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T1 - Taxonomic Keys

I can identify unknown organisms using a taxonomic key and use rules of classification to create a taxonomic key


Billions of years of evolution on Earth have resulted in a huge variety of different types of organisms. For more than two thousand years, humans have been trying to organize this great diversity of life. The classification system introduced by the Swedish botanist Carolus Linnaeus in the early 1700s has been the most widely used classification for almost 300 years.


Scientific classification is a method by which biologists organize living things into groups. It is also called taxonomy. Groups of organisms in taxonomy are called taxa (singular, taxon). You may already be familiar with commonly used taxa, such as the kingdom and species. A kingdom is a major grouping of organisms, such as plants or animals. A species includes only organisms of the same type, such as humans (Homo sapiens) or lions (Panthera leo). The modern biological definition of a species is a group of organisms that are similar enough to mate and produce fertile offspring together. In a classification system, kingdoms, species, and other taxa are typically arranged in a hierarchy of higher and lower levels. Higher levels include taxa such as kingdoms, which are more inclusive. Lower levels include taxa such as species, which are less inclusive.

This type of hierarchical classification can be demonstrated by classifying familiar objects. For example, a classification of cars is shown in Figure below. The highest level of the classification system includes all cars. The next highest level groups cars on the basis of size. Then, within each of the size categories, cars are grouped according to first one and then another trait. Higher taxa (for example, compact cars) include many different cars. Lower taxa (for example, compact cars that are blue and have two doors and cloth seats) contain far fewer cars. The cars in lower taxa are also much more similar to one another.


Cars can be classified, or grouped, on the basis of various traits. In this classification, the most inclusive groups are the size categories, such as all compact cars or all mid-size cars. The most exclusive groups in this classification share several additional traits, including color, number of doors, and type of seats. Note that just one group for each trait is further divided as an example.


Why do biologists classify organisms? The major reason is to make sense of the incredible diversity of life on Earth. Scientists have identified millions of different species of organisms. Among animals, the most diverse group of organisms is the insects. More than one million different species of insects have already been described. An estimated nine million insect species have yet to be identified. A tiny fraction of insect species is shown in the beetle collection inFigure below.

Only a few of the more than one million known species of insects are represented in this beetle collection. Beetles are a major subgroup of insects. They make up about 40 percent of all insect species and about 25 percent of all known species of organisms.


As diverse as insects are, there may be even more species of bacteria, another major group of organisms. Clearly, there is a need to organize the tremendous diversity of life. Classification allows scientists to organize and better understand the basic similarities and differences among organisms. This knowledge is necessary to understand the present diversity and the past evolutionary history of life on Earth.

Lesson Summary

  • Taxonomy is the scientific classification of organisms. Scientists classify organisms in order to make sense of the tremendous diversity of life on Earth.

Review Questions

  1. Define taxonomy.
  2. How does classification benefit scientists generally? 
  3. Explain why biologists need to classify organisms.
  4. Describe how to use a key.
  5. Describe the basic rule for creating a taxonomic key.
  6. Create a hierarchical taxonomy to classify writing implements, such as pens and pencils. Use a diagram to show your taxonomy.