Classification of Organisms (WJEC GCSE Biology: Combined Science)

• Living organisms show great variety, e.g.
  • Size
    • Bacteria are microscopic while blue whales are very large
  • Features
    • Plant cells have features which enable them to carry out photosynthesis while animal cells do not
  • Complexity
    • Single-celled organisms are more simple than multicellular organisms

• This variety can be used to place organisms into groups, e.g.
  • Plants can be grouped into:
    • Flowering plants that produce flowers
      • E.g. sunflowers, grasses and oak trees
    • Non-flowering plants that do not produce flowers
      • E.g. mosses and ferns
  • Animals can be grouped into:
    • Vertebrates that have a backbone
      • E.g. mammals and birds
    • Invertebrates that do not have a backbone
      • E.g. insects and arachnids

Plant and animal groups diagram

Plants can grouped into flowering and non-flowering plants while animals can be vertebrates or invertebrates

• The process of putting organisms into groups is known as classification
• Classification can be carried out on the basis of:
  • Similar morphological features
    • Morphology relates to the physical features of an organism
    • Organisms that are similar in appearance would be classified together under this system
  • DNA analysis
    • Organisms that show similarity in their DNA sequences are classified together
• Classification is important because it:
  • Aids scientific understanding of the variety of life
  • Allows scientists to learn about the relationships between living organisms, i.e. which species share a common ancestor
• One example of a classification system is the five kingdoms; organisms are grouped into these kingdoms on the basis of their morphology
• The five kingdoms are:
  • Bacteria
  • Eukaryotic single-celled organisms, also known as protists
  • Plants
  • Fungi
  • Animals

Five kingdoms diagram

The five kingdoms classification is based on morphological features

Scientific naming of organisms

• The scientific system of naming species uses a two-part Latin name, e.g. under this system:
  • Human = Homo sapiens
  • Wolf = Canis lupus
  • Tomato plant = Solanum lycopersicum
• The system ensures that species have a simple name, allowing easy identification
  • In the past species names could be very long, e.g. the old name for the tomato plant was Solanum caule inerme herbaceo, foliis pinnatis incisis, racemis simplicibus
• This system is universal, ensuring that scientists around the world all use the same name for each species
  • Species common names are often different in different parts of the world. e.g. the domestic cat is known as kitte, maow, chat, kissa, bili, and gato in different languages, but its scientific name is always Felis domesticus

Hierarchy of taxa

• The taxa (singular taxon) are the groups into which organisms can be classified
• The highest taxon in the hierarchy includes many different species while the lowest taxon contains only one species

Hierarchy of taxa diagram

Domain is the largest of the taxa while species is the smallest

• The two-part scientific name is taken from the two smallest taxa for any given species, e.g. for Homo sapiens
  • Homo = genus
  • sapiens = species
• Other examples include:

Hierarchy of taxa table