Constructing a Dichotomous Key
What is a Dichotomous Key?
• A system of identifying objects or organisms
in which a succession of paired choices leads
progressively to identification of the object
(the only remaining correct choice.)
Using a Dichotomous Key
• Study the organisms that will be classified.
Choose one
• Read the first set of choices in the
dichotomous key
• Decide which characteristic the organism has
• Follow the numbered items in the key and
name the organism
Finch
Identification
1. a. The beak is relatively long and slender.
2.
3.
Certhdea sp.
b. The beak is relatively stout and heavy.
Go to set 2
a. The bottom surface of the lower bill is flat and
straight.
Geospiza sp.
b. The bottom surface of the lower bill has a bend
Go to set 3
a. The lower edge of the upper bill has a distinct
bend.
Camarhynchus
sp.
b. The lower edge of the upper bill is mostly flat.
Platyspiza sp.
Seatwork
• Complete the Using a Dichotomous Key
assignment
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.
An Example
• As an example, we could construct a key using
the following stationary supplies taken from a
student's pencil case.
• pen, highlighter, pencil sharpener, paper clip
A Spider Key
• Some students find it easier to arrange their
information in a spider key before setting it up
in the traditional dichotomous key format.
A Traditional Dichotomous Key
• The same information presented in a traditional
dichotomous key format would look like this:
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. not constructed of plastic ……….pencil
3. a. green and grey ………………......highlighter
b. blue and clear …………………….pen
4. a. black and silver …………………..pencil sharpener
b. silver ……………………………….paper clip
Homework
• Create a dichotomous key for the insects on page 26 #16
• Number them from 1 – 7 starting on the top row left to right (1 – 3)
• Second row left to right (4 – 7 )