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.
