1.2 Concept and uses of classification systems
- Syllabus
- 0610–2026–2027
- Topic
- 1.2
- Level
- —
Learning objectives
1.2.1Organisms can be classified into• State that organisms can be classified into groups by the features that they share1.2.2Species as a group of organisms that• Describe a species as a group of organisms that can reproduce to produce fertile offspring1.2.3Binomial species names• Describe the binomial system of naming species as an internationally agreed system in which the scientific name of an organism is made up of two parts showing the genus and species1.2.4Dichotomous keys based on identifiable• Construct and use dichotomous keys based on identifiable features1.2.5Classification systems aim to reflect• Explain that classification systems aim to reflect evolutionary relationships1.2.6DNA base sequences for classification• Explain that the sequences of bases in DNA are used as a means of classification1.2.7Groups of organisms which share a more• Explain that groups of organisms which share a more recent ancestor (are more closely related) have base sequences in DNA that are more similar than those that share only a distant ancestor
Classify organisms by shared features
Classification organises organisms into groups according to features they share. A useful system makes similarities and differences explicit so organisms can be identified, compared and studied consistently.
| Evidence | What is compared |
|---|---|
| morphology | visible form, such as body shape or presence of wings |
| anatomy | internal structures, such as skeletons or organs |
| biochemical or molecular evidence | protein or DNA sequences |
| behaviour, ecology or distribution | courtship, habitat, niche or geographical pattern |
Choose features that are observable or measurable and apply them consistently. Place organisms together only when the stated shared feature is actually present; one superficial resemblance may be outweighed by several stronger differences.
Viruses could be grouped by shape, size, type of genetic material or host. Animals might first be grouped by a shared structural feature and then separated into smaller groups using further differences.
Classification is not simply arranging organisms by size or familiarity. The grouping rule must be stated, relevant and shared by every member of the group.
Define a species using fertile offspring
A species is a group of organisms that can reproduce with one another to produce fertile offspring.
Both parts matter: the adults must be able to reproduce with one another, and their offspring must be fertile—able to reproduce in turn. Similar appearance alone does not establish that organisms belong to the same species.
A donkey and a zebra can produce a zedonk, but the zedonk is infertile. Producing an offspring is therefore not enough: the fertility condition shows that the parents are different species.
Members of one species need not look identical. Individuals can vary in colour, size, sex or age while still belonging to the same species.
Do not define a species as organisms that merely look alike, share a genus, or can produce any offspring. The required phrase is fertile offspring.
Write and interpret binomial names
The binomial naming system gives every species an internationally agreed two-part scientific name: the genus name followed by the species name.
| Part | Rule | Example in Homo sapiens |
|---|---|---|
| genus | written first; initial capital letter | Homo |
| species | written second; lower-case initial | sapiens |
| whole name | italicised when printed, or underlined when handwritten | Homo sapiens |
Pan troglodytes belongs to the genus Pan. Panthera leo and Panthera tigris share the genus Panthera but have different complete binomial names and are different species.
After the genus has been written in full, it may be abbreviated where the meaning is unambiguous, for example Falco peregrinus then F. peregrinus.
The first word is not a kingdom and the second word alone is not the full species name. Capitalising both words, using a common name, or reversing the order breaks the convention.
Use and construct dichotomous keys
A dichotomous key identifies an organism through a sequence of paired, contrasting statements about observable features. Each choice sends the user to another pair or to an identity.
| Step | Action |
|---|---|
| 1 | observe the organism and read both statements in the current pair |
| 2 | choose the one statement that matches |
| 3 | follow its instruction to another numbered pair or a name |
| 4 | repeat until one identity is reached, then check the route against the specimen |
To construct a key, choose one identifiable feature at each branch and write mutually exclusive alternatives, such as ‘three pairs of legs’ versus ‘more than three pairs of legs’. Keep both statements about the same feature and make every specimen follow exactly one route.
For a spider: ‘three pairs of legs’ is false, so follow ‘more than three pairs’; ‘four pairs of legs’ is true, so the key reaches the spider group. The conclusion follows from the whole route, not from guessing the picture.
Avoid subjective choices such as ‘large’ versus ‘small’ unless a measurement defines them. Do not write overlapping alternatives, use hidden features, or skip reading the second statement.
Make classification reflect evolutionary relationships
Modern classification systems aim to reflect evolutionary relationships: how groups are related through descent from common ancestors.
On a classification or evolutionary tree, a branching point represents a common ancestor. Two groups whose branches meet at a more recent branching point share a more recent common ancestor and are more closely related.
Trace each pair of branches backwards until they meet. Compare the positions of those meeting points; the pair with the most recent meeting point is the closest relationship shown by the tree.
Classifications can change when new anatomical, biochemical or molecular evidence gives a better account of ancestry. The aim is not to preserve an old grouping but to represent the best-supported relationship.
Nearby tips on a page are not necessarily closely related. Relationship is determined by the branching pattern and common ancestor, not by visual spacing or superficial resemblance.
Use DNA base sequences for classification
DNA base sequences can be compared as molecular evidence for classification. Scientists compare the order of bases in corresponding DNA or genes from different organisms.
| Step | Method |
|---|---|
| 1 | obtain DNA from each organism |
| 2 | determine the base sequence of the same region or gene |
| 3 | align corresponding positions |
| 4 | count or evaluate matching bases and differences |
| 5 | use the pattern of similarity to support a classification |
DNA comparison can distinguish organisms that look similar and provides a large set of precisely measurable features. Reference sequences can also be used to identify an unknown specimen or strain.
For sequences ATGCC and ATACC, compare position by position: four bases match and one differs. Repeating this across a suitable sequence provides evidence that can be compared between several organisms.
Compare corresponding sequences, not the shape of a DNA molecule or merely the types of bases present—all DNA uses the same four bases. A conclusion must come from their order and pattern of differences.
Link recent ancestry to DNA similarity
Groups that share a more recent ancestor have more similar DNA base sequences. Conversely, more sequence differences generally indicate a more distant evolutionary relationship.
After lineages separate, mutations can accumulate independently. A recent split leaves less time for differences to build up, so corresponding DNA sequences tend to remain more alike.
| Observation | Inference |
|---|---|
| fewest base differences / most matches | most closely related; most recent common ancestor |
| most base differences / fewest matches | least closely related; more distant common ancestor |
| branches meet most recently on a tree | expect more similar base sequences |
When given several sequences, align them, count differences for each requested pair, and rank the pairs. Use the smallest difference count to identify the closest pair; use the largest to identify the most distant pair.
Do not infer ancestry from appearance alone or claim that closely related organisms must have identical DNA. Similarity is comparative evidence: fewer differences supports a more recent common ancestor.