Making Dichotomous Keys
Lesson Objectives:
• To construct a dichotomous key based on the characteristics of “specimens” (similar items).
• To prepare a dichotomous key showing classification system.
• To use a dichotomous key to help identify similar items.
• To recognize the validity of classmates’ classification systems.
A dichotomous key is a device that can be used to easily identify an unknown organism. The word dichotomous comes from
two Greek words that together mean, "divided in two parts". A dichotomous key consists of a series of two part statements that
describe characteristic of organisms. At each step of a dichotomous key the user is presented with two choices. As the user
makes a choice about a particular characteristic of an organism they are led to a new branch of the key. Eventually the user
will be led to the name of the organism that they are trying to identify.
Constructing Dichotomous Keys
As an example, we could construct a key using the following stationary supplies taken from a student's pencil case.
When constructing a dichotomous key, the first step is to look at the group of objects or organisms and separate them into two
groups based on a single distinguishing characteristic. Then continue to separate each of the groups until each object has its
own separate set of characteristics.
You may find it easier to arrange your information in a spider key before setting it up in the traditional dichotomous key
format. The following key has been created as an example using the stationary supplies pictured above.
Key to Stationary Supplies
Spider Key:
The
same
information
outlined
in
the
spider
key
above,
presented
in
a
traditional
dichotomous
key
format,
would
appear
as
follows:
1.
a.
long,
tubular
objects ....................................................................................................go
to
#2
b.
short,
non‐tubular
objects.........................................................................................go
to
#4
2.
a.
constructed
from
plastic ............................................................................................go
to
#3
b.
constructed
from
material
other
than
plastic...................................................pencil
3.
a.
green
&
gray ....................................................................................................................highlighter
b.
blue
&
clear ......................................................................................................................ball
point
pen
4.
a.
black
&
silver....................................................................................................................pencil
sharpener
b.
silver ...................................................................................................................................paper
clip
Assignment: Creating a Dichotomous Key
Create a dichotomous key based on the characteristics of a group of everyday items (specimens). Other students will evaluate
your dichotomous key by using the key to identify each of your ten items.
1.
Obtain a group of ten different items from a similar category (ex. pens, buttons, jewelry).
2.
PAGE 1: CLASSIFICATION TABLE
• Photograph or find pictures of each of your ten items (be sure pictures are clear).
• Incorporate photos/pictures into a Table similar to the “It’s Classified – Shark activity” and “Sample
Classifying Shoes” on my website.
• Give each item a number (Item #1, Item #2, etc.) and a blank line after it.
3.
Carefully examine the characteristics of the ten items and create a dichotomous key (similar to the one we did in class)
using some of these characteristics.
1.
2.
3.
Is the shoe a sneaker or a sandal?
Is it a high top or a low top?
Is it leather or canvas?
4.
PAGE 2: DICHOTOMOUS KEY
Create and type up your dichotomous key in a table format (see Shark activity and Classifying Shoes). Check my
website for a template you may use for this project.
5.
PAGE 3: ANSWER KEY
Make an answer key. Make a copy of your Classification Table and include the item identification names. The
answer key should be set up like the Sample Answer Key: Classifying Shoes that can be found on my website.
6.
Hand in your finished Classification Table, Dichotomous key, and Answer Key (3 pages total).
7.
STUDENT EVALUATION:
When you receive a key that has been created by another student, use the key to try and identify each of the items.
Fill in the names and/or identifying numbers for each of the items in the blanks provided.
8.
Using the rubric provided, assign each key that you work on a mark out of ten along with an explanation for the mark
you are assigning. Use the questions listed below to help you evaluate each of the keys.
✩ What difficulties did I have with the dichotomous key?
✩ What are the positive points about the dichotomous key?
✩ What kind of improvements might be made to this dichotomous key?
Classification
items:
Choose
one
of
the
following
categories
or
choose
one
of
your
own:
Cars
Electronics
Computers
Plates/Cups
Sports
equipment
Flowers
Buttons
Calculators
Clothing
Animals
Pens
Plants
Shells
Books
Jewelry
Chairs
Leaves
Cereal
Money
Pots/Pans
Silverware
String
instruments
Remotes
You
may
not
use
SHOES
as
your
items
for
this
activity!