Classifying living organisms

In a nutshell

Organisms used to be classified by their physical appearance. However, technology has since developed and scientists now use their DNA sequences to identify how closely related organisms are. Every organism has a binomial name that scientists from all over the world can use.

Artificial classification

Artificial classification is classifying organisms based on their visual characteristics. This method is very outdated but it is still used to make classification keys that help scientists identify organisms.

Examples of visual characteristics

How many legs they have.
The number of eyes they have.
Whether they lay eggs.
Whether they fly.

Natural classification

This classification method uses evolutionary relationships, information about common ancestors and common structural features to sort organisms. Using natural classification, Carl Linnaeus split all organisms into five kingdoms in the $1700s$ .

Animals	Fish, mammals and reptiles.
Plants	Grasses and trees.
Fungi	Mushrooms and yeast.
Prokaryotes	Single-celled organisms without a nucleus.
Protists	Eukaryotic single celled organisms.

Kingdoms (1.) are then progressively split into smaller groups. Those groups are the phylum (2.), class (3.), order (4.), family (5.), genus (6.) and species (7.). A species is a group of similar organisms that can interbreed to produce fertile offspring.

Biology; Natural selection and genetic modification; KS4 Year 10; Classifying living organisms

Technological developments improves classification

Improvements in technology such as the development of the microscope allowed scientists to look inside organisms and examine their structures more easily.

Molecular phylogenetics

Molecular phylogenetics is looking at the evolutionary history of an organism by looking at their molecules, like their DNA.

Molecular phylogenetics uses DNA sequencing to see how closely related organisms are based on the sequence of their DNA bases. The more similar the organisms DNA sequences are, the more closely related they are.

Using RNA sequence analysis, Carl Woese created a three-domain system in $1990$ . He found that some species that were thought to be closely related were actually not. He added another group before kingdoms called 'domains'. His three domains are archaea, bacteria and eukaryota.

Archaea	Organisms in this domain were thought to be early versions of bacteria but there are actually lots of differences between them and they were first found in extreme environments like hot vents and salt lakes.
Bacteria	True bacteria like Escherichia coli and Staphylococcus.
Eukaryota	Organisms including animals, plants, protists and fungi.

These are then further divided into the kingdom, phylum, class, order, family, genus and species as before.

Binomial naming system

Each organism has a binomial name which is a two part latin name. The first part is the genus of the organism and the second part is the species.

Example

Humans are Homo sapiens. 'Homo' is the genus and 'sapiens' is the species.

The first part of a binomial name must be capitalised and the second part is lowercase. It is always written in italics unless the sentence is in italics and then the binomial name can be underlined. Alternatively, if the sentence is in italics, the name can be in a regular font.

This name is used across the globe so it doesn't matter which language you speak, the binomial name will always be the same.

Evolutionary trees

Evolutionary trees use information such as classification data and fossils to show common ancestors between species and how different species are related. The more recent a common ancestor of two species is, the more closely related the two species are.

Example

(1.) shows a distant common ancestor between whales, dolphins and sharks. Whales and dolphins share a more recent common ancestor (2.) so they are more closely related to each other than they are to the shark.

Biology; Natural selection and genetic modification; KS4 Year 10; Classifying living organisms