Why classify, and problems with the traditional hierarchySpec A3.2.1, A3.2.2, A3.2.3
In short
Classification is needed because of the immense diversity of species, and once organisms are classified a broad range of further study is easier. The traditional hierarchy of kingdom, phylum, class, order, family, genus and species does not always match evolutionary divergence. The ideal classification follows evolutionary relationships, so characteristics of group members can be predicted.
The need for classification
Classification is needed because of the immense diversity of species: well over a million have been named and many more remain undescribed. Sorting species into groups makes this diversity manageable. After classification is completed, a broad range of further study is facilitated: researchers can find information about related species, identify new specimens, compare groups and plan conservation.
The traditional hierarchy of taxa
| Taxon | Lion |
|---|---|
| Kingdom | Animalia |
| Phylum | Chordata |
| Class | Mammalia |
| Order | Carnivora |
| Family | Felidae |
| Genus | Panthera |
| Species | leo |
This hierarchy does not always correspond to patterns of divergence generated by evolution. Evolution produces branching with gradual variation, not a fixed number of equal levels. Deciding whether a group is a family or an order is a matter of judgement, and taxa at the same rank in different groups are not equivalent in age or diversity.
Some traditional groups do not include all the descendants of their common ancestor. The class Reptilia excludes birds, yet birds evolved from within the reptiles: birds are more closely related to crocodiles than crocodiles are to lizards.
Nature of science: a fixed ranking of taxa is arbitrary because it does not reflect the gradation of variation. Cladistics replaces it with unranked clades, an example of a paradigm shift in scientific theories.
Advantages of classifying by evolutionary relationships
- In the ideal classification, all members of a group have evolved from a common ancestor.
- Characteristics of organisms in such a group can be predicted because they are shared within a clade. If one species in a clade makes a useful chemical, related species may make it too.
- The classification reflects real evolutionary history rather than human judgement about which traits matter.
- It can be tested and updated as new sequence evidence appears.
Written and checked against the IB Biology HL specification · Updated October 2026