LESSON 3: DICHOTOMOUS KEYS
Background Information
Dichotomous keys are a method used in taxonomy to categorize species.
It is a form of a single-access key where the sequence and structure of identification steps are fixed by the author. At each point in the decision process, alternatives are offered, each leading to a result or a further choice. If there are only two choices at each branching point, then it can be considered to be a dichotomous key. The majority of single-access keys are dichotomous.
A dichotomous organizes a large set of organisms in a structure that breaks them down into smaller subsets, with many keys leading to the smallest available classification unit, which is usually a species or a specific taxon.
The key questions can either be displayed as a lead style where statements are made, or as questions/answer style where a question is asked, with possible answers displayed underneath. The questions involved have yes or no answers that are based on either characteristics of the organisms or they can reflect taxonomic classification.
It is a form of a single-access key where the sequence and structure of identification steps are fixed by the author. At each point in the decision process, alternatives are offered, each leading to a result or a further choice. If there are only two choices at each branching point, then it can be considered to be a dichotomous key. The majority of single-access keys are dichotomous.
A dichotomous organizes a large set of organisms in a structure that breaks them down into smaller subsets, with many keys leading to the smallest available classification unit, which is usually a species or a specific taxon.
The key questions can either be displayed as a lead style where statements are made, or as questions/answer style where a question is asked, with possible answers displayed underneath. The questions involved have yes or no answers that are based on either characteristics of the organisms or they can reflect taxonomic classification.
Lesson Outline
HOOK
Use volunteers from the class. Ask the class how we could separate all the students so that we have a specific description for each student.
Could use descriptors such as: male or female, colour of hair, colour of eye, height, clothing, etc.
ACTION
How and why dichotomous keys are used
Instructional Videos on Dichotomous Keys
Practice making and using a dichotomous key as a class using the monster template. (Please see the Creative Pieces Tab to get more information on this activity).
CONSOLIDATION
Practice as a class using a dichotomous key using the online interactive
Use volunteers from the class. Ask the class how we could separate all the students so that we have a specific description for each student.
Could use descriptors such as: male or female, colour of hair, colour of eye, height, clothing, etc.
ACTION
How and why dichotomous keys are used
Instructional Videos on Dichotomous Keys
- Video: How to Organize Organisms in a Flow Chart
- Video: How to Make a Dichotomous Key from a Flow Chart
Practice making and using a dichotomous key as a class using the monster template. (Please see the Creative Pieces Tab to get more information on this activity).
- Students will work as a class to make the dichotomous key
- Once complete, students will each get a "monster" on their back
- They will have to ask their peers the series of questions that were developed for the dichotomous key in order to find out what type of monster they are
- Students will then be able able make their dichotomous key, they will hand it in as their exit slip at the end of the class.
CONSOLIDATION
Practice as a class using a dichotomous key using the online interactive
Additional Teaching Resources
References
Biology Junction. "Dichotomous Key to Salamanders." Dichotomous Keys. Retrieved on Feb 3, 2013 from: http://www.biologyjunction.com/salamander_key.htm
Di Giuseppe, M., Fraser, D, LeDrew, B, & Roberts, J. (2003). Biology 11: College Preparation. Nelson, Canada.
Ellis, C., Muller, M.R., Panayiotou, H. E., Sharp, J. C., Webb, P. (2011). Biology Source 11. Pearson Investigating Science, Toronto, Ontario.
Explore Learning. "Lesson Info: Dichotomous Keys Gizmo" Retrieved on Feb 3, 2013 from: http://www.explorelearning.com/index.cfm?method=cResource.dspDetail&ResourceID=1047
Kimmel, Elliott. "Dichotomous Key for Bugs." Zerobia. Retrieved on Feb 3, 2013 from: www.execulink.com/~ekimmel/dichotomous_bugs.swf
"Making a Dichotomous key - Part One.mp4 - YouTube." YouTube. Retrieved on Feb 3, 2013 from: http://www.youtube.com/watch?v=IFDuz92fzLE
"Making a Dichotomous key - Part Two.mp4 - YouTube." YouTube. Retrieved on Feb 3, 2013 from: http://www.youtube.com/watch?v=QEnTATHJ058
Nixon, Carolyn. "The Naturalist's Apprentice: So Many Fishes! How can You Tell Them Apart?." INHS Reports. University of Illinois, Retrieved on Feb 3, 2013 from: www.inhs.illinois.edu/inhsreports/may-jun99/fishkey.
Teaching Channel. "Dichotomous Key - YouTube." YouTube. Retrieved on Feb 3, 2013 from: http://www.youtube.com/watch?v=MtXspRBsrHs
Di Giuseppe, M., Fraser, D, LeDrew, B, & Roberts, J. (2003). Biology 11: College Preparation. Nelson, Canada.
Ellis, C., Muller, M.R., Panayiotou, H. E., Sharp, J. C., Webb, P. (2011). Biology Source 11. Pearson Investigating Science, Toronto, Ontario.
Explore Learning. "Lesson Info: Dichotomous Keys Gizmo" Retrieved on Feb 3, 2013 from: http://www.explorelearning.com/index.cfm?method=cResource.dspDetail&ResourceID=1047
Kimmel, Elliott. "Dichotomous Key for Bugs." Zerobia. Retrieved on Feb 3, 2013 from: www.execulink.com/~ekimmel/dichotomous_bugs.swf
"Making a Dichotomous key - Part One.mp4 - YouTube." YouTube. Retrieved on Feb 3, 2013 from: http://www.youtube.com/watch?v=IFDuz92fzLE
"Making a Dichotomous key - Part Two.mp4 - YouTube." YouTube. Retrieved on Feb 3, 2013 from: http://www.youtube.com/watch?v=QEnTATHJ058
Nixon, Carolyn. "The Naturalist's Apprentice: So Many Fishes! How can You Tell Them Apart?." INHS Reports. University of Illinois, Retrieved on Feb 3, 2013 from: www.inhs.illinois.edu/inhsreports/may-jun99/fishkey.
Teaching Channel. "Dichotomous Key - YouTube." YouTube. Retrieved on Feb 3, 2013 from: http://www.youtube.com/watch?v=MtXspRBsrHs