shells provided by your teacher (Blackline Masters 1- 3), as they will aid you in key construction. Continue to write the key until each individual shell can be identified. Make an answer key to accompany your dichotomous key.

Blackline Masters

  • BM 1 : Student Background: Mollusks and Their Shells (PDF)
  • BM 2 : Student Background: Anatomy of the Oyster Shell (PDF)
  • BM 3 : Student Instructions: Creating A Dichotomous Key (PDF)
  • BM 4 : Seashell Pictures for Dichotomous Key Activity — Page 1 (PDF), Page 2 (PDF), Page 3 (PDF)

Alternative Assessments

  • Create a concept map from the key.
  • Students will design a dichotomous key using a particular item (examples: coke tabs, ketchup bottles, paper clips, other shells) and be graded on accuracy and completeness.

Answer Key(s)

Each student or student group will submit a dichotomous key and answer key; therefore, each answer key will be different.

Extension Ideas

  • Have students create a field guide using dichotomous keys to help with identification of a particular item (examples: plants on the school campus, or macro-invertebrates in a local stream, etc). Give students examples of field guides to generate ideas.
  • Using the key they have created, develop an outline of the format used in their English class, that students will understand what the divisions in an outline represent and the relationships among the terms.

Resources and Web Links

Timme, Stephen, 1991, Association for Biology Laboratory Education website, How to Construct and Use a Dichotomous Key, accessed 02/16/01, www.zoo.utoronto.ca/able/volumes/vol-12/7-timme/7-timme.htm

Description: An excellent web-based activity on the construction and use of a dichotomous key that also describes the use of a dichotomous key in the field and provides a key for prairie plants.

Frontier High School, Red Rock, OK, The Dichotomous Key, accessed 02/16/01, http://pc65.frontier.osrhe.edu/hs/science/hbotkey.htm

Description: Provides instructions on the two methods of constructing a dichotomous key as well as several online dichotomous keys. Grade level: High School.

Detka, Jon , California State University at Monteray Bay, Designing and Using a Dichotomous Key, accessed 02/16/01, www.monterey.edu/students/ Students_D-H/ detkajon/world/ron/dichotdesign.html

Description: Students first construct a simple dichotomous key and then use a basic key to identify some of the native plants and the most unwanted invasive weeds of California. Grade level: 3-5.

Santa Cruz Productions, Wastewater Filamentous Bacteria Dichotomous Key, accessed 02/16/01, http://home1.gte.net/vsjslsk1/gramstainflowchart.htm

Description: A completely web-based dichotomous key designed to assist students in identifying wastewater bacteria.

How to make a dichotomous key

A dichotomous key is a tool that can be used to identify organisms or objects in the natural world, such as plants, animals, or rocks. The key consists of a series of paired statements or clues about features or characteristics, providing a stepwise guide toward identifying each entity. As the user proceeds from one step to the next, the clues gradually narrow down the list of possible entities until all are identified. Dichotomous keys are useful for biological classification as well as identification.

The word dichotomous is derived from the Greek words meaning “two parts.” Each step in the key provides two contrasting choices about a characteristic. Each alternative choice then leads to another pair of clues and then another, until the items are identified. A group of leaves, for example, might first be separated on the basis of vein patterns; each group could then be further subdivided on the basis of other traits such as shape and stem arrangement.

The number of steps in a key depends on how many entities are being identified. There is generally one less step than the total number of entities to be identified; thus a key used to identify four unknown entities would have three steps, whereas identifying eight entities would require seven pairs of clues. The clues may be presented as questions or statements.

The paired clues below are an example of a simple dichotomous key that might be used to identify five types of beans. Note that each set of clues concerns a single feature; the first clue concerns the shape of the beans, with the successive clues addressing hues, patterns, and finally specific colors.

How to make a dichotomous key

The most important elements of a good key are that each set of clues offers two contrasting choices and that the clues gradually narrow down the list of possible objects or organisms so that identification is possible. The following key could be used to identify four familiar insects.

How to make a dichotomous key

Dichotomous keys can be used to identify nonliving things as well as living things. Such keys are helpful in identifying rocks and minerals.

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How to make a dichotomous key

Classification and identification are very important in any scientific field – especially biology. Therefore, scientists have created a simple yet effective system to identify and group organisms in the right categories. This tool is known as a dichotomous key.

What is a Dichotomous Key?

A dichotomous key is a visual tool that allows users to determine and identify different creatures in the natural world like mammals, trees, reptiles, fish, rocks, and wildflowers. Dichotomous keys identify objects and organisms.

They are typically used for identifying plants and animal species on the basis of their characteristics. Moreover, they can also be used to identify minerals and any other object with observable traits.

How to make a dichotomous key

Image Source: ib.bioninja.com.au

• Uses of Dichotomous Keys

A dichotomous key is used in the field of biology and other sciences. As new species are discovered every day, it’s important to have a technique to identify and classify them easily. So, the dichotomous key is the perfect tool for this.

Some common uses of the dichotomous key are:

  1. To identify and categories organisms.
  2. To help students easily understand complex scientific concepts.
  3. To simplify and organize large amounts of information for identification.

Types of Dichotomous Keys

Dichotomous” means “divided into two.” Hence, according to the dichotomous key definition, it always presents two choices in each step. The option you select in each step gives rise to new choices in the next step.

All dichotomous keys function in the same way, but there are several different ways of presenting them. Therefore, there are different types of dichotomous keys:

1. Nested Style

In a nested dichotomous key, the next question in a series is presented “nested” under the answer that leads to that question. Indentations are used to distinguish between the questions.

How to make a dichotomous key

Image Source: slideplayer.com

2. Linked style

In the linked style of the dichotomous key, the questions are laid out in the form of a list. Each answer directs you to the next question, and the user has to find the correct item to identify the object or the organism correctly.

How to make a dichotomous key

Image Source: www.twinkl.it

3. Branching Tree

In a branching tree layout of a dichotomous key, the characteristics of each organism are laid out like a flowchart.

Each characteristic or question gives birth to a new branch and carries a subsequent question as a sub-branch. The identification is revealed at the end of each branch.

How to make a dichotomous key

Image Source: evolvingsciences.com

4. Computer Program

Dichotomous keys are now being converted into computer program forms thanks to the global digitization. This allows users to answer questions one at a time to get the correct follow-up question automatically.

It is similar to an online quiz where the user reaches the correct identification after answering all the questions.

How to make a dichotomous key

Image Source: researchgate.net

How to Make a Dichotomous Key?

To create a dichotomous key, both qualitative and quantitative factors are considered. It can be done in visual form (flowchart) or a written format (quiz).

Here is a step-by-step guide to help you create a dichotomous key:

Step 1: List the Characteristic

Attention to the organism you are trying to identify with your dichotomous. Note down all the visible characteristics. For example, if you are trying to identify a group of plants, you may notice that some have large flowers, whereas others have tiny leaves.

Step 2: Organize the Characteristics

To create an effective dichotomous key, organize the characteristics. Start with the most common features first and then move to more specific ones. This process will help you identify the more obvious and less obvious contrasting characteristics of different species.

Step 3: Divide the Organisms

Create categories and divide your organisms into groups based on similarities in characteristics. For example, animals can be divided into those with feathers and without feathers. You can further subdivide your specimen by asking more questions and tell each organism is identified.

Step 4: Draw Dichotomous Key Diagram

Either you can create a text-based dichotomous key for a visual one. The graphic dichotomous key can also use images of the organisms you are trying to identify. Here you can choose between different types of dichotomous keys like branch tree or nested.

Step 5: Test Your Dichotomous Key

Once your dichotomous key is ready to test it out to check if it works. Focus on the organism you are trying to identify and answer each question in your dichotomous diagram. If the questions need rearrangement, then make necessary adjustments until you are satisfied.

Recommended Dichotomous Key Makers

Dichotomous keys make it simple and easy to identify biological organisms and other objects. However, creating a dichotomous key by hand can be a tedious and time-consuming task. This is why we recommend using a dichotomous key maker that is easy to learn and use for both beginners and professionals.

How to make a dichotomous key

EdrawMax helps you create professional-looking diagrams of all types. You can effortlessly use the software to visualize dichotomous keys using its advanced drawing features:

Construction and Use of Dichotomous Keys
• Biological keys are sets of statements that act as clues leading to the identification of an organism.
• By following the keys we can be able to place an organism in its group.
• The most common key is the dichotomous key.
• This is a biological tool for identification of unknown organisms.
• The word dichotomous means branching into two.
• A single characteristic is considered at a time.
• Two contrasting statements are put forward to describe the characteristics in such a way as to separate the organisms.
• This continues until all the organisms have been identified.

Rules Used to Construct a Dichotomous Key
• Use morphological characteristics as far as possible e.g. type of leaf – simple or compound.
• Select a single characteristic at a time and identify it by number. 1. Type of leaf. .
• Use identical forms of words for two contrasting statements e.g.:
a) Flowers scented.
b) Flowers not scented.
• Start with a major characteristic that divide the organisms into two large groups then proceed to lesser variations that would separate the organisms further into smaller groups.
• Use positive statements especially the first one.
• Avoid generalizations e.g. short plants. Be specific in your description e.g.:
a) Plants above 1m tall.
b) Plants below 1m tall.

Some Common Features Used for Identification
In Plants
Leaves
1. Type of leaf Leaf
Compound leaves.
• Simple leaf
• Trifoliate
• Pinnate
• Type ofleaf margin.
Type of venation.
• Type ofleaf arrangement on stem.
• The colour of leaf.
• The texture ofleaf; whether hairy or smooth.
• Shape of the leaf e.g. palmate.

Stem
• Type of stem – woody or herbaceous.
• Shape of stem – cylindrical or rectangular.
• Texture of stem smooth or spiny.

Infloresence
• Are flowers terminal or lateral
• For each flower:
• Is the flower regular or irregular?
• Number of floral parts for each whorl.
• Are floral parts free or fused?

Roots
• Type of root system- Taproot or fibrous?
• Function of the root.

Features used to identify animals:
• Type of mouthparts.
• Type of skeleton.
• Presence or absence of antennae.
• Body segmentation.
• Body covering: scales, fur, hair or feathers.
• Number of body parts.
• Locomotory structures: legs, wings and fins.

• Presence or absence of vertebral column.
• Presence and type of eves.

What is a dichotomous key?

• A biological device (tool) which enables one to identify an organism by progressively opting between two alternative observable characteristics

Necessity of using a dichotomous key

• used to identify organisms quickly and accurately
• by following the statements in the key we are able to identify each organism on the basis of a characteristic which is not to be found in other specimens

Rules followed in constructing a dichotomous key

• use observable characteristics only
• start with major characteristics, placing organisms into two groups at each stage
• use a single characteristics at a time
• use contrasting characteristics at each stage e.g 1(a) short, 1(b) tall
• avoid repeating the same characteristics

Procedure of using a dichotomous key and a list of major features of the characteristics to be identified

• look at the features of similarities
• look at the features of differences between the organisms
• we can then be able to identify the organisms by distinguishing one from another
• the key uses a method of elimination by following statements that are correct only for the organism

Example
i)You are provided with a specimen leaves(Araicaria, Cynodon, Grevellea, Kalanchoa, Brassica, Tradescantia, Commelina). Use the dichotomous key below to identify the taxonomic group to which the specimen belongs. Show the steps (number and letter) in the key that you followed to arrive at the identify of the specimen

1 a) leaf broad . go to 2
b) leaf narrow . Araicaria

2 a) leaf parallel veined . Cynodon
b) leaf net-veined . go to 3

3 a) leaf with one lobe (simple) . go to 4
b) leaf with many lobes (compound) . Grevellea

4 a) leaf fleshy . Kalanchoa
b) leaf not fleshy . go to 5

5 a) leaf petiole modified to form sheath. go to 6
b) leaf petiole not modified to form sheath . Brassica

6 a) leaf purple . Tradescantia
b) leaf green . Commelina

Steps – 1a, 2b, 3a, 4b, 5b Identify – Brassica

ii) You have been provided with four animals labeled K (mature adult housefly), L (mature adult grasshopper, M(maize flour beetle) and N(worker termite) use the dichotomous key below to identify the specimens. Write down in the correct order, the steps (number and letter) in the key that you followed to arrive at your answer.

1 a) animal with wings. go to 2
b) animal without wings. go to 7

2 a) with two pairs of wings . go to 3
b) with one pair of wings . Diptera

3 a) with membranous wings . go to 4
b) hind pair of membranous wings . go to 6

4 a) with long abdomen . Odontata
b) medium sized abdomen . go to 5

5 a) wings with coloured scales. Lepidoptera
b) wings without scales. hymenoptera

6 a) forewings hard and shell-like . coleopteran
b) forewings hard but not shell-like. Orthoptera

7 a) body horizontally flattened . Isoptera
b) body laterally flattened. Symphonoptera

How to make a dichotomous key

Making your own plant dichotomous key is a little like playing twenty questions to identify a plant. A dichotomous key is guide, sort of like a map of characteristics, to lead the user to the correct identification of the plant.

The word dichotomous comes from two Greek words that mean divide in two parts. To make a dichotomous key you will choose physical characteristics that can be used to divide a collection into two parts. Possible physical characteristics to use include plant size, plant shape, leaf shape, stem type, color, and presence of seeds, fruits or flowers. You will continue to divide each group in two until all of the groups have only one member.

The Process

Step 1: Begin by gathering your plants (or pictures of plants) to be identified. For example: a corn plant, hosta, oak tree and dandelion.

Step 2: Decide how to split your group in two parts. Carefully observe your specimens to pick a characteristic which can be used to decide which plant goes into which group. Record the chosen characteristic and the groups formed. For example, our group of corn, hosta, oak tree and dandelion could be divided by leaf color:

A. The leaves are solid green. See #2.

B. The leaves have a white margin. Your plant is a hosta.

Step 3: After the collection has been divided into two groups, divide the first group (group A) into two more groups based on one characteristic. Record your characteristic and groups. When you form a group with only one plant, name it. The next division in our example is leaf edge:

A. The leaves have a straight edge. Your plant is corn.

B. The leaves have a curvy edge. See #3.

Step 4: Continue to identify characteristics and divide groups until all of the plants are named. Be sure to record each division made. When the first group is complete, repeat the steps until all of the plants in the second group (group B) have been named. Your completed key should look something like this:

A. The leaves are solid green. See #2.

B. The leaves have a white margin. Your plant is a hosta.

A. The leaves have a straight edge. Your plant is corn.

B. The leaves have a curvy edge. See #3.

A. The plant has a woody stem. Your plant is an oak tree.

B. The plant has soft stems. Your plant is a dandelion.

The sample key is fairly short and simple. The more items your beginning collection has, the longer your key will be. Have a friend try out your completed key to see that it leads users to the correct choices and identification.

Creating a Dichotomous Key for Families of Idaho Butterflies
suggested grade levels: 9-12, College
view Idaho achievement standards for this lesson Overview:
Teachers should explain how to make a dichotomous key and make a simple one together as a class. As a precursor to this butterfly activity, it would be a good idea to have students make a dichotomous key using inanimate objects such as pencils, paperclips, coins, etc. At each stage in the dichotomous key all remaining “species” must be separated into two clearly distinguishable groups. The splitting must continue until all “species” have been separated out. To see a good example of how this is done, refer to the How to identify species using morphology and creating a dichotomous key in the biology exercise site. This kind of activity works better if students work in groups of three or four, but larger groups can be used depending on how many computers are available in the classroom.

Objectives:
1. Students will learn how to use the butterfly section of the Digital Atlas of Idaho.
2. Students will learn how to make a dichotomous key.
3. Students will hone their skills of observation and learn how to recognize defining characteristics that distinguish different families.

Procedure:
Have your students read through the section on “How to identify species using morphology and creating a dichotomous key”. Once they have become familiar with dichotomous keys they can go the butterfly family pages to see the characteristics of each family.

  1. To get there: Click on Atlas Home, Biology, Butterflies, and then on Family Tree.
  2. From the family tree page, just click on each of the eight families to obtain information on the families of butterflies.
  3. Emphasize to the students that they must pay careful attention to detail. Groups should use these pages to obtain defining characteristics of the families of butterflies. Use these defining characteristics to make a dichotomous key to distinguish these families.
  4. If there is time, groups should switch papers and try to follow the dichotomous keys made by other groups.

Handouts/Activities:
These are links to access the handouts and printable materials.
Dichotomous Key

Related Lesson Topics:
Biology: Butterflies
Biology: Biology Topics

Lesson plan by James Scannell and Stefan Sommer, 2001
Idaho Achievement Standards (as of 7/2001) met by completing this activity:
351.01.02.05
352.01
355.01
361.01

Presentation on theme: “how to make a dichotomous key”— Presentation transcript:

1 how to make a dichotomous key

2 What is a dichotomous key?
A key for the identification of organisms using paired statements These statements must be opposites For example, if the first statement is “has blue eyes,” then the paired statement would be: Has green/brown/hazel eyes Does not have blue eyes

3 So how do I make my own dichotomous key?
Make a list of things that you want to classify: Let’s classify restaurants Red Lobster McDonalds Chili’s Cracker Barrel Chick Fil A

4 So how do I make my own dichotomous key?
Group the restaurants into two groups those with drive-thrus those without Write paired statements, one reflecting each group type those with drive-thrus = “has a drive-thru” those without drive-thrus = “has no drive-thru”

5 So how do I make my own dichotomous key?
Each statement is like a map. It tells you where to go next. Read the statement and look at the things that you are trying to classify. How many items match that statement? If it is one, write the name of the item If it is more than one, you need another way to divide the group Go to #(next available number)

6 So how do I make my own dichotomous key?
So in our example, the first statement “has a drive-thru” matches two of the restaurants: McDonalds Chick Fil A So it needs another way to divide them Go to #2

7 So how do I make my own dichotomous key?
The second statement “has no drive-thru” matches three restaurants: Red Lobster Chili’s Cracker Barrel So we need another way to divide them: Go to #3

8 So how do I make my own dichotomous key?
For #2, we need to divide McDonalds and Chick Fil A serves hamburgers doesn’t serve hamburgers Each statement has exactly one match, so we are done with those two restaurants serves hamburgers = McDonalds doesn’t serve hamburgers = Chick Fil A

9 So how do I make my own dichotomous key?
For #3, we need to divide Red Lobster, Chili’s, and Cracker Barrel Serves breakfast Doesn’t serve breakfast One statement has exactly one match Serves breakfast = Cracker Barrel The other statement has two matches, so we need another category Doesn’t serve breakfast = Go to #4

10 So how do I make my own dichotomous key?
For #4, we need to separate Chili’s and Red Lobster Serves whole lobsters= Red Lobster Doesn’t serve whole lobster = Chili’s Now all five restaurants are in groups by themselves. Our dichotomous key is complete!

11 I know how to make one.. Now how do I read one?
Pick one of the items that you are trying to classify ALWAYS start with #1! Determine which statement matches the item that you are trying to classify, and follow the direction that statement gives you Once you have classified that item, pick another item until the key is complete!

How to make a dichotomous key

One of my guilty pleasures is taking personality quizzes online with silly titles like, “Which Flavor of Ice Cream are you?” or “Which Teen Heartthrob is Your One True Bae?”. They ask questions like “Do you like long walks on the beach?” and you answer “yes” or “no” before it leads you to a new question, ultimately ending (you hope) by telling that yes, indeed, Zayn Malik is your One True Bae! These quizzes, though often frivolous, are utilizing a very scientific process to lead you to an answer. This tool is called a Dichotomous Key.

A Dichotomous Key is a tool used to categorize living organisms based off their characteristics using simple yes / no questions. The process of using a dichotomous key is simple; begin by observing the whole organism you want to identify. The key will ask questions that lead to alternate possibilities or end in your desired identification. Let’s walk through our example below!

  1. How to make a dichotomous keyChoose an animal you want to identify
  2. Does this animal have a backbone?
    1. If NO – Octopus
    2. If YES – continue
    1. If NO – Catalina Island Fox
    2. If YES – continue
    1. If NO – continue
    2. If YES – Giant Kelpfish
    1. If NO – Juvenile Garibaldi
    2. If YES – Juvenile California Sheepshead

    A dichotomous key is an important scientific tool, used to identify different organisms, based the organism’s observable traits. Dichotomous keys consist of a series of statements with two choices in each step that will lead users to the correct identification.

    Background

    Scientists have catalogued 1.5 million species on Earth and estimate that there might be as many as 100 million more unidentified species (Giller, 2014). Classification keys are tools that provide a means to correctly identify different organisms, based on observable traits. Most such keys are created in a dichotomous format. Dichotomous literally means dividing into two parts (Merriam-Webster, 2014). A dichotomous key provides users with a series of statements with two choices that will eventually lead to the correct identification of the organism. To use a dichotomous key, one must be able to make accurate observations and follow directions carefully.

    Focus Questions:
    How can you identify an unknown organism?
    What types of characteristics can be used to identify an organism?
    How do you use a dichotomous key and why is a dichotomous key important?

    Learning Target:
    I can construct and use a dichotomous key based on different organisms’ characteristics.

    Preparation

    30-60 minutes to review activity, collect materials, and make copies; Extension: 1-2 hours to collect materials

    (10-15 minutes)
    The instructor will find ten pictures of different plants and animals that depict the complete body of the organism rather than a partial or face shot. Pictures can be projected on a smart board or overhead projector, or magnets can be placed on the back of pictures for placement on the blackboard. Pictures should be diverse and have evident easily seen characteristics. The instructor will ask the students to observe traits of the displayed organisms. Dichotomous keys are based on observations, so the instructor should encourage students to use observable characteristics, rather than behaviors. For example, if the picture depicts an owl, accept the observation “they have eyes that face toward the front.” Do not accept the observation “they hunt at night.”

    For the students’ dichotomous keys to work, the observations must be ones that the students can see in the pictures because not all students are at the same level of knowledge. Students will then use the pictures to model the process of creating a dichotomous key.

    Materials

    Lesson Hook/Preview

    A dichotomous key is an important scientific tool, used to identify different organisms, based the organism’s observable traits. Dichotomous keys consist of a series of statements with two choices in each step that will lead users to the correct identification. This type of key is not hard to use, but makes more sense when students undertake the process of making one before using it. The instructor will model the process before students begin work on their own keys. After the demonstration, students will first make a Branching Key using a picture of an organism of their choice. Once the Branching Key is completed, students will convert it into a written dichotomous key. The end product will be a poster displaying the student pictures, along with their written dichotomous keys.

    Procedure

    1. Prior to the class, direct students to collect ten pictures of plants and animals, stressing that the photos should depict the entire body of the organism. These will be used after the instructor’s demonstration.

    3. After modeling the process, direct students to take out the ten pictures that they collected for their project. Students will then go through the same process to create a dichotomous key unique to their pictures.

    4. Have students check another student’s key to make sure they “work” and are correct before working on their final presentation.

    Vocabulary

    dichotomous key, organism

    Assessment Materials

    Students will make a poster with a written dichotomous key and pictures on the front and their branching keys (rough draft) on the back. Give each student a copy of the Worksheet 1, “Classification 1: Using Dichotomous Keys” for a grading rubric to guide them on the requirements for their project. After grading, students should display their dichotomous keys around the room for all students to observe.

    Extension:

    Students will be given different unknown organisms with keys for identification and will work through the identification process. Sources of keys: internet or textbooks.

    Instructors may also wish to use field guides for local areas, using pictures of animals or plants that students might see in their backyards or parks. Good sources are the Peterson First Guides. Also, the internet can be searched for “sample dichotomous keys” and will yield multiple examples.