Introduction to Classification

Lessons

Senior 4 · Biology

Introduction to Classification

5 lessons

Lesson 5 of 5

Dichotomous keys: using and constructing them

25 min

Naming what you find

A student turns over a rotting log on the edge of a school compound and finds six kinds of small animal. A laboratory technician grows an unknown bacterium from a patient's sample. Both face the same question: what is it? Neither can compare the specimen with every picture in every book. Instead they use a dichotomous key, a series of paired choices that narrows the possibilities step by step until only one name is left.

In S1 you used and wrote simple keys. At A-level you must use keys that rely on the classification features of this unit, construct efficient keys for any set of organisms, and judge whether a key is well made.

Definition

Dichotomous key: an identification tool made of numbered couplets, each a pair of contrasting statements about one feature; choosing the statement that fits the specimen leads either to the next couplet or to a name. "Dichotomous" means dividing into two.

Couplet: a pair of statements, such as "1a Legs present" and "1b Legs absent", that between them cover every specimen still being considered.

Bracketed key: a key set out as a list of couplets, each ending in a name or a couplet number. A branching (spider) key shows the same choices as a tree diagram.

Using a key

Start at couplet 1 every time. Read both statements before choosing. Follow the number at the end of the statement that fits, and stop when you reach a name. Then check the name against a description or picture: if the specimen does not match, go back and look again at the features you chose.

Worked example 1: a key to bacteria

Use this key, built on the features in Lesson 3, to identify (a) Gram-positive rods that form spores and grow only where there is no oxygen; (b) pink, straight rods that grow with or without oxygen.

Couplet	Statement	Go to
1a	Gram-positive (stains purple)	2
1b	Gram-negative (stains pink)	4
2a	Cells spherical (cocci)	3
2b	Cells rod-shaped; form spores	5
3a	Cocci in grape-like clusters	Staphylococcus
3b	Cocci in chains	Streptococcus
4a	Cocci in pairs	Neisseria
4b	Rods	6
5a	Grows in air	Bacillus
5b	Grows only without oxygen	Clostridium
6a	Rods curved like a comma	Vibrio
6b	Rods straight	Escherichia

(a) 1a (Gram-positive) → 2b (rods with spores) → 5b (only without oxygen): Clostridium.
(b) 1b (Gram-negative) → 4b (rods) → 6b (straight): Escherichia. Growing with or without oxygen fits E. coli, a facultative anaerobe, which confirms the result.
Notice that 7 genera need 6 couplets: a strictly dichotomous key for $n$ organisms always has $n - 1$ couplets.

Constructing a key

List the specimens and make a table of their observable features.
Choose a feature that splits the whole group into two, ideally into two groups of similar size.
Write the couplet as two contrasting statements about the same feature: "Shell present" and "Shell absent", not "Shell present" and "Body soft".
Repeat for each subgroup until every specimen stands alone.
Test the key on every specimen, and ask someone else to test it.

Good couplets use features that are permanent and easy to see: number of legs, presence of a shell, leaf arrangement. Avoid features that vary or that need comparison with something absent, such as size ("large"), colour that changes with age, or behaviour that the specimen may not show.

Worked example 2: building a key for six animals

The student's six animals were a spider, a woodlouse, a centipede, a millipede, a snail and an earthworm. Construct a dichotomous key.

Animal	Legs	Pairs of legs	Legs per body segment	Shell
Spider	Yes	4	(no clear segments)	No
Woodlouse	Yes	7	1 pair	No
Centipede	Yes	Many	1 pair	No
Millipede	Yes	Many	2 pairs on most	No
Snail	No	0	(none)	Yes
Earthworm	No	0	(none)	No

1a Legs present … go to 2. 1b Legs absent … go to 5.
2a Four pairs of legs … spider. 2b More than four pairs of legs … go to 3.
3a Seven pairs of legs; body oval and flattened … woodlouse. 3b Many pairs of legs; body long and narrow … go to 4.
4a One pair of legs on each body segment … centipede. 4b Two pairs of legs on most body segments … millipede.
5a Shell present … snail. 5b Shell absent; body divided into rings … earthworm.

Six animals, five couplets, as expected. Figure 1 shows the same key as a branching diagram.

Judging a key

A well-made key	A poorly made key
Each couplet contrasts one feature: "wings present" or "wings absent"	Couplets mix features: "has wings" or "is black"
Uses features that every specimen shows, whatever its age or sex	Uses features found only in some individuals, or only at some seasons (flowers, fruits)
Uses measurable statements: "legs longer than the body"	Uses vague comparisons: "long legs", "large"
Positive statements, so the user sees what to look for	Negatives that are hard to confirm: "no hairs visible"

A dichotomous key also has limits: it identifies only the organisms it was written for, a single wrong choice early on sends the user down the wrong branch, and a damaged specimen may lack the feature that a couplet needs. Multi-access keys, often on computers, let the user start with whichever feature is visible.

Amoeba Sisters: "Dichotomous Keys: Identification Achievement Unlocked". Follow the worked key in the video, and note why scientists use scientific names rather than common names.

Warning

Common mistakes

Writing couplets that are not true opposites, so that a specimen fits both statements or neither.
Using features that vary within a species, such as size or colour, or that the specimen may not show at the time, such as flowers.
Writing a question ("Does it have legs?") instead of two statements; most examiners ask for paired statements.
Stopping before every specimen has its own end point, or leaving one specimen with two routes to it.

Tip

Exam technique. Before writing a key, draw a table of features. Count your couplets at the end: for $n$ organisms there should be $n - 1$ . Then run each organism through your key once, as the examiner will.

✦

Practice: dichotomous keys

Using the key in Worked example 1, identify Gram-positive cocci arranged in chains.
Using the same key, a student identified a Gram-negative organism as Staphylococcus. Explain the error.
State three features of a good couplet.
Construct a dichotomous key to separate these five organisms: a mushroom, a moss, yeast, Amoeba and E. coli. Use the features: nucleus present or absent; cell wall present or absent; chlorophyll present or absent; single-celled or multicellular.
How many couplets will a dichotomous key for 12 tree species need?
Suggest why a key to trees that uses flower colour is less useful than one that uses leaf arrangement.
Evaluate the use of DNA barcoding (comparing a short, standard DNA sequence with a database) as an alternative to keys.

Show answersHide answers

1a → 2a → 3b: Streptococcus.
Staphylococcus can only be reached through 1a (Gram-positive). The student must have chosen the wrong statement at couplet 1, perhaps by reading a pink stain as purple or decolourising too little; a Gram-negative organism goes to couplet 4.
Any three: the two statements are about the same feature; they are true opposites; the feature is permanent and visible in every specimen; statements are precise or measurable rather than vague; they are positive statements.
One possible key: 1a Nucleus absent … E. coli; 1b Nucleus present … 2. 2a Cell wall absent … Amoeba; 2b Cell wall present … 3. 3a Chlorophyll present … moss; 3b Chlorophyll absent … 4. 4a Single-celled … yeast; 4b Multicellular … mushroom. (Four couplets for five organisms.)
$12 - 1 = 11$ couplets.
Flowers are present only for part of the year, and flower colour can vary; leaves are present most of the year and their arrangement is a fixed feature of the species.
Advantages: works on any stage or fragment (larva, seed, damaged specimen), needs no expert knowledge of anatomy, and can separate species that look identical. Disadvantages: needs laboratory equipment, money and electricity, gives no result in the field, and depends on the species being in the database. Keys stay useful for quick field identification; the two methods complement each other.

Quick check

Which pair of statements makes a good couplet?

Key points

A dichotomous key is a series of couplets of contrasting statements about one feature; follow it from couplet 1 and check the result.
To build one: tabulate features, split the group in two, write true-opposite statements, repeat, then test.
Use permanent, visible, precise features; avoid size, variable colour and seasonal features.
$n$ organisms need $n - 1$ couplets; keys work only for the organisms they were written for.

1Naming what you find

2Using a key

3Constructing a key

4Judging a key

Naming what you find

Using a key

Constructing a key

Judging a key