Classifying Organisms | Cambridge (CIE) O Level Biology Revision Notes 2021

How Organisms are Classified

• Millions of species of organisms on Earth.
• Species: Group of organisms that can reproduce to produce fertile offspring.
• Classification based on shared features (e.g., mammals have hair, feed young with mammary glands, have external ears).

The Binomial System

• Developed by Swedish naturalist Linnaeus.
• Organisms classified into groups with increasing common features.
• Binomial System: Scientific name in two parts:
  • Genus: Capital letter.
  • Species: Lowercase letter.
• Example: Homo sapiens (italicized as Latin).
• Sequence: Kingdom > Phylum > Class > Order > Family > Genus > Species.

Classification System Diagram

• Illustrates Linnaeus’s classification system.
• Mnemonic for classification order: KING PHILIP CAME OVER FOR GRAN'S SPAGHETTI.

Dichotomous Keys

• Used to identify organisms through a series of questions about features.
• Dichotomous: Branching into two; offers two descriptions at a time.
• Navigate by starting with a single organism and using the key until the organism is identified.
• Repeat the process for each organism.

Example of a Dichotomous Key

• Provides step-by-step approach to identifying organisms based on features.

Examiner Tips and Tricks

• Focus on using dichotomous keys rather than constructing them, as exams typically require usage.

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