Part 1 of 6Purpose of a Key
Purpose of a Key
Identification
- A key helps someone who is not an expert to identify an unfamiliar species in the field.
Accuracy
- Correct identification is essential, because mistakes make the results of an investigation inaccurate.
Another name
- A paired-statement key is also called a dichotomous key.
Structure
- It is made of a series of numbered pairs of statements, each describing a different feature.
Part 2 of 6Using a Paired-Statement Key
1
Start at the first pair of statements and read both of them.
2
Decide which statement correctly describes the organism, since one will be true and the other false.
3
Follow the number or name at the end of the true statement.
4
If it gives a number, go to that pair of statements and repeat the process.
5
When the true statement gives the name of an organism, you have identified it.
Part 3 of 6Example Key
This simple key identifies five common invertebrates.
Pair 1
- 1a Has legs go to 2; 1b No legs go to 4.
Pair 2
- 2a Three pairs of legs, ladybird; 2b More than three pairs of legs, go to 3.
Pair 3
- 3a Four pairs of legs, spider; 3b More than four pairs of legs, woodlouse.
Pair 4
- 4a Has a shell, snail; 4b No shell, earthworm.
Part 4 of 6Constructing a Key
Constructing a Key
Clear features
- Use features that can be seen and counted, such as three pairs of legs, rather than opinions such as small.
Yes or no
- Each statement should be either true or false for an organism, with no in-between answer.
Two groups
- Each pair should divide the organisms into two distinct groups.
Narrowing down
- Each later pair should split the remaining organisms into smaller groups until each has been named.
Directions
- Every statement should end with either the number of the next pair or the name of the organism.
Part 5 of 6Worked Example
Question Use the example key to identify an animal that has legs and four pairs of legs.
- Pair 1: the animal has legs, so go to pair 2.
- Pair 2: it does not have three pairs of legs but more than three, so go to pair 3.
- Pair 3: it has four pairs of legs, so statement 3a is true.
Answer The animal is a spider.
Part 6 of 6Exam Checklist & Tips
3.2.5 – Key Points for the Exam
- Paired-statement key A series of pairs of statements used to identify an organism.
- Dichotomous key Another name for a paired-statement key.
- Species A group of organisms that can interbreed to produce fertile offspring.
- Feature An observable characteristic used to identify an organism.
Common Mistakes
- Using subjective features such as small or large instead of countable ones in a key.
- Choosing the statement that seems likely without checking that the other one is false.
- Writing a key where a statement can be both partly true and partly false.
- Forgetting to give the next number or organism name after each statement.