Dichotomous Keys
Using a dichotomous key
- 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
- Correctly identifying species may be important for a researcher studying biodiversity or looking at the impacts of a changing environment on a community of organisms
- Someone seeking to identify species with which they are not already familiar may use a tool known as a dichotomous key
- A dichotomous key contains a series of paired statements
- The term 'dichotomous' refers to these pairs of statements
- An example of such a pair of statements might read:
- The organism shows radial symmetry
- The organism shows bilateral symmetry
- Or:
- The organism has one pair of wings
- The organism has two pairs of wings
- To work through a dichotomous key, you start with the first pair of statements and apply them to the unknown species; one statement will be clearly false, 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 first pair of statements applies
The organism has an exoskeleton, so we can ignore the first statement in pair 1 and move onto the third pair of statements as instructed in the key
Step 2: Decide which of the third pair of statements applies
The organism has 4 pairs of legs, so we move on to the fourth pair of statements as instructed
Step 3: Decide which of the fourth pair of statements applies
The organism has no tail, so we know that it is not a scorpion, and we move onto the sixth pair of statements as instructed
Step 4: Decide which of the sixth pair of statements applies
The organism cannot produce silk, so it must be a species of mite
Constructing a dichotomous key
- When constructing a key, the following should be considered
- 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 e.g. the organism is small
- A leg count gives an objective number, but size is relative and depends on what the species is being compared to
- 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 an exoskeleton 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
- 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 e.g. the organism is small
Dichotomous keys can also be represented using a branched diagram as shown here. This visual representation can be easier to understand, but it is limited in the number of organisms it can include
Exam Tip
Get some practice at using a dichotomous key by identifying the remaining organisms featured in the key above, all of which are described below. Note: they are all quite easily recognisable animals so you should be able to tell whether you have used the key correctly or not.
- The organism has an exoskeleton, a segmented body, and 4 pairs of legs. It has a tail with a sting
- The organism has an exoskeleton, a segmented body, and 3 pairs of legs. Its body is uniform in colour
- The organism has an exoskeleton, a segmented body, and 3 pairs of legs. It's body is striped black and yellow
- The organism is soft bodied with a muscular foot and a hard shell
- The organism has an exoskeleton, a segmented body, and 4 pairs of legs. It has no tail but can produce silk fibres from a specialised organ on its abdomen
- The organism is soft bodied with a muscular foot and no hard shell