Lesson 9. Dichotomous Keys
Estimated Time: 1 period
Objective
To use structural characteristics in order to identify an organism (taxonomy and classification)
EQ: How can I use characteristics and a specific scientific tool (dichotomous key) to identify a
specimen?
Bridge
We have been doing a lot with characteristics and relationships. What have we looked at to
identify these relationships?
Mini Lesson
Share out responses from the bridge.
Characteristics or traits are the easiest way to identify organisms. Scientists know that there are
millions of different species on Earth, but they also know that there are many more they have yet
to discover. Observation of an organism alone is enough to give the species a name and to place
it into one of the kingdoms, but there needs to be a way for them to identify differences between
species that look very much alike or to identify an organism they come across to find out if it is a
new species or an already existing one.
Scientists use a tool to help them do this. This tool is a dichotomous key. The idea behind this
tool is to take characteristics and methodically separate organisms from each other based on
these traits until you have the specific name of the organism.
Let’s take a look at a dichotomous key.
Take your fish cards and place them into groups based on their characteristics (do this in pairs).
Since we still don’t know their names, we can use these same characteristics that we used to
group them to figure out what they are. Use the fish key on the next page with your pictures to
identify each kind of fish and see if you can find the trick to reading a dichotomous key! We
will then label each of them as a class (do not label them below until we go over them)
Fish key
Step 1
If fish shape is long and skinny, then go to step 2
If fish shape is not long and skinny, then go to step 3
Step 2
If fish has pointed fins, it is a trumpet fish
If fish has smooth fins, it is a spotted moray eel
Step 3
If fish has both eyes on top of the head, then go to step 4
If fish has one eye on each side of the head, then go to step 5
Step 4
If fish has long whip-like tail, it is a spotted eagle ray
If fish has short, blunt tail, it is a peacock flounder
Step 5
If fish has spots, then go to step 6
If fish does not have spots, then go to step 7
Step 6
If fish has chin "whiskers," it is a spotted goat fish
If fish does not have chin "whiskers," it is a band-tail puffer
Step 7
If fish has stripes, then go to step 8
If fish does not have stripes, it is a glassy sweeper
Step 8
If fish has a v-shaped tail, it is a squirrel fish
If fish has a blunt tail, it is a glass-eye snapper
Fish Cards…. Cut them out ahead of time, 1 set for each pair of students.
Work Period
Create a dichotomous key for the 8 organisms from the activity yesterday.
Summary
Answer the EQ
SPED and ELL Modifications:
For students using translation- this activity is online and can be simply ‘cut and paste’ into
translation app instead of retyping or recording directions
SpeakText app: is able to open the webpage and would make this very easy.
http://www.pbs.org/wgbh/nova/teachers/activities/pdf/2215_reef.pdf
Apps and Internet activities:
1. Dichotomous Key activities to use as additional practice.
http://www.amaisd.org/sites/nheights/dichotomous_key.htm
2. http://www.mdsg.umd.edu/programs/education/interactive_lessons/key/student1.htm
Closing, answer all of the following questions
1. A dichotomous key is shown below.
Use the dichotomous key to identify the scientific name of the organism represented
below. [1]
__________________________________
Independent Practice
Regents questions on dichotomous keys…. Specifically the one with the chart and the fish and
the one with the spotted bugs and wings