Lesson Explainer: Dichotomous Keys Biology • First Year of Secondary School

In this explainer, we will learn how to design and use a dichotomous key.

Sometimes, the correct identification of a species can mean the difference between life and death. For people who like to hunt for edible mushrooms, incorrect identification could lead to accidental poisoning. Similarly, determining if a snake is venomous will impact the method of treatment. When scientists need to quickly and accurately identify species based on their characteristics, they often turn to a tool called a dichotomous key.

A dichotomous key is a series of descriptions separated into pairs that can be used to identify organisms by their characteristics. The word dichotomous means “something that is made of two parts.” These keys are designed to be used with a defined set of specimens by answering a series of simple yes or no questions. It is important to remember that a dichotomous key is not a classification tool but is designed specifically to identify a particular specimen from a defined group of specimens.

Example 1: Describing a Dichotomous Key

The following statement explains what a dichotomous key is: A dichotomous key is a series of , grouped into pairs, designed to identify an organism by its ?

1. Which word is most appropriate to replace the first blank?
   1. Descriptions
   2. Organisms
   3. Taxonomic ranks
2. Which word is most appropriate to replace the second blank?
   1. Peers
   2. Characteristics
   3. Genetics

Answer

This question is asking us to fill in the blanks in a statement that describes what a dichotomous key is and what it is used for. A dichotomous key is a tool that is used to identify organisms. Dichotomous is a word that means “two parts” (remember that the prefix di- means “two”). A dichotomous key uses a series of statements, descriptions, or yes or no questions grouped into pairs, which eventually lead to the correct identity of a specimen or organism. An example of a dichotomous key designed to be used by children to classify common invertebrates they may find in a garden is shown below.

We can see that each juncture has two options, either possession or absence of a certain physical trait. We can also add that all of the traits listed in the dichotomous key in orange are observable, physical characteristics.

Part 1

Using the explanation above, we can conclude that the word that best fills the first blank is A: descriptions.

Part 2

Using the explanation above, we can conclude that the word that best fills the second blank is B: characteristics.

The typical dichotomous key comes in one of two forms: one that looks like a list, and one that looks like a chart. The list version is more useful for extensive dichotomous keys that can identify many different specimens. The chart version is more visual, but generally more effective if fewer specimens are included. In order to use either version, you start at the top and continue downward, making decisions based on the descriptions and following the direction that is the result.

The dichotomous keys shown in Figure 1 and Figure 2 present nearly the same information in different arrangements. Either one can be used to identify the species of the tropical trees within a particular park. Let’s imagine that we take a walk in this park and take the photograph of a single tree shown below.

Let’s identify the species of this tree using the dichotomous key in Figure 2. Statement 1 says “leaves are simple and flat.” Since this is not the case, we follow the instructions at 1b and continue to statement 2. Statement 2 says “tree is short and bushy.” Since the tree in the picture is quite tall, we will follow the instructions at 2b to go to statement 3. Statement 3 says “leaves are palmate (fan) shaped.” Since this is true, we can follow the instructions at 3a which tell us that this tree is a fan palm, which has the scientific name W. robusta.

How To: Using a List-Style Dichotomous Key

Outline of a List-Style Dichotomous Key

The following are the steps to use a list-style dichotomous key to identify a specimen:

1. Observe the specimen you are going to identify.
2. Start at the top of the list.
3. Read the description in row 1 of the dichotomous key.
4. Read the two options, 1a and 1b, in the dichotomous key.
5. Decide which option best describes your specimen.
6. Follow the direction that corresponds to the appropriate description.
7. Repeat steps 3 through 6 from this list for all of the remaining rows of the dichotomous key until you reach a taxon.
8. Your specimen has been identified!

Now, let’s identify the species of this tree using the dichotomous key in Figure 1. Tracing down from the top, the first juncture says “compound leaves/simple leaves.” Since the tree in our picture has compound leaves, we will follow the branch to the left. The next juncture says “tall/short.” Since this tree is tall, we will follow the branch to the left. The next juncture says “pinnate/palmate.” Since the leaves in our tree spread out in a shape like fingers from a palm, we will follow the branch to the right. The end of this branch tells us that our tree is a “fan palm.”

How To: Using a Chart-Style Dichotomous Key

Outline of a Chart-Style Dichotomous Key

The following are the steps to use a chart-style dichotomous key to identify a specimen:

1. Observe the specimen you are going to identify.
2. Start at the top of the chart.
3. Trace downward until you reach the first junction.
4. Read the two options.
5. Decide which option best describes your specimen.
6. Follow the direction, left or right, that corresponds to step 5.
7. Repeat steps 3 to 6 until you have reached a taxon at the end of a branch.
8. Your specimen has been identified!

Example 2: Using a Dichotomous Key to Identify a Common Garden Invertebrate

A simple dichotomous key used to identify invertebrates commonly found in gardens is provided. A child finds an organism that has no wings, no shell, and 8 legs. Using this dichotomous key, determine which organism it is most likely to be.

Answer

The question informs us that this dichotomous key was designed to identify common garden invertebrates. An invertebrate is an organism that does not have a spine. Invertebrates typically possess soft, fluid-filled bodies or an exoskeleton or shell. The question goes on to describe an invertebrate organism that was found in a garden by a child. In order to identify the organism, we need to read the dichotomous key using the description provided. In order to read the dichotomous key, we start at the top and trace downward until we reach our first junction. The junction reads “wings” to the left and “no wings” to the right. Since we were told that the mystery organism has no wings, we will follow the branch to the right. The next junction we reach reads “no shell” to the left and “shell” to the right. Since we were told that the organism in question has no shell, we will follow the branch to the left. The next junction reads “no legs” to the left and “8 legs” to the right. Since we were told in the question that this organism has 8 legs, we will follow the branch to the right. This branch ends with the identity of the organism the child found in their garden.

Following these steps, we learn that the organism is most likely a spider.

Example 3: Using a Dichotomous Key to Identify Groups of Vertebrates

A simple dichotomous key used to identify groups of vertebrates is provided. An organism that does not have fur but has feathers is discovered. Using this dichotomous key, determine which group it is most likely to belong to.

Answer

The question informs us that this dichotomous key was designed to identify groups of vertebrates. A vertebrate is an organism that possesses a spine. Examples of vertebrates include reptiles, amphibians, birds, and fish. The question goes on to describe an organism that was “discovered.” In order to identify the organism, we need to read the dichotomous key using the description provided. In order to read the dichotomous key, we start at the top and trace downward until we reach the first junction. The junction reads “fur/hair” to the left and “no fur/hair” to the right. Since we were told that the mystery organism “does not have fur,” we will follow the branch to the right. The next junction we reach reads “no feathers” to the left and “feathers” to the right. Since we were told that the organism in question has feathers, we will follow the branch to the right. This branch ends with the identity of the organism described in the question.

Following these steps, we learn that the organism is most likely a bird.

Dichotomous keys are easy and intuitive to use, which is what makes them such useful and powerful tools for identification. Next, let’s learn how to make a dichotomous key of our own.

There are two slightly different methods that you can use to design and construct your own dichotomous key.

For the first method, let’s imagine the following scenario.

A student is participating in a lab activity in which they observe several specimens of protists using a microscope. The student completes the exercise and takes the notes shown in Figure 4.

Imagine that we would like to make a dichotomous key to help other students identify the same specimens when they try this lab activity. How should we proceed? First, we need to organize our specimens based on their observable traits. Then, we need to construct our dichotomous key using this information.

In order to plan our dichotomous key and organize our information, we will start by making a table like the one shown in Table 1. We will divide the specimens into two groups, A and B, based on some distinguishing characteristic. In this case, we have chosen the presence of cilia. Cilia are short extensions that protrude from the cell body. We will divide the specimens so the ciliated ones are placed into group A and the nonciliated are placed into group B.

Next, we will divide the specimens in group A into two further groups based on their distinguishing characteristics. In this case, the characteristic is whether the specimen was observed swimming freely or attached to a substrate. We will call these new, smaller groups C and D. Finally, we will do the same for group B, creating group E and group F based on the presence of a flagellum.

When we make our dichotomous key, we start with general or broad characteristics and then continue to make more and more specific distinctions.

If we had more specimens, we would continue this process of dividing groups into smaller groups based on a particular trait until each of them was in a group by itself. Since we only have four specimens to identify, we are done with the planning portion of making our dichotomous key. Next, we just need to organize the information into a chart format. An example of what that would look like is shown in Figure 5.

This method of dividing the groups into smaller, even groups is easier to use when the total number of specimens is relatively small and you intend to make a chart-style dichotomous key.

How To: Making a Chart-Style Dichotomous Key

The following are the steps to make a chart-style dichotomous key

1. Observe your specimens and take notes on their individual characteristics.
2. Organize your information into a table like the one shown below:
   1. Divide your specimens into two even groups based on an observable characteristic.
   2. Fill in these specimens as group a and group b in your table.
   3. Divide group a into two more groups, group c and group d, based on an observable characteristic.
   4. Divide group b into two more groups, group e and group f, based on an observable characteristic.
   5. Continue dividing each group into smaller groups until each specimen is in a group of its own.

      Group a: - Specimen A - Specimen B		Group b: - Specimen C - Specimen D
      Group c: - Specimen A ()	Group d: - Specimen B ()	Group e: - Specimen C ()	Group f: - Specimen D ()

3. Use the information in your table to draw a dichotomous key like the one shown below.

This second method is slightly different. It is more useful for when the number of specimens is larger or you are specifically interested in making a list-style dichotomous key.

To practice the second method, let’s imagine the following scenario.

A student in the UK has spent the summer making observations about the different species of bumblebees in their local ecosystem. Their notes are shown in Figure 6.

Imagine that we would like to make a list-style dichotomous key to identify these different species of bumblebees. How should we proceed? Whenever we are making a dichotomous key, we start by organizing our information into a table and then completing the dichotomous key based on that table.

In order to begin filling in our table and organizing our information, we will find one specimen that has a feature that sets it apart from all the others. We will add the feature and the species to the first row of a table. In this case, we have listed “elongated head shape” since that identifies the garden bumblebee from the other 5 species. Next, we will look at the remaining 5 species and pick one that has a trait that stands out from the other 4. We will fill in row 2 of our table with this information. We will continue separating one specimen from all the rest that remain, adding rows to the table until there is just one specimen remaining. You can see an example of a completed table in Table 2.

Now, we will write our dichotomous key in list format. Statement 1 in Figure 7 is the characteristic we used in the first row of the organization in Table 2. Option 1a is the species that is identified by this statement. Option 1b will instruct the user to “continue to step 2.” Statement 2 is the characteristic we used in the second row of the organization table. Option 2a is the species that is identified by that statement. Option 2b will instruct the user to “continue to step 3.” If you complete this dichotomous key by repeating this pattern, you will eventually come up with the finished product shown in Figure 7.

Let’s review what we have learned in this explainer.