Introduction
and Application
Classification
systems are used constantly in everyday life. In a grocery store, for example,
items are classified into food or non-food items; food items into perishable or
non-perishable; perishable into meats, produce, dairy or frozen; meats into
chicken, beef, pork, lamb or fish; produce into fruits or vegetables; etc. When
you go for a walk outside you automatically classify things you see (examples:
trees, flowers, birds, insects, etc.). Classification by using a dichotomous
key is a way of identifying distinct traits specific to one individual object
or organism.
In
science, as well as in everyday life, people attempt to organize objects, ideas
or information. Placing objects, ideas or information into groups on the basis
of similarity is called classification. We classify things in order to locate
them more easily, see how they are similar or different, find order in the
world and so we can communicate with each other by knowing we are talking about
the same thing.
A
classification key is a tool which organizes the traits of a particular group
of organisms so that these traits may be used to identify a particular organism
in the group. One type of classification device is a dichotomous key.
Dichotomous keys have a series of numbered steps which take the user from very
general to more specific traits. Each step usually consists of two statements
(there are exceptional times when ther may be more than two choices) about a
single trait. Only one of these statements can be true about a single organism
or object. The key helps the user narrow his choices until he arrives at the
step which identifies the organism or object. Each key is specific for a group
of organisms or objects. For example, you may have a key for wildflowers in a
particular
region, trees in a particular region,
freshwater fish, insects, etc. A key can only be used to identify organisms or
objects that are present in that key.
The process of using a dichotomous key
is the same from key to key even though they identify different organisms or
objects. Begin by observing the organism or object in question and noting the
general characteristics.
Using the key, read both statements in
the first step. Decide which of the statements is true. This will lead you to
another step in the key. Continue to read the statements in the steps to which
your choices lead you until you arrive at the name of the organism.
In this particular exercise you are
going to create your own classification key for the objects provided.
Materials and Methods:
1. Students will be arranged into small
groups of 2-4 students per group.
2. Each group will receive the
following materials: different pieces
of
hardware, rulers.
3. Instructions:
a.
Remove all the hardware pieces from the plastic bag and observe
some general traits.
b.
Divide the objects into at least two groups based on the presence
or absence of one trait. (Example: Objects with
threads and
objects without threads.)
c.
Place each object into the proper group.
d.
Write the statements at the bottom of a piece of paper, one to the
left side and one to the right. (Keep in mind it is
possible to have
more than two statements).
e.
Draw 2 diverging lines above each statement on the paper. (See
diagram.)
f.
Observe the objects in one group at a time. Further divide the
objects based on a single trait for each group. Write
the
statements above each group and place the objects in
the proper
group. (Example: In the group of objects without
threads, you may
observe that some have heads and some do not.)
g.
Continue to divide objects in each group into two subgroups until
you arrive at the level where there is only one object
in each
group.
h.
At the final level, draw or trace each object next to the
appropriate statement.
i.
Place all hardware back into the bags and write the names of all
the group members on the back of your papers.
Sample
Diagram
with without
heads
heads
objects with objects without
threads
threads
The
following diagrams and terms are designed to provide some clues to your
investigation.
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Binomial Nomenclature: A two-name
system of naming organisms.
Classify: To put objects, ideas or
organisms into groups based on
similarity.
Dichotomous: Divided into two parts;
branched or forked into two more or
less equal divisions.
Genus: A group of similar species.
Organism: Any living thing.
Scientific Name: The two-part name of
an organism consisting of the
genus and species.
Species: A group of closely related
organisms capable of mating and
producing viable offspring.
Trait: An inherited characteristic.