Classification (AQA GCSE Biology: Combined Science)

• There are millions of species of organisms on Earth
• A species is defined as a group of organisms that can reproduce to produce fertile offspring
• These species can be classified into groups by the features that they share eg. all mammals have bodies covered in hair, feed young from mammary glands and have external ears (pinnas)
• Traditionally living things have been classified into groups depending on their structure and characteristics in a system developed by Carl Linnaeus.
• Organisms were first classified by a Swedish naturalist called Linnaeus in a way that allows the subdivision of living organisms into smaller and more specialised groups
• The species in these groups have more and more features in common the more subdivided they get
• He named organisms in Latin using the binomial system where the scientific name of an organism is made up of two parts starting with the genus (always given a capital letter) and followed by the species (starting with a lower case letter)
• When typed binomial names are always in italics (which indicates they are Latin) e.g. Homo sapiens
• The sequence of classification is: Kingdom, Phylum, Class, Order, Family, Genus, Species

Linnaeus’s system of classification

• Organisms share features because they originally descend from a common ancestor
• Example: all mammals have bodies covered in hair, feed young from mammary glands and have external ears (pinnas)
• Originally, organisms were classified using morphology (the overall form and shape of the organism, e.g. whether it had wings or legs) and anatomy (the detailed body structure as determined by dissection)
• As evidence of internal structures became more developed due to improvements in microscopes, and the understanding of biochemical processes progressed, new models of classification were proposed
• As technology advanced, DNA sequencing allowed us to classify organisms using a more scientific approach
• Studies of DNA sequences of different species show that the more similar the base sequences in the DNA of two species, the more closely related those two species are (and the more recent in time their common ancestor is)
• This means that the base sequences in a mammal’s DNA are more closely related to all other mammals than to any other vertebrate groups

DNA sequences can show how closely related different species are

• The sequences above show that Brachinus armiger and Brachinus hirsutus are more closely related than any other species in the list as their DNA sequences are identical except for the last but one base (B.armiger has a T in that position whereas B.hirsutus has an A)
• As DNA base sequences are used to code for amino acid sequences in proteins, the similarities in amino acid sequences can also be used to determine how closely related organisms are

• 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 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 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 we 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)

An evolutionary tree for five species of primate

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