Dichotomous Keys
PSI AP Biology
Name: _________________________
Objective
Students will learn how dichotomous keys are used in biology. They will create either a textual or a
pictorial dichotomous key that accurately identifies the genus and species of six seashells found on the
beaches of the southeastern United States.
Standard
Essential Knowledge:
1B : Organisms are linked by lines of descent from common ancestry.
Materials
Every student needs:
Activity worksheet
Blank sheet of paper
Procedure
1. Read the information about dichotomous keys.
2. Observe the textual and pictorial formats for the candy dichotomous key.
3. Create a dichotomous key for the provided shells.
a. Read each shell description thoroughly.
b. On the Brainstorming Table, list different shell characteristics as well as each type of shell
that shares that characteristic. Use this as a tool to organize the shells into different
categories.
c. Looking at the shell characteristics as a whole, pick one trait that separates the shells into
two distinct groups. This will be your first level of separation.
d. Continue to create couplets for the remaining shells until all shells have been identified in
your key.
4. Once finished, trade keys with a neighboring group and check their work.
5. Answer the Analysis and Application questions.
www.njctl.org
PSI AP Biology
Evolution & Classification
Dichotomous Keys
Dichotomous keys are tools that are used to identify unknown items or organisms. A series of paired
statements, called couplets, enable the item to be identified through the observation of characteristics.
Each couplet requires the observer to answer a question about the item. The answer will lead them to
additional couplets until the item is identified.
Dichotomous keys can be written in a textual or pictorial format. Below are dichotomous keys for
different types of candy: peanut M&M’s, lollipop, Reese’s peanut butter cup, M&M’s, gum, and skittles.
Choose one candy and work through both keys. Observe how each dichotomous key is written in a
different format, but it relays the same information.
Textual Format
1a. Has chocolate component………go to 2
1b. No chocolate component……….go to 3
2a. Peanut in the middle………………peanut M&M’s
2b. No peanut………………………………go to 4
3a. Chewy…………………………………….go to 5
3b. Hard……………………………………….lollipop
4a. Peanut butter in the middle……Reese’s peanut butter cup
4b. No peanut butter……………………M&M’s
5a. Individually wrapped………………gum
5b. Several in one package…………..skittles
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PSI AP Biology
Evolution & Classification
Pictorial Format
Candy
Chocolate
No peanut
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No chocolate
Peanut
Peanut butter
No peanut
butter
Reese's
peanut butter
cup
M&M's
Peanut
M&M's
Chewy
Hard
Individually
wrapped
Several in a
package
Gum
Skittles
PSI AP Biology
Lollipop
Evolution & Classification
Seashells of the Southeastern United States
Lettered Olive – Oliva sayana
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Snail
Reddish brown zigzag markings
Up to 7cm tall
Low spire
Long, smooth aperture
Aperture opens on right
Source: http://commons.wikimedia.org/wiki/File:Oliva_sayana_02.JPG
Lightning Whelk - Busycon contrarium
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Snail
Gray or tan with brown streaks
Up to 40 cm tall
Knobs on whorls
Large aperture
Aperture opens on left
Source: http://commons.wikimedia.org/wiki/File:Busycon-contrarium.jpg
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PSI AP Biology
Evolution & Classification
Knobbed Whelk – Busycon carica
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Snail
Gray or tan with brown streaks
Up to 30 cm tall
Knobs on whorls
Large orange/red aperture
Aperture opens on right
Source: http://commons.wikimedia.org/wiki/File:Busycon_carica.jpg
Coquina – Donax variabilis
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Bivalve
Exterior color: variable
Interior color: purple
Up to 2.5 cm
Wedge-shaped
Radial grooves along surface
Source: http://commons.wikimedia.org/wiki/File:Coquina_variation3.jpg
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PSI AP Biology
Evolution & Classification
Giant Atlantic Cockle – Dinocardium robustum
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Bivalve
Exterior: tan with brown blotches
Interior: red, purple, or brown
Up to 13 cm
Exterior has about 35 vertical ribs
Source: http://commons.wikimedia.org/wiki/File:Dinocardium_robustum.jpg
Ribbed Mussel – Geukensia demissa
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Bivalve
Exterior: brown
Interior: silvery white
Up to 13 cm
Strong radiating ribs
Source: http://commons.wikimedia.org/wiki/File:Atlantic_ribbed_mussel.jpg
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PSI AP Biology
Evolution & Classification
BRAINSTORMING TABLE
Characteristic
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Shells with characteristic
PSI AP Biology
Evolution & Classification
Analysis
1. Is it possible for more than one dichotomous key to be used for a group of items? (Hint: Was
your dichotomous key exactly the same as your neighbor’s?).
2. In what ways do you think dichotomous keys would be helpful in the field of biology?
3. Phylogenetic trees are illustrations that show common ancestry between different species.
While dichotomous keys are based on characteristics, not common ancestry, is it possible for
similar trends to be found on some keys. Why?
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PSI AP Biology
Evolution & Classification
Application
1. Two of the shells that you distinguished between in your dichotomous key were called
whelks. There are several types of whelks, including one called the knobbed whelk. Knobbed
whelks that live off the coast of Oregon and Washington have been found to attach their
eggs to nearby rocks. The observed result is that newborn whelks are born and grow up
within a few yards of where their parents lived. Different whelk populations are continually
doing this, leading to genetic isolation of the distinct populations. If this continues, what will
be the result?
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PSI AP Biology
Evolution & Classification
Answer Key
Analysis
1. There can be a variety of dichotomous keys for one group of items, depending on what
characteristics the creator chose to use in the individual couplets.
2. Dichotomous keys are used in a variety of ways. Scientists create them so that others can
identify organisms with which they are working.
3. Because common characteristics sometimes indicate evolutionary history and/or common
ancestry, some dichotomous key show patterns that match common ancestry.
Application
1. Allopatric speciation would result as a result of the different populations being geographically
isolated from each other.
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PSI AP Biology
Evolution & Classification