Dichotomous Keys and Phylogenetic Classification

A dichotomous key is a tool for identifying organisms through a series of two-choice questions, while phylogenetic (evolutionary) classification groups organisms according to their evolutionary relationships. In brief: a dichotomous key leads you step by step from a choice such as “has wings / has no wings” to a species name. Identification (finding out what something is) and classification (deciding how groups are related) are different tasks: keys are practical tools that use visible features, and phylogenetic classification uses evidence of common ancestry such as DNA.

Keys are used by field biologists, students and gardeners, and evolutionary classification is used by taxonomists. This guide explains how a dichotomous key works, shows how to build and use one, describes the principles of evolutionary classification and the evidence that it uses, works through an example with a key for leaves and compares the approaches.

What is a dichotomous key?

The word dichotomous means “divided into two parts”. A dichotomous key is a series of numbered steps, each offering two statements (a couplet) about a feature. You choose the statement that fits the specimen, and it either gives the name of the organism or sends you to another step. Each step eliminates part of the group, until only one choice remains.

Keys work best with features that are easy to see and are reliable, such as the number of legs, the leaf shape or the presence of fur. They should avoid features that vary a lot, such as size or color, or that are subjective, such as “large” or “pretty”.

How do you use a key?

1. Start at step 1 and read both statements.
2. Choose the one that matches your specimen. If neither fits, check the specimen again.
3. Follow the instruction: go to the number indicated or read the name.
4. Continue until a name is reached.
5. Check the result against a description or a picture.

How do you build a key?

1. List the organisms to be identified and their features in a table.
2. Choose a feature that divides the group into two roughly equal sets.
3. Write a couplet: two clear and opposite statements.
4. Repeat with each set until each organism is separated.
5. Number the steps and test the key on a person who has not seen it before.

For n organisms, a key needs exactly n − 1 couplets, if each couplet divides one set into two. Five organisms need 4 couplets. This is a useful check. A key can be written as numbered text, as shown below, or as a flow chart (branching diagram).

Worked example: a key to five trees

Five trees have the following leaves: pine (needle-shaped, in bundles), spruce (needle-shaped, single), oak (broad, lobed), beech (broad, smooth edge, oval) and maple (broad, lobed, opposite pairs on the twig).

1. Check the number of couplets: 4 couplets (steps 1, 2, 3 and 4) for 5 trees: n − 1 = 4. Correct.
2. Test with a specimen: a tree has broad, lobed leaves in pairs. Step 1: broad, so 1b, go to 3. Step 3: lobed, so 3b, go to 4. Step 4: opposite pairs, so 4a: maple.
3. Test another: a tree has needles in bundles. Step 1a, go to 2. Step 2a: pine.

Notice that each statement is the opposite of the other in a couplet, so the answer is always clear. The key relies only on features that can be observed directly.

What is phylogenetic classification?

Evolutionary or phylogenetic classification arranges organisms into groups that reflect their evolutionary history. Groups should be clades (monophyletic): an ancestor and all of its descendants. The relationships are shown in phylogenetic trees and cladograms, as described in an earlier article.

The main evidence used includes:

• Anatomy and physiology: especially homologous structures, and shared derived characters.
• Embryology: the similarities of early development.
• Molecular data: DNA and protein sequences, which are now the main source of evidence. The more similar the sequences, the more recently two species shared a common ancestor.
• Fossils: to date the divergence and provide intermediate forms.

This approach has led to changes in traditional classifications. Birds are now considered to be reptiles in the evolutionary sense, since they descend from dinosaurs. The giant panda was long debated, and DNA showed that it is a bear. Whales are most closely related to hippopotamuses. Fungi are the closest relatives of animals among the large groups, not of plants. Some groups that look alike, such as the Old World vultures and the New World vultures, evolved independently and are not each other’s closest relatives.

Keys and phylogenetic classification compared

How did evolutionary classification develop?

Before Darwin, groups were based on similarity and thought to be fixed. After 1859, many biologists wanted classifications to reflect common descent. Willi Hennig’s cladistic method (1950) gave a rigorous way to do this. Comparisons of DNA from the 1970s and 1980s and large-scale sequencing from the 1990s transformed the field. Dichotomous keys were used in the eighteenth century and have been widely used since Jean-Baptiste Lamarck’s key of 1778. Today they are often available as interactive computer programs.

Which mistakes are common?

Mistake 1: Using vague features in a key

Words like “big” or “dark” are not reliable. Use features that can be counted or clearly observed.

Mistake 2: Writing couplets that are not opposites

Each step must have two clear alternatives, so only one can apply.

Mistake 3: Assuming that a key shows evolutionary relationships

A key is a tool for identification. Organisms that look alike in a key may not be closely related.

Mistake 4: Grouping organisms by appearance alone

Evolutionary classification requires shared ancestry. Convergent evolution can make unrelated organisms look similar.