Classification Systems

A kingdom contains many different organisms. Moving down the hierarchy narrows the group, so its members have more characteristics in common. Species is the smallest group in this system, and organisms within a species share the most features.

Humans provide a concrete example:

These are not seven separate groups that humans must choose between. Humans belong to all seven: the species sits within the genus, which sits within the family, and so on. For example, Mammalia is a narrower group within Animalia.

When using classification information, compare the levels organisms share. Two organisms in the same genus also share the broader groups above it, but they need not belong to the same species.

Naming a species with two words

The binomial naming system gives each species a two-part scientific name. The first word is the genus; the second identifies the species within that genus. Together, the two words identify the species.

For humans, the name is Homo sapiens: Homo is the genus and sapiens is the species-identifying word. The second word alone is not the complete scientific name. This naming system provides a consistent way to identify organisms, rather than relying on common names that may vary between places or languages.

Why classification systems changed

Classification depends on the evidence available. Early scientists could compare visible features and dissected body structures, but later developments allowed much more detailed comparisons.

Improved microscopes revealed internal structures that had previously been difficult or impossible to see. Scientists could therefore identify similarities and differences that were hidden when organisms were compared only by their outward appearance.

A better understanding of biochemical processes—the chemical reactions occurring in organisms—provided further evidence. Chemical analysis allowed scientists to compare organisms at a molecular level, leading to new classification models.

DNA sequence comparisons are another important form of evidence. A DNA base sequence is the order of bases along DNA. In general, greater similarity between comparable DNA sequences indicates a closer evolutionary relationship and a more recent common ancestor. Classification can therefore reflect evolutionary relationships, rather than just visible resemblance.

Carl Woese’s three-domain system

Chemical evidence led Carl Woese to develop the three-domain system, proposed in 1990. Domains are larger groups than kingdoms. The three domains are:

• Archaea: organisms described in the GCSE classification model as primitive bacteria, usually living in extreme environments such as hot springs and salt lakes. They form a separate domain from Bacteria.
• Bacteria: true bacteria, including E. coli and Staphylococcus.
• Eukaryota: the domain containing protists, fungi, plants and animals.

The important change was recognising Archaea and Bacteria as distinct groups on the basis of chemical evidence. Domains are then subdivided into smaller classification groups, including kingdom, phylum, class, order, family, genus and species.

Reading evolutionary trees

An evolutionary tree is a branching diagram showing how scientists believe organisms are related. Branching points represent shared ancestors and the separation of evolutionary lineages as new species evolve. The ends of branches identify the organisms being compared.

To compare two species, follow their branches backwards until they meet. This meeting point represents their most recent common ancestor. Species sharing a more recent common ancestor are more closely related.

Chimpanzees and bonobos share a recent common ancestor, so they are more closely related to each other than either is to the other great apes shown. Humans share a more recent common ancestor with gorillas than with orangutans, so humans are more closely related to gorillas than to orangutans. All five groups share an older common ancestor.

Scientists construct evolutionary trees using current classification data for living organisms, including structural similarities and DNA comparisons, alongside fossil data for extinct organisms. Trees represent interpretations of this evidence: new discoveries can change scientists’ understanding of the relationships.

• Improved microscopes revealed more detail of internal structures.
• Understanding biochemical processes and chemical analysis supplied new evidence.
• More similar comparable DNA sequences generally indicate closer relationships.

Three domains

Carl Woese’s system was based on chemical evidence:

• Archaea: described as primitive bacteria, usually in extreme environments.
• Bacteria: true bacteria.
• Eukaryota: protists, fungi, plants and animals.

Domain sits above kingdom.

Evolutionary trees

• Branching points represent common ancestors and the separation of lineages.
• A more recent shared ancestor means a closer evolutionary relationship.
• Trees use current classification data for living organisms and fossil data for extinct organisms.