Levels of Biological Organization

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

This lesson is about the organization of living things, or biological organization. Just like it can be helpful to organize your closet, scientists use a classification system to organize living things. This allows for clarity when talking about a species and provides information about an organism, just through the name!

The Linnaean System

Scientists need a way to explain the relatedness of species, and they do this by grouping them together based on shared characteristics, ancestry, and DNA sequences. The currently used classification system is called the Linnaean System. This system was designed by Carl Linnaeus in the mid 1700's. Linnaeus used physical characteristics to design his system, but today we use more advanced methods to group organisms, such as DNA sequences. The organization that Linnaeus formed has been modified based on new discoveries about cells and DNA over time. The categories of the Linnaean system (listed from broadest to most specific) are domain, kingdom, phylum, class, order, family, genus, and species. We will look at each of these in more detail below. A diagram of how the categories nest together is also shown below.

Nested categories of the Linnaean classification system
Linnaean classification nesting


The principle behind the Linnaean system is to organize things from broad categories, which contain a lot of species, to small categories that are in fact the only name for one particular species. An addition to the traditional system was domains. Domains are now the largest category. Think of it like rooms in your house. You might refer to the bathroom, kitchen, and bedroom. Things in these rooms go together, but aren't necessarily similar. In biological classification, there are three domains, Eukarya, Archae, and Bacteria. Below is the comparison of each room to each biological domain.

Comparison of different rooms in the house to the biological domains
Comparison of rooms to domains

Let's use the example of the leopard for us to classify. This animal belongs in the Eukarya domain. Below is an image of the leopard.

Leopard to classify


Next we might divide the items in the bedroom into categories. This is the kingdom in the Linnaean classification system. We might choose clothing, bedding, and books, depending on your room. There are six kingdoms in biological classification: Bacteria, Archaea, Fungi, Protista, Animalia, and Plantae. Our leopard is classified in kingdom Animalia.


After kingdoms there are phylums. In this group, the kingdom is sorted into smaller groups that are more similar. In our example, the shoe category might be sorted into flat shoes and high heels. The leopard is classified into phylum Chordata. In reality, the kingdoms of living things are sorted into dozens of different phylums and those into dozens of classes. For simplicity, I will only talk about splitting the piles into two groups each time for our shoes.


After phylum comes class. Again, we sort the shoes into more similar piles. Our flat shoes might get sorted into dress shoes and sneakers. Our leopard is now sorted into class Mammalia. Other mammals include all warm blooded animals that give birth to live young with mammary glands. Humans are mammals!


Next, organisms are sorted into orders. This is like our sneakers being sorted into brands, like Nike or Converse. Our leopard is sorted into order Carnivora.


We continue to sort our organisms and shoes even more, and they become more related. After class, the next level of organization is family. For example, our Nikes could be sorted into particular styles, like Jordan's or Air Max. The leopard is now sorted into Felidae, which contains other large cats.


Next is the category of genus. Organisms with the same genus are very similar. They have similar physical features, similar DNA, and probably live in similar habitats. This would be like different models of Jordan's or Jordan's that came out in different years. The leopard is in genus Panthera. Other big cats in genus Panthera are lions, jaguars, and tigers.

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