Classification » Classification of Living Organisms

Why do we classify living things?

There are millions of different species on Earth. To study them, scientists sort them into groups. This is called classification. Traditionally, living things were put into groups depending on their structure and characteristics, using a system developed by a scientist called Carl Linnaeus.

Key terms:

• Classification: sorting living things into groups.
• Kingdom, phylum, class, order, family, genus, species: the seven levels of the Linnaean system.
• Binomial system: naming an organism using its genus and species.

The Linnaean system

Linnaeus classified living things into seven levels. The biggest group is the kingdom. Each kingdom is split into phylum groups, each phylum into classes, and so on, down to the smallest group, the species.

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

As you go down the list, the groups get smaller. Organisms in the same kingdom have only a few features in common. Organisms in the same genus have a lot in common.

Worked example: lion and tiger

You may be given a table of information and asked to show that you understand the system. Here is the classification of two big cats.

The lion and the tiger share every group down to genus. They only differ at species level. This tells us they are very closely related. A wolf would share the kingdom, phylum, class and order with them, but would be in a different family. So the wolf is less closely related to the lion than the tiger is.

The binomial system

Common names such as 'robin' or 'daisy' can mean different things in different places. So scientists name organisms by the binomial system. 'Binomial' means two names. The first name is the genus and the second is the species.

• The genus name starts with a capital letter.
• The species name starts with a small letter.
• Both are written in italics, or underlined if you are handwriting.

The lion is Panthera leo and the tiger is Panthera tigris. The first name tells you they are in the same genus. Humans are Homo sapiens.

How classification has changed

Classification is not fixed. Scientific ideas develop over time as new evidence is found. Linnaeus based his system on what organisms look like. Later, two developments gave scientists much more evidence.

As this evidence developed, new models of classification were proposed. These ideas have to be tested against the evidence, and they can be changed or replaced when better evidence appears.

The three-domain system

Chemical analysis gave strong new evidence. Because of it, a scientist called Carl Woese developed the three-domain system. A domain is a group even bigger than a kingdom. In this system, organisms are divided into three domains.

Notice that the system puts bacteria into two separate domains. The chemical evidence showed that archaea are different from the true bacteria. All the plants and animals you know are together in the Eukaryota domain.

Key terms:

• Three-domain system: a model of classification by Carl Woese that divides organisms into archaea, bacteria and eukaryota.
• Archaea: primitive bacteria, usually living in extreme environments.
• Eukaryota: the domain that includes protists, fungi, plants and animals.

Evolutionary trees

Scientists use evolutionary trees to show how they believe organisms are related. They are built using two types of evidence:

• Classification data for organisms that are alive today.
• Fossil data for organisms that are extinct.

Remember that each branching point shows a common ancestor. The more recently two organisms branched from a common ancestor, the more closely related they are.

Exam-style question

(a) Name the two parts of the name Panthera leo and say which one is the genus. [2 marks]

(b) Describe how improvements in microscopes led to new models of classification. [2 marks]

(c) Name the three domains in the system developed by Carl Woese. [3 marks]

(d) Which two types of data are used to build an evolutionary tree? [2 marks]