6.4: Classification of living organisms
Not started yet — this one needs some love.
Traditionally living things have been classified into groups depending on their structure and characteristics in a system developed by Carl Linnaeus.
Linnaeus classified living things into kingdom, phylum, class, order, family, genus and species.
Organisms are named by the binomial system of genus and species.
Improved evidence from microscopes, internal structures and biochemical processes led to new models of classification.
Carl Woese developed the three-domain system using evidence from chemical analysis.
In the three-domain system, organisms are divided into archaea, bacteria and eukaryota.
Archaea are primitive bacteria usually living in extreme environments.
Bacteria are true bacteria.
Eukaryota includes protists, fungi, plants and animals.
Evolutionary trees are a method used by scientists to show how they believe organisms are related.
Evolutionary trees use current classification data for living organisms and fossil data for extinct organisms.
Common exam mistakes
The classification order from broadest to most specific is kingdom, phylum, class, order, family, genus, species.
In a binomial name, the genus is capitalised and the species is lower case, for example *Homo sapiens*.
Do not confuse the three-domain system with the five-kingdom system; domains are archaea, bacteria and eukaryota.
On an evolutionary tree, the branching point is a common ancestor; do not say one living species at the branch tip evolved from the other branch-tip species.
Species with a more recent common ancestor are more closely related.