Dichotomous Keys

Dichotomous Keys

›Dichotomous Keys are a series of branching, two-part statements used to identify organisms or objects. Organisms are classified into various taxa with defined criteria which provides a framework for identifying organisms. Dichotomous Keys are basically a structure in which a large set of items is broken down into smaller subsets, ultimately leading to the smallest available classification level.


If we pick a bird in the above image, we can follow the dichotomous key to figure out which species it is. for example, if we choose Bird X: 1b to 2b to 3a (Camarthyncus)

A key challenge that traditional taxonomists face is deciding and agreeing on what criteria to use to define each taxon. The choice of criteria is often hotly debated but most modern taxonomists no longer use similarity as the basis for grouping organisms. Now, biologists use the principle of relatedness to classify organisms and this is based on the evolutionary history of a species. This new approach has significant advantages but many biologists still us the traditional and familiar classification system.


Dichotomous keys can come in many shapes and forms. The one above shows classification in a neat chart formation. As long as the species can be identified, written or chart formats of keys are useful.

Teaching Activity: Create a dichotomous key with the students in your class. Randomly choose 6 students to come to the front of the class. A good way to start your key is to distinguish between males and females. Then you can use features like hair colour, if they are wearing glasses or not, clothing etc…). Once the students watch you create a dichotomous key, they are then ready to work on their own in their table groups.


dichoactivity2Use the slide above on the Smart Board and write the dichotomous key out while the students are at the front of the class.

Student Activity: Create Your Own Dichotomous Key

Print off the sets of species and cut them out. You should print about 5 of every sheet so students can work in groups. Each group will receive a red level (fish), yellow level (animals) and green level (monsters) set. Using chart paper, students will create a dichotomous key that they will exchange with another group at the end. Groups will be checking to see if the dichotomous keys that their peers made work. Students can choose which level they wish to tackle depending on how much they want to challenge themselves (red=hardest, yellow=medium, green=easiest). Remind students that it is always best to push themselves out of their comfort zone but that it is great to start with a green and then try to work their way up. If there is extra time in class or if students are working well that day, they can do the remaining keys.

Dichotomous Key- Green Level (Choose 4 monsters and cut them out)


Dichotomous Key: Yellow Level (Write the binomial nomenclature name of each species on the back and cut out)


Dichotomous Key- Red Level (Scroll down to the #5 fish key)



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