Dichotomous Keys
Introduction
Taxonomic systems can be used to help identify unknown organisms. A taxonomist, working on
a particular group of organisms, will devise and publish a taxonomic key to aid others in the
identification of specimens. The keys are usually written for a particular group of organisms and a
particular geographic region. They have a simple design to serve those who are not experts in the
group or region.
Taxonomic keys are based on phenetic systems of classification. As such, they do not necessarily
group closely related species. Keys utilize the different states of distinguishing, anatomical
characteristics that are shared by the species under consideration. The characteristics are chosen for
reliability and ease of examination. Traits useful for identification of species must be measurable,
describable, and relatively invariable within a species, regardless of the environment in which the
organism grows. Consequently, taxonomists often do not use coloration as a key characteristic
because species are often polymorphic for color in order to blend into the various habitats in which
they live.
The most useful keys are dichotomous. At each step the remaining species are divided into two
groups on the basis of the state of one or more key characteristics (the presence or absence or the
degree of development of one or more traits). In a dichotomous key, the characteristic states are
listed in the form of couplets. Each couplet contains a pair of distinguishing statements (often a
statement and its negation). The couplets are numbered consecutively and the two statements within
each couplet are labeled a. and b. Each statement is followed by an instruction about which couplet
should be read next or by a species name identifying the specimen.
Preparing a dichotomous key takes a great deal of forethought and deliberation. It is easier if you
carefully choose those characteristics which will be useful in dividing the species into smaller and
smaller groups. Then, produce a chart that lists the species under consideration and the characteristic
states that are shared by some species but not others. Next, use the chosen characteristics to produce
couplets. Then, make a flow chart from the couplets to determine the best way to organize them into
a key. It is best to start with couplets that will divide the species into large groups. Next, order the
couplets so that all the species will eventually be distinguished. As you order the couplets number
them and add the instructions and species names.
Activities
1. Plant Anatomy
With your partners, examine the diagrams of plant anatomy paying special attention to the
vocabulary. Find these organs in the plant specimens on your lab table. Use your new found
vocabulary to develop characters for making a key to your plants. Now, sort the plants into 2 groups
based upon shared character states. Now, sort each of the 2 groups into 2 smaller groups based upon
shared character states. If you are careful, these groups may correspond to taxa. Continue the
subdividing until you’re left with individual plants. Show the result to your instructor. Explain why
you subdivided the plants the way you did. After talking with your instructor proceed to Activity 2.
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2. Create a characteristic State Chart for 6 Plants.
Species 1 Species 2 Species 3 Species 4 Species 5 Species 6
Characters ________ ________ ________ ________ ________ ________
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3. Create a flow chart from your characteristic state chart.
Characters
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4. Create a dichotomous key from your flow chart.
1. a.
1. b.
2. a.
2. b.
3. a.
3. b.
4. a.
4. b.
5. a.
5. b.
6. a.
6. b.
5. After completing your key, leave your key on your lab table, circulate through
the lab, try using the keys made by your fellow students, and leave some
constructive comments on your experience using them.
Comments:
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