# 5.2.7 Skills: Classification & Cladistics

### Dichotomous 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
• 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

The organism has an exoskeleton, a segmented body, and 4 pairs of legs. It has no tail and cannot produce silk

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
• 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

#### Exam Tip

Get some practice at using a dichotomous key by identifying the remaining organisms featured in the key, 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.

1. The organism has an exoskeleton, a segmented body, and 4 pairs of legs. It has a tail with a sting
2. The organism has an exoskeleton, a segmented body, and 3 pairs of legs. Its body is uniform in colour
3. The organism has an exoskeleton, a segmented body, and 3 pairs of legs. It’s body is striped black and yellow
4. The organism is soft bodied with a muscular foot and a hard shell
5. 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
6. The organism is soft bodied with a muscular foot and no hard shell

• Evolutionary relationships between species can be represented visually using a diagram called a cladogram
• Cladograms are evolutionary trees that show probable order of divergence from ancestral species and therefore probable relationships between species
• Analysis of a cladogram can provide several pieces of information
• The point at which two branches separate is known as a node, and represents common ancestor species
• A node immediately adjacent to a pair of clades indicates that these two clades share a recent common ancestor
• This shows that the two clades are more closely related to each other than they are to any other clade in the cladogram
• If several nodes need to be traced back before two clades can be joined, this indicates a more distant relationship between two clades
• Cladograms sometimes show numbers along the branches; these indicate the number of base or amino acid changes that have occurred between one node and the next or between a node and an emerging clade or species
• The constant rate at which mutations accumulate means that these numbers can be used as a molecular clock to calculate how much time has passed
• Some cladograms have a time scale to show how many millions have years have passed
• Computers use the information from sequence data to build the most likely cladogram
• This is done using the principle of parsimony which states that the simplest explanation is preferred
• The computer builds the shortest possible cladogram with the smallest number of divergence events to fit the available data
• Cladograms provide the most likely estimate of the evolutionary progress of organisms
• The reliability of a cladogram may vary depending on the amount of sequence data used to construct it
• A cladogram based on the sequencing of one gene will be less reliable than a cladogram based on the sequencing of several genes
• Cladograms are subject to change when new sequence data becomes available

### Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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