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Classification is the process of sorting living organisms into groups based on shared features. Taxonomy is the branch of biology concerned with naming and classifying organisms. Understanding how and why we classify organisms is fundamental to biology — it allows us to communicate clearly about species, predict characteristics of newly discovered organisms, and understand evolutionary relationships.
There are an estimated 8.7 million species on Earth, with only around 1.5 million formally described. Without a system of classification, studying this enormous diversity would be chaotic. Classification provides:
Exam Tip: When explaining why classification is important, always link to the idea that it reflects evolutionary relationships, not just convenience. Examiners reward answers that connect classification to phylogeny.
Carl Linnaeus was a Swedish botanist who developed the hierarchical system of classification still used (in modified form) today. He published Systema Naturae in 1735, which laid the foundations for modern taxonomy.
Linnaeus organised living things into a nested hierarchy of groups called taxa (singular: taxon). Each level is called a taxonomic rank. The standard ranks, from broadest to most specific, are:
| Rank | Description | Example (Human) | Example (Lion) |
|---|---|---|---|
| Kingdom | The broadest grouping of organisms | Animalia | Animalia |
| Phylum | Major body plan divisions within a kingdom | Chordata | Chordata |
| Class | Divisions within a phylum | Mammalia | Mammalia |
| Order | Divisions within a class | Primates | Carnivora |
| Family | Divisions within an order | Hominidae | Felidae |
| Genus | A group of closely related species | Homo | Panthera |
| Species | The most specific rank — organisms that can interbreed to produce fertile offspring | Homo sapiens | Panthera leo |
A useful mnemonic for remembering the order is: King Philip Came Over For Good Spaghetti.
The following diagram shows the taxonomic hierarchy from the broadest to the most specific rank:
graph TD
A["Domain"] --> B["Kingdom"]
B --> C["Phylum"]
C --> D["Class"]
D --> E["Order"]
E --> F["Family"]
F --> G["Genus"]
G --> H["Species"]
Exam Tip: You may be asked to place an organism into its correct taxonomic groups or to state which taxonomic rank two organisms first diverge at. Practise reading taxonomic tables and identifying the level of shared classification.
Linnaeus introduced binomial nomenclature — a two-part naming system for every species. The name consists of:
The full name is always written in italics (or underlined if handwritten): Homo sapiens.
Latin was the language of science in Linnaeus's time. Using Latin ensures names are:
The most widely used definition of a species is the biological species concept: a species is a group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other such groups.
While useful, this definition has significant limitations:
| Limitation | Explanation | Example |
|---|---|---|
| Asexual organisms | Organisms that reproduce only asexually never interbreed, so the concept cannot apply | Bacteria, many protists |
| Fossils | We cannot test whether extinct organisms could have interbred | Classifying dinosaur species |
| Ring species | Adjacent populations can interbreed, but populations at opposite ends of the range cannot | Herring gulls and lesser black-backed gulls around the Arctic |
| Hybridisation | Some distinct species can interbreed and produce fertile offspring | Grizzly bears and polar bears producing fertile "grolar bears" |
Because of these limitations, biologists sometimes use other definitions:
Exam Tip: The Edexcel specification expects you to know the biological species concept and be able to discuss its limitations. Be prepared to explain why classification of some organisms (e.g., bacteria, ring species) is problematic using this definition.
When a new species is described, a type specimen is designated — a single preserved individual that serves as the permanent reference point for that species name. Type specimens are stored in museums and herbaria around the world. If there is ever a dispute about whether an organism belongs to a particular species, the type specimen is consulted.
The scientist who first formally describes and names a species is recorded as the taxonomic authority. For example, Homo sapiens Linnaeus, 1758 — Linnaeus described the species in 1758.
Natural classification groups organisms based on their evolutionary relationships (phylogeny). It uses a wide range of evidence including:
Natural classification aims to reflect how organisms are truly related through common descent. Groups formed by natural classification are called clades — each clade includes an ancestor and all its descendants.
Artificial classification groups organisms based on superficial or convenient features that do not necessarily reflect evolutionary relationships. For example:
| Feature | Natural Classification | Artificial Classification |
|---|---|---|
| Basis | Evolutionary relationships | Convenient observable features |
| Reflects phylogeny? | Yes | Not necessarily |
| Uses multiple evidence sources? | Yes — DNA, anatomy, fossils, etc. | Often uses one or few features |
| Stability | Relatively stable; updated with new evidence | May change depending on which features are chosen |
| Predictive power | High — shared ancestry predicts shared features | Low — grouping by one feature does not predict others |
Exam Tip: If an exam question asks you to distinguish between natural and artificial classification, always make the point that natural classification reflects evolutionary relationships while artificial classification is based on observable features chosen for convenience.
As you move down the taxonomic hierarchy from kingdom to species:
This pattern arises because classification reflects the branching pattern of evolution. Species within the same genus diverged relatively recently and share many features. Species in different kingdoms diverged billions of years ago and share relatively few features.
Modern taxonomy has been transformed by molecular biology. Techniques that have revolutionised classification include:
These molecular methods have led to major reclassifications. For example, the traditional five-kingdom system has been supplemented by the three-domain system based on ribosomal RNA comparisons.
| Key Concept | Detail |
|---|---|
| Classification | Sorting organisms into groups based on shared features |
| Taxonomy | The branch of biology dealing with naming and classifying organisms |
| Linnaean hierarchy | Kingdom, Phylum, Class, Order, Family, Genus, Species |
| Binomial nomenclature | Two-part Latin name: Genus species |
| Biological species concept | Organisms that interbreed to produce fertile offspring |
| Natural classification | Based on evolutionary relationships |
| Artificial classification | Based on convenient observable features |
| Molecular techniques | DNA sequencing, hybridisation, protein and immunological comparisons |
Exam Tip: Classification questions often carry extended-response marks. Structure your answer clearly: define the key term, give the principle behind the classification system, provide a specific example, and link back to evolutionary relationships.