CIE IGCSE Biology: 复习笔记:1.1.2 Concept & Uses of Classification Systems

CIE IGCSE Biology: 复习笔记:1.1.2 Concept & Uses of Classification Systems

How Organisms are Classified


  • 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: KingdomPhylumClassOrderFamilyGenusSpecies



Linnaeus’s system of classification



Exam Tip

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





Dichotomous Keys

  • Keys are used to identify organisms based on a series of questions about their features
  • Dichotomous means ‘branching into two’ and it leads the user through to the name of the organism by giving two descriptions at a time and asking them to choose
  • Each choice leads the user onto another two descriptions
  • In order to successfully navigate a key, you need to pick a single organism to start with and follow the statements from the beginning until you find the name
  • You then pick another organism and start at the beginning of the key again, repeating until all organisms are named

Example of a dichotomous key #1

Example of a dichotomous key #2

Exam Tip

Simple dichotomous keys almost always come up in the multiple choice paper, so make sure you can use one.Very occasionally they show up in the theory paper, and when they do you almost always have to use one instead of constructing one, so focus on this rather than spending hours learning to construct them yourself!