Classification of Living Organisms¶

Part of 4.6 Inheritance, Variation and Evolution.

Classification helps biologists organise the diversity of life into groups that reflect evolutionary relationships. The system is not fixed — it changes as new evidence emerges, particularly from genetics.

Learning Objectives¶

The Linnaean System¶

Carl Linnaeus (1707–1778) devised a hierarchical system for classifying organisms based on shared physical features. He grouped organisms from the broadest level down to the most specific:

A useful mnemonic: King Phillip Came Over For Good Soup.

The more levels two organisms share, the more closely related they are.

The Five Kingdoms¶

Linnaeus originally divided all living things into two kingdoms (plants and animals). Today, five kingdoms are recognised:

Binomial Naming¶

The binomial system gives every species a two-part Latin name: - The first part is the genus name — written with a capital letter. - The second part is the species name — written in lower case. - Both parts are written in italics (or underlined if handwritten).

Examples: - Homo sapiens — modern human - Panthera leo — lion - Bellis perennis — common daisy

Why use binomial names?¶

• Common names vary between countries and languages (e.g. "bluebells" means different plants in England and Scotland).
• Latin names are universal — scientists anywhere in the world can be certain they are discussing the same organism.
• The genus name shows which organisms are closely related (e.g. Panthera tigris, Panthera leo and Panthera pardus are all big cats in the same genus).

The Three-Domain System¶

In the 1970s–1990s, Carl Woese used ribosomal RNA (rRNA) sequences to compare organisms at a molecular level. He discovered that the prokaryotes were actually two very distinct groups, leading him to propose a three-domain system that sits above kingdom level:

Archaea are particularly interesting: they often live in extreme environments (volcanic vents, very salty water) and have some biochemical features more similar to eukaryotes than to bacteria, even though they look like bacteria under a microscope. This shows that visible appearance can be misleading as a basis for classification.

Modern Classification: Molecular Phylogenetics¶

Modern classification relies increasingly on comparing DNA base sequences and amino acid sequences of proteins rather than just visible features. This approach is called molecular phylogenetics.

The greater the similarity between two organisms' DNA sequences, the more recently they shared a common ancestor. For example: - Chimpanzees and humans share approximately 98–99% of their DNA base sequences, confirming very recent common ancestry. - Organisms that look similar but have very different DNA (e.g. dolphins and fish) may have evolved similar features independently (convergent evolution) rather than sharing a recent common ancestor.

This molecular evidence has led to several reclassifications: groups that appeared related based on appearance have been split, while others that looked different have been merged.

Evolutionary Trees (Phylogenetic Trees)¶

An evolutionary tree (also called a phylogenetic tree) is a branching diagram that represents the evolutionary relationships between groups of organisms.

How to read an evolutionary tree: - The tips of the branches represent current species or groups. - The points where branches split (nodes) represent common ancestors. - The further back a node is, the longer ago that common ancestor lived. - Two species on branches that split recently share a more recent common ancestor (i.e. they are more closely related). - Closely related species share more DNA in common than distantly related ones.

Evolutionary trees are hypotheses — they are revised when new fossil evidence or genetic data is found. They are different from a simple ladder or "chain of being" because evolution is a branching process, not a straight line of progress.

Common Confusions¶

• Prokaryotes vs Archaea: Students sometimes say all bacteria are prokaryotes — this is true, but the three-domain system reveals that not all prokaryotes are the same. Archaea and bacteria are as different from each other at the molecular level as either is from eukaryotes.
• Binomial naming format: Both parts of a binomial name are Latin, not English — they must be italicised (e.g. Homo sapiens, not Homo Sapiens or homo sapiens). The species name does not get a capital letter.
• Appearance vs classification: The five kingdoms and three domains are based partly on evolutionary relationships, not just what organisms look like. Dolphins are mammals (Kingdom Animalia), not fish, because of their evolutionary origin and physiology, despite their streamlined shape.
• Species definition: Two organisms are the same species only if they can interbreed and produce fertile offspring. A mule (horse × donkey) is a hybrid but is infertile, so horses and donkeys remain separate species.
• Evolutionary trees vs food chains: An evolutionary tree shows ancestry and common descent — it does not show what eats what. These are completely different diagrams.

Key Terms¶

• Classification: the organisation of living organisms into groups based on shared features and evolutionary relationships.
• Species: a group of organisms that can interbreed to produce fertile offspring.
• Genus: the classification level immediately above species; organisms in the same genus are closely related.
• Binomial system: the two-part naming system where each species is given a genus name and a species name in Latin.
• Kingdom: the broadest level of classification in the Linnaean system; the five kingdoms are animals, plants, fungi, protists and prokaryotes.
• Phylum: the classification level below kingdom and above class.
• Domain: the level of classification above kingdom, introduced by Carl Woese; the three domains are Bacteria, Archaea and Eukaryota.
• Archaea: a group of ancient prokaryotes with biochemical features distinct from true bacteria, forming their own domain.
• Evolutionary tree: a branching diagram showing evolutionary relationships between organisms through common ancestors.
• Molecular phylogenetics: the use of DNA and protein sequences to determine evolutionary relationships.
• Carl Woese: the scientist who proposed the three-domain system based on ribosomal RNA comparisons.
• Convergent evolution: the independent evolution of similar features in unrelated organisms due to similar environmental pressures (e.g. streamlined body in dolphins and fish).

Connected Pages¶

• 4.6.3 The Development of Understanding of Genetics and Evolution
• GCSE Biology Key Terms Glossary