Using and Constructing a Dichotomous Key
Background
Knowing the similarities and
differences in organisms can
help not only to figure out their
evolutionary history, but also to
help sort and identify them.
Suppose you find a large
colorful wildflower while walking
through the woods. Chances
are the flower has already been
named and classified, but how
can you learn its identity? As
an aid to help others identify
unknown organisms, biologists
have developed classification
keys.
Many classification keys have
been developed to help identify
wildflowers and many other
kinds of plants and animals.
Although these keys may vary
in purpose and complexity, they
have
certain
features
in
common. These classification
keys
are
often
called
dichotomous keys. The word
dichotomous comes from the
word dichotomy, meaning “two
opposite parts or categories.” A
dichotomous classification key
presents the user with two
opposite
statements
about
some trait or characteristic of an
organism.
By choosing the
statement that best describes
the unknown organism; the user
is led to further pairs of
statements. By going from one
set of statements to another,
the name of the organism or its
classification group is finally
determined.
Do not confuse a dichotomous key with a phylogenetic tree. A phylogenetic tree represents
evolutional relationships, whereas a dichotomous key does not. However, both analyze the
similarities and differences and therefore help in the classification of organisms. `
In this investigation, you will use a classification key to identify several salamanders (Part A). You will
then write a classification key (Part B) for the Beasties you analyzed in Part 1.
Name: ______________________________
Part A: Using a Dichotomous Key
Use the dichotomous key for salamanders to identify each of the 11 salamanders, writing both their
scientific name and their common name. Remember to write the scientific name in the correct
format!
Scientific Name
1.
Common Name
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Part B: Constructing a Dichotomous Key
1. Use your data regarding the characteristics present/absent in salamanders to construct your
own dichotomous key.
2. Remember that a dichotomous key includes pairs of opposing descriptions. At the end of the
descriptions, the key should either identify the organism for you or lead you to another pair of
opposing descriptions.
Conclusion Questions
1. As you used the classification key to identify the salamanders, did you go from general to
specific characteristics or from specific to general characteristics?
2. What two groupings (levels of classification) do the scientific names of the salamanders
represent?
3. Was the classification key you constructed exactly like those of other students? Explain why
or why not.
4. If you were using actual living organisms, what other characteristics could you use to identify
them?
Critical Thinking and Application Questions
1. Do you think that there may be some closely related species of organisms that cannot be
identified with a classification key? Explain your answer.
2. Why do you think biological classification keys always present two, rather than some other
number, of choices at each step?
3. What types of problems would scientists have today if Carolus Linnaeus had not developed his
classification and naming system of organisms?