Lesson Video: Dichotomous Keys Biology

Video Transcript

In this video, we’ll learn about the dichotomous key, what it is, and how it’s used. After that, we’ll practice making and using our own dichotomous keys. So now that we’ve identified what this lesson’s about, let’s get started.

A dichotomous key is a tool used for the identification of organisms. It’s not to be confused with a phylogenetic tree, which shows classifications and evolutionary relationships. A single dichotomous key is meant to be used with a specific set of specimens, not a wide range of different organisms. They’re designed to allow scientists to identify an organism from a group of different organisms by answering a set of yes or no questions. The word dichotomous actually means something that’s divided into two parts, which both describes where dichotomous keys get their name and how they’re made. Now we’re ready to try an example.

A curious biology student is interested in the plants growing around their school and draws a sketch of the tree we see here. Their teacher provides them with this dichotomous key, which should allow them to identify the tree. Our student reads the first statement: broad, flat leaves. They decide that this does not apply to the tree they drew. They follow the instructions to move on to step two. Is the tree short and bushy? Again, our student decides no and moves on to step three. This statement, palmate or fan-shaped leaves, our student can answer yes to. And they have identified their tree as a fan palm.

A dichotomous key in this form can continue for pages and pages, depending on how many different organisms is designed to identify. If a dichotomous key is only designed to identify a few organisms, sometimes it’s organized more visually, like this. In this case, our student would start at the top and move down the tree, making decisions at each branch, following the path in the chart until they’ve reached the correct identification.

Dichotomous keys can also be used to identify organisms in another way. Let’s say that this student would like to find a Mazari palm near their school. Well, according to the dichotomous key, they know they’re looking for a short, bushy tree that has compound leaves. Dichotomous keys are pretty intuitive to use, which is what makes them such a powerful tool for identification.

Next, let’s take a look at how we can make our own dichotomous key. Let’s imagine that a biology student is using a microscope to observe protists during a lab exercise. During the activity, they drew sketches and took notes about a Paramecium, a Stentor, an Amoeba, and a Euglena. Our biology student is interested in making a dichotomous key to help other students to identify these organisms. So how does she get started? Well, she can start by dividing these organisms into two even groups based on some defining characteristic. For example, the Paramecium and the Stentor possess cilia, while the Amoeba and the Euglena do not.

Next, we’ll need to further distinguish the two organisms in group A based on their individual characteristics. For example, the Paramecium was observed swimming freely, while the Stentor was observed attached to a stationary object. Next, we need to do the same thing for the two organisms in group B. For example, the Euglena swims around with the use of a flagellum, but the Amoeba does not. Now, we have all the information we need to draw our dichotomous key. We can fill in our characteristics and then add our organisms. And now, future students have a tool that they can use to quickly identify these four protists.

Let’s try an example with a few more organisms. Let’s imagine that a student in the UK spent their summer observing and identifying different species of bumblebees. These are their sketches and their notes. Now, the student would like to make a dichotomous key to allow others to quickly identify these common bumblebees. Here, I’ve listed the steps that we followed in our last example. Our biology student will start by dividing the organisms into two groups based on their characteristics. Then, they’ll continue to divide the groups further until each specimen is in its own category.

The last step is to draw, label, and fill in the new dichotomous key. At this point, you might want to pause the video and see if you can make this dichotomous key on your own. Go ahead and see what you come up with. There are several different correct answers.

I started by dividing the bees into a group that’s mostly solid color and a group that possesses bold, yellow, and black stripes. The mostly solid color group is distinguished further into bees that are mostly ginger colored and bees that are mostly black with a red tail. Next, for the yellow striped group, I divided those bees further into bees that possess a bright white tail and those that have a tale of a different color. Between the two white tail species, they can be told apart because one has a longer head shape than the other. And between the two remaining species that do not have bright white tails, one of them has a buff colored tail, and one of them has a bright red tail.

Let’s check our work by putting our dichotomous key to the test. We’ll try to identify the species of this bee specimen using our own dichotomous key. In the key that I’ve made, I’ll start by recognizing that this bee is not solid colored. It has bright yellow and black stripes, and its tail does not appear to be bright white. Its tail is also more of a buff or beige color than it is red. So we can conclude that this bee is a member of the species Bombus terrestris, also known as a buff-tailed bumble bee.

Your dichotomous key might look completely different than mine, but it should lead you to the same conclusion. Now that we’re familiar with dichotomous keys, their function, and the basics of how they’re constructed and used, let’s try a practice question.

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 the dichotomous key, determine which group it is most likely to belong to.

This question is asking us to use the dichotomous key provided to identify the group that a particular vertebrate must belong to. A dichotomous key uses the observable characteristics that an organism possesses in order to determine the identity of that organism. So what characteristics does our mystery organism possess? We’re told that this organism does not have fur, but it does possess feathers. Using this information, we should be able to identify its group.

We’ll start at the top, knowing that our organism is some type of vertebrate. And we’ve been told that this organism does not have fur. Continuing downwards, we reach another branch. And this time, we know that our mystery organism has feathers. Here our path ends, and we find the group bird. So the group that an organism that does not have fur but does have feathers is most likely to belong to, according to this dichotomous key, is bird.

Next, let’s wrap up our lesson by reviewing what we’ve learned. In this video, we learned about dichotomous keys, which are tools used by scientists to identify organisms based on their characteristics. We looked at several examples of dichotomous keys. And then we practiced making and using dichotomous keys to identify specimens in some examples.