18.1 Classification

2026 Syllabus Objectives

By the end of this subtopic, you should be able to:

  1. Discuss the meaning of the term species, limited to the biological species concept, morphological species concept and ecological species concept
  2. Describe the classification of organisms into three domains: Archaea, Bacteria and Eukarya
  3. State that Archaea and Bacteria are prokaryotes and that there are differences between them, limited to differences in membrane lipids, ribosomal RNA and composition of cell walls
  4. Describe the classification of organisms in the Eukarya domain into the taxonomic hierarchy of kingdom, phylum, class, order, family, genus and species
  5. Outline the characteristic features of the kingdoms Protoctista, Fungi, Plantae and Animalia
  6. Outline how viruses are classified, limited to the type of nucleic acid (RNA or DNA) and whether this is single stranded or double stranded

Understanding Classification

https://cambridgeassistant.com

Classification is the process of arranging organisms into groups based on their similarities. Scientists group organisms that share similar body structures (morphology) and similar ways their bodies work (physiology). This makes it much easier to study and understand the millions of different organisms on Earth.

Taxonomy is the scientific study and practice of classification. It involves placing organisms into a series of groups, called taxonomic groups or taxa (singular: taxon). Each taxon is like a category that contains organisms with shared characteristics.

Why do we classify organisms? Imagine trying to learn about every single living thing individually - it would be impossible! By grouping similar organisms together, we can understand their key features more easily and see how different groups relate to each other.

What is a Species?

https://cambridgeassistant.com

A species is a fundamental unit of classification. But defining exactly what a species is can be tricky, and scientists use different concepts depending on the situation.

The Three Species Concepts

1. Biological Species Concept

This is the most widely used definition. According to this concept, a species is a group of organisms that can interbreed to produce fertile offspring. The key words here are "fertile offspring" - the babies must be able to reproduce themselves.

For example, horses and donkeys can mate and produce offspring called mules, but mules are sterile (cannot reproduce). This means horses and donkeys are different species, not the same species.

Organisms of the same species are reproductively isolated from other species. This means they don't naturally breed with organisms from different species to produce fertile young.

2. Morphological Species Concept

This concept defines a species based on physical appearance and structure (morphology). Organisms that look very similar and share the same structural features are grouped as the same species.

This is the easiest way to identify species, especially for organisms that are dead, preserved, or impossible to observe breeding. Scientists can examine body shape, size, color, and anatomical features to determine if organisms belong to the same species.

3. Ecological Species Concept

This concept defines a species based on the ecological niche it occupies - meaning the role it plays in its environment and the resources it uses. Organisms that occupy the same ecological niche and interact with their environment in the same way are considered the same species.

Other Features Used to Define Species

Beyond these three main concepts, scientists also look at:

  • Physiological features: How the body functions normally, such as muscle contraction, enzyme reactions, nervous system communication, and hormonal systems
  • Biochemical features: The sequence of amino acids in proteins and the sequence of nucleotides in DNA. Greater similarity in these sequences indicates organisms are more closely related and share more recent ancestors
  • Behavioral features: Patterns of behavior, especially breeding seasons and mating behavior

Challenges in Identifying Species

Using the biological species concept (interbreeding) isn't always practical because:

  • The organisms might both be the same sex
  • The organisms might be dead or preserved
  • The male and female reproductive organs might be incompatible
  • Scientists might not have the time or resources to observe whether organisms can successfully breed

This is why scientists sometimes rely on the morphological or ecological concepts instead.

Sign in to view full notes