Students sometimes struggle to make a dichotomous key on their own. Creating one together as a class can be very helpful. This activity takes the fun one step further by actually using classmates as the subject of the Key itself!
First, show students a simple dichotomous key, and explain how it works. I use something like this –
Then, explain that we are going to make a dichotomous key for our classroom!
Make a Dichotomous Key with your Students
We will start with the tree form of the key. It is easier for students to follow and much more visually intuitive.
Have all of the boys come to the front of the room. Invite students to offer suggestions for characteristics or attributes that would roughly split the group of boys in half (we usually start with ‘wearing shorts’ vs ‘not wearing shorts’). Have the boys divide into those two groups, while you draw the first part of the Dichotomous Key on the board:
I use the boys for this activity, rather than girls or the whole class, for two reasons. It would take much longer to key the whole class! Also, girls (in my experience) tend to be more uncomfortable standing in front of the room being scrutinized for their attributes.
Addressing Common Student Misconceptions
Now that we have split our large group in two, focus on just one group. We will start with the shorts group for our example. Have students suggest another attribute that will divide this shorts group into two parts. Making a Dichotomous Key with the whole class allows you to guide students’ thinking processes. Help them choose binary options (the ‘di’ part of Dichotomous Key!). For example, they may suggest ‘red shirt’ vs ‘green shirt’ vs ‘blue shirt’. For Dichotomous Keys we only want two options for each level, so we could instead say ‘red shirt’ vs ‘not red shirt’.
Let’s say you now only have two boys that are wearing shorts and not-red shirts – Jackson and Sam. Jackson happens to be a lot taller than Sam. Students will likely suggest the subjective attributes ‘tall’ vs ‘short’. Height can be used, but only if it is objective. Get out a yardstick and choose where the measurement will be. For example, ’5’11 or more’ vs ‘5’10 or less’.
This way, if another boy walked into the room who was wearing shorts and a not-red shirt, and was 6’1’’, he could easily be placed into our Key. We would only then need to add one more level, some difference between the new boy and Jackson.
Finishing the Key
As you go along, once a student is fully categorized, he may sit down. Go back up a few levels and characterize the next smallest group (red shirts in our example). Then, once all of the ‘shorts’ are finished, move back over to the ‘not shorts’ and work them out the same way.
When the opportunity comes up, take the chance to reinforce the sequential structure of a dichotomous key. For example, Greg is wearing a red shirt. Why would he not go under the ‘red shirt’ group? Explain that it is not just a ‘red shirt’ group, it is a ‘wearing shorts and a red shirt’ group. Greg has a red shirt, but he is not wearing shorts, so he does not belong in that group.
This can bring you to another important point, which is that we can make a dichotomous key in many different ways! You could choose any number of attributes as you go along.
What is the Purpose of a Dichotomous Key?
The ultimate purpose of a dichotomous key is for identification. Explain and show that now any stranger can walk into Mrs. Parker’s classroom and use the Key to identify the name of any boy in the room!
Keys are also useful for categorizing new species. To demonstrate, once your Key is complete, invite another boy or male teacher to enter the room and challenge your students to place him properly into the Key! Or, pull up an image of any random male on the internet and do the same. You will need to add an additional level somewhere on the Key. For example, if the guest student is not wearing shorts and is wearing a hoodie, you will need a new attribute to distinguish between the new person and Jeff.
Dichotomous Keys can also be used to figure out traits of an individual. If someone outside the room wanted to know about Greg, he could trace backwards up the Key and see that Greg was wearing flip flops, no hoodie, and shorts.
Make a Dichotomous Key into the Chart Form
Once students are comfortable with the Key this way, they are ready to convert it into a chart form. I find it easiest to understand if you number the brackets/levels first, then put them into the chart. I follow the same path we took to create the tree form to make it easier for students to track with the process.
First we will drop straight down to Jackson and Sam. We will have to leave some blanks in the chart that we will come back to later.
Now we are ready to go back to finish out Choice 2, wearing a red shirt, starting with the next available row on the chart: row 4.
And finally we go back to our first choice and drill down from there:
Students Make a Dichotomous Key On Their Own!
Now students are ready to make their own Dichotomous Key! There are tons of ways you can set this up. When my own children were young, I would bring in a huge bag of their little toys (small stuffed animals, McDonalds doo-dads, toy cars, action figures, etc). Each pair of students got five or so toys to make a Key with.
My toy stash got too low once my babies grew up, so I swapped them out for Mystery Creatures! These are imaginary creatures that are printed and cut out. Students work in pairs to make a dichotomous key with six different creatures.
Just for fun, I challenge students to give a scientific name to the creatures before making the Key. It gives them a chance to be creative and reinforce the format of a scientific name! Once they are done with the Key, pairs swap their Keys and creatures, challenging each other to use the Key and identify (figure out the name) of each creature!
I would love to share the Creature Key Activity with you! Click here and get the Dichotomous Key with Creatures activity as a free download!