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BIOLOGY · Stage 7 · Chapter 1: Classification and Life

Biology 1.5 Simple keys — Dichotomous Identification Keys

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Biology 1.5 Simple keys — Dichotomous Identification Keys — Alhussan International Academy — Biology (Stages 7–9) notes: Use a dichotomous key to identify an organism by following a series of questions about its features.Key = a tool that identifies organisms using simple questio…

  • Why classify
  • Example key to fish
  • Observable features
  • Measurements
  • Stable features
  • Reliability

Section 1 of 4What a key is

  • Key = a tool that identifies organisms using simple questions about their features.
  • Dichotomous key
    • it is made of paired choices, since dichotomous means divided into two, and each answer gives a name or sends you to another question.
  • Why classify
    • living things are classified by their similarities and differences, and a key identifies a strange animal or plant even if you have never seen it.
  • Use
    • biologists use keys to identify organisms they find in fieldwork, to measure biodiversity and to monitor rare organisms.

Section 2 of 4Using a key step by step

  1. Look closely at the specimen and note its features, such as legs, wings, colour or leaf shape.
  2. Start at question 1 and read both statements or answers.
  3. Choose the one that matches the specimen, and look again or measure if unsure.
  4. Follow the instruction or arrow to the next numbered question or to a name.
  5. Continue until you reach a name.
  6. Check the answer against any description or picture provided.
  • Example key to fish
    • question 1 asks whether the body is flattened; for fish A the answer is no,
    • which leads to question 3 , and the yes answer identifies a gulper eel.
      • (is the tail long and thin?)
    • The same key then identifies other fish, which shows a key can be reused.
  • Recording
    • note the path taken, such as 1a, 2a, 3b, so another person can check the identification.

Section 3 of 4Good key questions

  • Observable features
    • use features that are clear, countable and easy to observe, such as the number of legs, presence of antennae or a smooth or toothed leaf edge.
  • Measurements
    • avoid vague words such as big or colourful; if size is used, give a measurement such as longer than 5 cm.
  • Stable features
    • avoid features that change with age or season.
  • Complete
    • each pair of statements must cover every possible specimen.
  • Forms
    • keys may be written as numbered statements or as a branching diagram with the same information.

Section 4 of 4Keys for groups and species

  • Insects
    • most animals are invertebrates, most invertebrates are arthropods and most arthropods are insects, with three body parts and six legs.
    • Nearly a million insect species are named and about a third are beetles.
  • Beetles
    • all have a tough cover over their wings, so they look hard and often shiny.
  • Ladybirds
    • about five thousand beetle species are ladybirds, small beetles with round, brightly coloured bodies, often red with black spots.
    • A ladybird key uses features such as the number of spots to name species.
  • Levels
    • some keys sort animals into large groups such as insects or worms, and others identify a particular species.
  • Limits
    • a key works only for the group it was designed for, and a specimen that fits no statement may belong to another group.
  • Reliability
    • if two people follow the same path and reach the same name, the identification is reliable.

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