Classification (AQA GCSE Biology): Revision Note

Linnaeus classification

• Classification is the process of putting living organisms into groups

• Traditionally, living things have been classified according to their structures and characteristics, in a system developed by Carl Linnaeus

• Linnaean classification sorts living organisms into the following groups:

  • Kingdom

  • Phylum

  • Class

  • Order

  • Family

  • Genus

  • Species

• Kingdom is the largest group, while species is the smallest

• The organisms within a species share the most common features with each other

Development of classification

• Early classification was done on the basis of:

  • overall form and shape of the organism, e.g. whether it had wings or legs

  • internal anatomy, i.e. the detailed body structure as determined by dissection

• New models of classification have emerged due to:

  • improvements in microscopes; this has provided detail of internal structures for comparison

  • understanding of biochemical processes

• Modern DNA sequencing now forms the basis of classification

  • The more similar the base sequences in the DNA of two species, the more closely related the two species are (and the more recent in time their common ancestor is)

  • E.g. the base sequences in a mammal’s DNA are more similar to all other mammals than to any other vertebrate groups

Three domain system

• Due to evidence available from chemical analysis, there is now a ‘three-domain system’ of classification

• This was developed by Carl Woese in 1990

• In this system, organisms are divided into three large groups called domains

• These domains are:

  • Archaea (primitive bacteria usually living in extreme environments such as hot springs and salt lakes)

  • Bacteria (true bacteria such as E. coli and Staphylococcus)

  • Eukaryota (which includes protists, fungi, plants and animals)

• These domains are then subdivided into the smaller groups: kingdom, phylum, class, order, family, genus, species

Evolutionary trees

• Evolutionary trees are diagrams that show the relationship between species over evolutionary time

• A new branch in the tree shows where speciation has occurred (when a new species has evolved)

• In the evolutionary tree below, for example:

  • Chimpanzees and bonobos share a recent common ancestor. Chimpanzees are therefore most similar to bonobos (more similar than they are to any other primate species)

  • Humans share a more recent common ancestor with gorillas than they do with orangutans – this means humans are more closely related to gorillas than we are to orangutans

  • All five primate species shown here share a common ancestor (from the distant past)

Use this image

• Evolutionary trees are created using current classification data for living organisms (such as DNA analysis and structural similarities) and fossil data for extinct organisms