Revision Notes

1.2 Classification

How Organisms are Classified: Basics

  • 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 e.g. all mammals have bodies covered in hair, feed young from mammary glands and have external ears (pinnas)

The Binomial System

  • 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 Classifcation, IGCSE & GCSE Biology revision notes

Linnaeus’s system of classification


Exam Tip

The order of classification can be remembered by using a mnemonic like:


Extended Only

How Organisms are Classified

  • 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 technology advanced, microscopes, knowledge of biochemistry and eventually 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, IGCSE & GCSE Biology revision notes

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

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Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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