Dichotomous Keys

What is a dichotomous key?• A tool that scientists can use to help

identify a particular specimen.

• Based on the organisms

characteristics (Ex. Eye color)

• Never based on the organisms

behaviors (Ex. Hibernates in winter)

Examples of the specimens

• Specimens could be trees,

wildflowers, mammals,

reptiles, rocks, and fish.

Think of three other examples of things that

could be used using a dichotomous key.

What does the prefix “di”

mean?

Two

“Dichotomous”

means “divided into

two parts”.

How to use a dichotomous key?

Dichotomous keys always give two choices in

each step. Often they are opposites

Examples Black/white; blue/green;

pointed/rounded

• The dichotomous key allows for scientists

to ask a series of questions with yes or no

answers.

Rules For Dichotomous Keys

• Read both choices in a couplet carefully.

Although the first description may seem to

fit your sample, the second may apply

even better

• If you are unsure of which choice to make

in a couplet, follow both forks (one at a

time). After working through a couple of

more couplets, it may become apparent

that one fork does not fit your sample at all

• When you have keyed out an organism,

do not take your effort as the final result.

Find a type specimen (if available) and

compare your unknown to the type

specimen.

• When a measurement is indicated, make

sure that you take the measurement using

a calibrated scale. Do not “eyeball” it or

take a guess

RULES IN A NUTSHELL

1.Read both choices before you make your

decision

2. If you are unsure – Follow both paths. The

correct path will become clearer once you

move on further down the couplets.

3. If possible, compare your unknown to an

actual specimen

Here are creatures we don’t know!

Lets choose one

How to use a Dichotomous Key?

Choose only one creature at a time.

How to use a Dichotomous Key?

Read steps 1a and 1b

Decide which statement is true

How to use a Dichotomous Key?

1b is true

Then follow the directions after that step.

How to use a Dichotomous Key?

At choice 5, you make another dichotomous choice

How to use a Dichotomous Key?

5a is true

Keep going until you come to a step

that gives you the creature’s name.

How to use a Dichotomous Key?

6 a. The creature has one antennae Go to Step 7.

b. The creature has two antennae Mr. Mitchell

1. a. wings covered by an exoskeleton – go to step 2

b. wings freely observed – Go to step 3

2. a. body has a round shape ……….ladybug, a red beetle with black spots

b. body has an elongated shape ……….grasshopper, a green insect that hops

3. a. wings point out from the side of the body ……….dragonfly, an insect that is 10- 15 cm long and lives in marshes

b. wings point to the posterior of the body ……….housefly, a flying insect with red eyes and an annoying buzz

Dichotomous key

1 With flower.

Without flower

Angiosperms

Go to 2

2 Seedless.

Seed bearing

Go to 3

Gymnosperms

3 Plant body do not differentiated into root,

stem and leaves.

Plant body differentiated into root, stem and

leaves

Go to 4

Go to 5

4 Without photosynthetic pigment.

With chlorophyll or other photosynthetic

pigment

Algae

Fungi

5 No vascular tissues.

With vascular tissues

Mosses

Fern

Another Example of a dichotomous key…

Dichotomous key

the dichotomous key can also be expressed in a diagram form

Non-flowering plants

Without photsyn-

thetic pigment

With chlorophyll or

other photosynthetic

pigment

Algae Fungi

No vascular

tissuesWith vascular

tissues

Mosses Gymnosperms

Plant body not

differentiated into

root, stem and

leaves

Plant body

differentiated

into stem and

leaves

Seedless Seed-bearing

Ferns

Flowering Plants

(Angiosperm)

Without flowerWith flower

Plants