Using Keys (SQA National 5 Biology): Revision Note
Exam code: X807 75
Paired-statement keys
For anyone who doesn't specialise in studying a particular group of organisms, it can be very difficult to identify, or ID, a species when working in the field
Correct ID is essential to ensure that results are accurate
Someone seeking to identify unfamiliar species may use a tool known as a paired-statement key
These keys are also known as dichotomous keys
Using a paired-statement key
Paired-statement keys contain a series of paired statements, e.g.:
(1) the organism has three pairs of legs OR (2) the organism has more than three pairs of legs
(1) leaves are opposite each other on the stem OR (2) leaves alternate on the stem
For each pair of statements. one will be false when applied to the species being identified, while the other will be a correct description of the species
The correct statement leads to another pair of statements, and so on until the final correct statement leads to the name of the species
Worked Example
Use the dichotomous key provided to identify the type of organism below
Answer:
Step 1: decide which of the 1st pair of statements applies
The organism has an exoskeleton, so we can ignore the first statement in pair 1 and move onto the 3rd pair of statements as instructed in the key
Step 2: decide which of the 3rd pair of statements applies
The organism has 4 pairs of legs, so we move on to the 4th pair of statements as instructed
Step 3: decide which of the 4th pair of statements applies
The organism has no tail, so we know that it is not a scorpion, and we move on to the 6th pair of statements as instructed
Step 4: decide which of the 6th pair of statements applies
The organism cannot produce silk, so it must be a species of mite
Constructing paired-statement keys
When constructing a key, there are some important rules to consider
Each pair of statements should contain features that are clearly identifiable and not subject to opinion, e.g. "the organism has 3 pairs of legs" rather than "the organism is small"
Statements must be 'yes' or 'no' in style
Each pair of statements should divide the organisms being identified into two distinct groups, e.g. a group that has hairs on its leaves and a group that doesn't
Each subsequent pair of statements should divide the organisms into smaller and smaller groups
Each statement should be followed by either a number to continue the process of narrowing down the options or should name the organism to which it applies
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