Dichotomous Keys: Identification Achievement Unlocked

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Do you like to organize things?

One of us is very organized…maybe a little bit too


As for Pinky, it’s more like she lost a game of Jumanji.

But scientists, they are often trying to classify and organize different

organisms based on shared characteristics.

Having a classification system for organisms is important when trying to investigate common

ancestry and also to have a common understanding for scientists worldwide to use in classification.

It’s also important to have tools to differentiate among different organisms.

In this particular clip, we are going to use a tool called a

dichotomous key to identify 5 different organisms that are each from five different taxonomic


So what’s a dichotomous key?

Well it allows you to identify organisms based on

a series of statements that are typically organized in


We are going to use the dichotomous key to determine scientific names for five mystery


Often scientific names have Latin or Greek roots, and they can be used across different

languages by scientists all over the world.

This unification in the naming system is important because common names vary by language and location.

They're often are far less reliable.

For example, this scientific name has a lot of common names

such as mountain lion, Texas panther, puma, or cougar.

Although those were fun to say.

Ok, so let's go ahead and get started!

Here are our mystery organisms with clues: We are going to use this dichotomous key.

Now we’re going to point out that this dichotomous key has been created for use with these 5

specific organisms in the chart and only these specific organisms.

Let’s start with…mystery organism A, an amoeba!

With a dichotomous key, you always want to start with #1.

If you look at the clues, you notice this organism contains a nucleus.

A reminder from our prokaryote/eukaryote video…this means it is an eukaryote.

This tells us to go to #3.

So is this organism an autotroph or a heterotroph?

Well autotrophs make their own food.

An example is a photosynthetic organism making food from light energy.

Heterotrophs must rely on external food sources.

Since our clue states that this organism feeds on

other organisms, it must be a heterotroph.

This tells us to go to 4.

Now in 4, we have to think about this.

There’s actually a lot of different species of amoeba and we

don’t know what kind it is.

But it mentions that this specimen is small.

And this particular specimen is definitely microscopic meaning you need a microscope

to see it well.

Now not all microscopic organisms are necessarily single-celled, but

if you look at our organism here, this is one cell.

A single celled organism is unicellular, which means it only has one cell.

Therefore, 4B is more appropriate meaning that our organism

is Amoeba Proteus.

This one kind of had a giveaway because the scientific name had “amoeba”

in it, didn’t it?

But be careful as you will find that’s not always the case.

One thing about unicellular and multicellular.

Generally, organisms that are large enough for you to see easily with the naked eye,

with a few exceptions, are multicellular.

Organisms that can be seen easily with the naked eye tend to have structures that are

made of many cells---think of plants, animals, mushrooms…

For the purposes of this exercise, we have illustrations to hopefully make it

very clear.

If you are ever unsure, you can always research online!

Ok, let’s do one more.

Mystery organism B, a plant.

Again, always start with #1.

The details mention nuclei---remember that’s the plural

of nucleus because you don’t say nucleusus---you say nuclei.

This organism is made up of many cells that all have a nucleus.

Like the previous example, that means it’s an eukaryote!

So that takes us to #3.

So are plants autotrophs or heterotrophs?

Well they make their food from the sun right by photosynthesis?

They do not need to consume other organisms.

That makes them autotrophs, because they make their

own food.

That means according to this key---this plant here is Chlorophytum comosum.

Just a fun fact, the common name for this plant is

a spider plant.

Now you can try with the other mystery organisms on our accompanying handout.

A few big things we want to remind you: (1) Remember to always

start with #1 on the key and so you will start with #1 for every organism.

(2) You have to be careful not to just pick out random phrases

in the dichotomous key.

You may think you’re taking a shortcut but there are no shortcuts!

You need to go through the sequence.

For example, there is more than one organism on this chart

that is unicellular in that phrase 4B!

If you just picked out that phrase, you’d have

more than one organism in that category.

Our handout also includes a challenge.

This dichotomous key is really only useful with

these 5 organisms.

What if we add in another organism--- like this cat here?

Its scientific name is Felis catus.

But now our key doesn’t work correctly!

Your challenge is to redesign the dichotomous key so that it would also

work if including this cat.

You may just need a few revisions or additions.

On our handout, we have room for you to create the modified

dichotomous key so that the cat is included.

You can even design it in a differentiating Google form if you prefer, and we have instructions

for that on our handout.

One point to remember when making a dichotomous key: you need to use clues that an observer

would have access to.

If we started putting habitat information in the dichotomous key,

you wouldn’t necessarily know that information unless it was included.

Well that’s it for the Amoeba Sisters and we remind you to stay