Concept & Uses of Classification Systems (Cambridge (CIE) IGCSE Biology)
Revision Note
Written by: Lára Marie McIvor
Reviewed by: Lucy Kirkham
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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: Kingdom, Phylum, Class, Order, Family, Genus, Species
Linnaeus’s system of classification
Examiner Tips and Tricks
The order of classification can be remembered by using a mnemonic like:
KING PHILIP CAME OVER FOR GRAN’S SPAGHETTI
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, or you may be presented with an unfamiliar one as part of an exam questions
Follow the statements from the beginning. Each statement or question you should be able to answer using the information provided in the question or an image given as part of the question.
Eventually there will be no more statements or questions left and you will have the name of the organism
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
Examiner Tips and Tricks
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!
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