Classification and evolutionary trees – GCSE Combined Science Biology

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There are millions of different living things on Earth. Classification means sorting them into groups, so we can study them and see how they are related. This lesson shows how scientists do it.

It matters because a sensible system lets scientists around the world talk about the same organism without confusion. It also shows how all life is connected.

By the end you will be able to name the groups scientists use. You will write a binomial name correctly. You will also read an evolutionary tree to say which organisms are most closely related.

The Linnaean groups

Carl Linnaeus developed a way of sorting organisms into groups, based on their structure and characteristics. Big groups contain smaller groups, and those contain smaller groups still.

It works like an address. A country contains a city, which contains a street, which contains a house. The Linnaean groups, from biggest to smallest, are kingdom, phylum, class, order, family, genus and species.

A phrase can help you remember the order. King Philip Came Over For Good Soup is one example.

What is a species

A species is the smallest group in the Linnaean system. It is a group of similar organisms that can breed together to produce fertile offspring. Fertile means able to have young of their own.

A horse and a donkey can breed. Their offspring is a mule, but a mule is infertile. So a horse and a donkey are different species.

Members of the same species share most of their features. This is why species is a useful group to name.

Binomial names

Linnaeus gave each organism a two-part Latin name. This is the binomial system. Binomial means two names.

The first part is the genus. It has a capital letter. The second part is the species. It has a small letter. For humans it is Homo sapiens. It is written in italics, or underlined if handwritten.

Latin names mean scientists everywhere use the same name. Common names change between languages, and they can cause confusion.

Why classification has changed

At first, organisms were sorted by what they look like. As microscopes improved, scientists saw more detail inside cells.

Later, chemical analysis of organisms, for example of their DNA, showed how closely related they are. So the classification system has been changed to fit this new evidence.

New evidence does not make the old system wrong. It makes our picture more accurate. Think of a map being updated as better surveys come in.

The three-domain system

Carl Woese proposed a system with three domains above the kingdoms. A domain is a very large group. He used chemical analysis of the RNA in cells.

Archaea are single-celled organisms with no nucleus. Many live in extreme places, such as hot springs. Bacteria are the other group of single-celled organisms with no nucleus. Eukaryota are organisms whose cells have a nucleus. They include protists, fungi, plants and animals.

This fits the evidence better than a system with only five kingdoms.

Evolutionary trees

An evolutionary tree shows how scientists think organisms are related. Each branching point is a common ancestor, which is an organism that two or more groups are descended from.

To compare two organisms, trace back to where their branches meet. The nearer that point is to the present, the more closely related they are. In this tree, the cat and dog share a more recent ancestor than the cat and the eagle.

Trees are made using fossils and DNA. They may change when new evidence is found.

Recap

Linnaean groups, big to small: kingdom, phylum, class, order, family, genus, species.

A binomial name is genus plus species, such as Homo sapiens. The genus has a capital letter.

The three domains are Archaea, Bacteria and Eukaryota. Classification changes as new evidence appears.

On an evolutionary tree, the most recent shared ancestor shows the closest relatives.