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Classification of Living Things: Taxonomy

Now that you know more about what makes something alive, the next question that biology tries to answer is: how do we study all these different organisms?

One way to study living things is to classify them into different categories. Think about the many different organisms that exist in the world. What makes a bird a bird? What makes humans different from apes? What is the difference between a slug and a snail? In fact, how do you tell the difference between one living thing and another living thing?

Or, think about it this way: You and your friend are different living things. You and a bird are also different living things. But you and your friend are a lot more similar, than you and a bird. 

How do you explain the magnitude of these differences? There are more differences between a bird and a human being than two human beings. This is where taxonomy is used. Taxonomy is the process of organizing living things into groups that have common characteristics.


Imagine all the species that exist in the world. How would you organize them so that you can better understand them all? For example, you could list all the species in alphabetical order. That wouldn’t tell you very much about how each species is related though. You could also list each species by colour, or size, or even smell! That probably won’t help you either - a killer whale and an elephant are about the same size, but are very different from each other.

Biologists use taxonomy as the method of classification. The basic idea of taxonomy is based on how species share certain characteristics with each other (which are often due to evolving from the same species). There are many characteristics that biologists use to organize species into a taxonomy tree. Here are some criteria that we use:

  • physical appearance
  • method of movement
  • how they reproduce
  • habitat
  • what and how they eat
  • hereditary information
  • internal structures (vertebrates vs invertebrates)
  • etc.

For example, Cats and Bears have broad characteristics in common (both use sexual reproduction, circulate blood, etc.). Thus we can group them together as ‘Animals.’

Biologists use a dichotomous key to help them create these classifications. A dichotomous key is a series of ‘yes’ and ‘no’ responses that address the presence or absence of a single trait. So any new organism either has or doesn’t have these traits. A key should never describe more than one possibility (e.g., 2 legs or 4 legs: but what if you run into  something that has 6 legs or no legs?)

Let’s take a look at a sample dichotomous key for four different animals, the jellyfish, the sea star, the tiger, and the snake. 

What happens when we discover a new organism?

When scientists discover a new organism, they compare the new organism to known organisms so they can be classified, but this can sometimes be difficult. This is because new organisms might share some characteristics with known organisms, but have major differences that make them very different.

A single taxonomic group is called a taxon. A number of taxonomic groups are called taxa which is the plural of taxon.

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  Sep 27, 2021