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Biology LibreTexts

Dichotomous keys

  • Page ID
    23801
  • Diagnostics is a practical science which helps to determine living organisms. One of the best way of determining was invented in the end of 18 century by famous French naturalist, Jean-Baptiste Lamarck. He created the dichotomous key (sometimes called descriptive key, or descriptive table). The legend says that when Lamarck demonstrated this key for the first time, he gave it to the random stranger (who had no idea about plants and their names), and plant were determined without problems! How to make such a key? The example is below:

    1. We need to start with “players”. In this example, it will be same three plant families:

    Alphaceae

    Betaceae

    Gammaceae

    2. Assess descriptions of these three groups (we copy this from the above):

    Alphaceae: Flowers red, petioles short, leaves whole, spines absent

    Betaceae: Flowers red, petioles long, leaves whole, spines absent

    Gammaceae: Flowers green, petioles short, leaves dissected, spines present

    3. Start with a character which let to split the list into two nearly equal groups. Then add other character(s). It is always good to use more characters!

    1. Petioles long......................................................................................... Betaceae.

    -Petioles short ........................................................................................................2.

    2. Flowers red, leaves whole, spines absent.......................................... Alphaceae.

    Flowers green, leaves dissected, spines present ................................Gammaceae.

    As you see here, key consists of steps. Every step has a number and typically two choices. Number is attached to the first choice whereas the second choice is marked with minus “–”. The choice will lead either to the name, or to another step. The choice sentence might contain several phrases, the first is the most important and the last is the least important.