Creating aCreating aDichotomous KeyDichotomous Key

© S. Tanner / 1999

What is a Classification Key?

• Classification keys are used to identify an unknown biological specimen.

• One of the most frequently used types of keys is the “Dichotomous Key”

• What does “Dichotomous” mean?

Dichotomous

“Di”

(two)

“Chotomy”

(choices)

A dichotomous key always presents you with two choices

Two choices of what?

The choices are just different characteristics, or features, of the specimen. For example, is the shape of a leaf tip pointed or round?

Or, for the characteristic “leaf type”, is the leaf...

•A broadleaf type

•Or needlelike?

It’s a little tricky to make a dichotomous key for another person to use, but there is a process you can follow to

develop a key that works well.

Start by gathering a sample set of specimens

You will create a key for a group of seven specimens. They are all broadleafed

deciduous trees (they lose their leaves at the end of each growing season).

Look at the sample specimens shown on the next slide. Study them carefully, then write a list of some of the general leaf characteristics

that could be used to discuss differences between the specimens.

These should just be general features, not the specific descriptions you will

use in your dichotomous key.

generalfeatures?

How many of the following leaf characteristics did you come up

with?

•Leaf arrangement

•Leaf shape

•Margin (edge) features

•Tip shape

•Vein pattern

•Length of stalk (stem)

There are two general types of leaf arrangements:

Single leaves - one leaf on each leaf stalk.

Compound leaves - multiple leaves on oneleaf stalk.

Here are typical descriptions of the different leaf shapes:

These are the different leaf margin (edge) features:

Leaf tip shapes:

Leaf vein patterns:

Leaf stalk lengths:

There are other features of trees that could have been used if you had more information about the

trees, for example:

•bark features•tree silhouette shape•seed and cone characteristics

Tree silhouette shapes:

Tree seedsand cones:

Now it’s time to create a key for the seven tree specimens.

Remember that the key must be dichotomous, and we’ll start by

using a spider key format to organize the trees into

successively smaller and smaller groups.

Animal Group

Has no legs Has legs

Has two legs More than two legs

Horse

No feathersHas feathers

PersonBird

Has finsand scales

Doesn’thave finsand scales

FishSnake

For example, for the animal group - person, fish, horse, bird,and snake - a dichotomous spider key might look like this:

Animal Group

Has no legs Has legs

Has two legs More than two legs

Horse

No feathersHas feathers

PersonBird

Has finsand scales

Doesn’thave finsand scales

FishSnake

Once you have created the spider key, you must number the choices- each group of two has the same number, and either an a) or a b), for example:

1 a) 1 b)

2 a) 2 b) 3 a) 3 b)

4 a)4 b)

Animal Group

Has no legs Has legs

Has two legs More than two legs

Horse

No feathersHas feathers

PersonBird

Has finsand scales

Doesn’thave finsand scales

FishSnake

The first set of choices is always 1a) and b), but notice that youcan number differently from there. Compare the numbering onthis slide to the previous slide. Both are equally correct.

1 a) 1 b)

4 a) 4 b) 2 a) 2 b)

3 a)3 b)

Now that you have created a numbered spider key, rewrite the key by arranging the choices in a list format so that they fit nicely one below the other

on the page. The list key puts the numbers in sequential order, and gives “go to” directions or

identifications after each choice:1a) Has no legs …………………………..go to 21b) Has legs………………………………go to 3

2a) Has fins and scales……………………Fish2b) Doesn’t have fins and scales………… Snake

3a) Has two legs………………………….go to 43b) More than two legs…………………..Horse

… and so on

So that’s it!

Your teacher will now give you thematerials to makea dichotomous key of your own.

Good Luck!

Hickory Maple

Horse chestnut

Eastern redbud

Oak

Poplar

Birch