1.2 Concept and uses of classification systems

Objective: Explain that classification systems aim to reflect evolutionary relationships.

Classification systems are used by biologists to organise the vast diversity of life on Earth. These systems aim to reflect the evolutionary relationships between different organisms. Instead of simply grouping organisms based on superficial similarities, modern classification systems try to reflect a common ancestry.

Why are classification systems important?

Classification systems provide a framework for understanding the relationships between organisms. They allow biologists to:

• Identify organisms.
• Organise organisms into groups based on shared characteristics.
• Understand evolutionary history.
• Communicate clearly about organisms using a universal naming system.

Evolutionary Relationships and Classification

The key principle behind modern classification is that organisms with more recent common ancestors are grouped together. This reflects the evolutionary history of life. Think of a family tree – closer relatives share more recent ancestors than distant cousins.

Different levels of classification represent different degrees of relatedness. The main levels, from broadest to most specific, are:

1. Domain
2. Kingdom
3. Phylum
4. Class
5. Order
6. Family
7. Genus
8. Species

This hierarchical system is often remembered using the mnemonic: Dear King Philip Came Over For Good Soup.

Table of Classification Ranks

The following table summarises the different ranks in a classification system and what they generally indicate about the relatedness of organisms.

Rank	Description	Example (Humans)
Domain	The broadest category; divides life into 3 main groups.	Eukarya
Kingdom	Groups organisms with similar general characteristics.	Animalia
Phylum	Groups organisms with a similar body plan.	Chordata
Class	Groups organisms within a phylum with more specific characteristics.	Mammalia
Order	Groups organisms within a class with shared features.	Primates
Family	Groups organisms within an order that are closely related.	Hominidae
Genus	A group of closely related species.	Homo
Species	A group of organisms that can interbreed and produce fertile offspring.	Homo sapiens

The classification system is constantly being refined as new data (e.g., DNA analysis) become available. This leads to changes in the relationships between organisms and the way they are grouped.

Figure: A simple illustration of the taxonomic hierarchy.