Classifying Organisms (Cambridge O Level Biology)

• 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

• 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

Classification System Diagram

Linnaeus’s system of classification

• 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

Dichotomous key Diagram

Example of a dichotomous key #1

Example of a dichotomous key #2