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Organization of Living Things

Discusses how we classify life and the definition of a species.

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Organization of Living Things

How would you classify a horse?

It's easy enough to classify the horse in the animal kingdom. That's one level of classification. But what other groups does the horse belong to? Horses also belong to a class—the mammals. These animals all have fur and nurse their young.

Classification of Life

When you see an organism that you have never seen before, you probably put it into a group without even thinking. If it is green and leafy, you probably call it a plant. If it is long and slithers, you probably call it as a snake. How do you make these decisions? You look at the physical features of the organism and think about what it has in common with other organisms.

Scientists do the same thing when they classify, or put into categories, living things. Scientists classify organisms not only by their physical features, but also by how closely related they are. Lions and tigers look like each other more than they look like bears, but are lions and tigers related? Evolutionarily speaking, yes. Evolution is the change in a species over time. Lions and tigers both evolved from a common ancestor. So it turns out that the two cats are actually more closely related to each other than to bears. How an organism looks and how it is related to other organisms determines how it is classified.

Linnaean System of Classification

People have been concerned with classifying organisms for thousands of years. Over 2,000 years ago, the Greek philosopher Aristotle developed a classification system that divided living things into several groups that we still use today, including mammals, insects, and reptiles.

Carolus (Carl) Linnaeus (1707-1778) (Figure below) built on Aristotle’s work to create his own classification system. He invented the way we name organisms today, with each organism having a two word name. Linnaeus is considered the inventor of modern taxonomy, the science of naming and grouping organisms.

Portrait of Carl Linnaeus, the inventor of modern taxonomy

In the 18th century, Carl Linnaeus invented the two-name system of naming organisms (genus and species) and introduced the most complete classification system then known.

Linnaeus developed binomial nomenclature, a way to give a scientific name to every organism. In this system, each organism receives a two-part name in which the first word is the genus (a group of species), and the second word refers to one species in that genus. For example, a coyote's species name is Canis latrans. Latrans is the species and Canis is the genus, a larger group that includes dogs, wolves, and other dog-like animals. Here is another example: the red maple, Acer rubra, and the sugar maple, Acer saccharum, are both in the same genus and they look similar (Figure below). Notice that the genus is capitalized and the species is not, and that the whole scientific name is in italics. Tigers (Panthera tigris) and lions (Panthera leo) have the same genus name, but are obviously different species. The names may seem strange, but the names are written in a language called Latin.

Leaves from the green and red maple tree, and a maple seed

These leaves are from two different species of trees in the Acer, or maple, genus. The green leaf (far left) is from the sugar maple, and the red leaf (center) are from the red maple. One of the characteristics of the maple genus is winged seeds (far right).

Modern Classification

Modern taxonomists have reordered many groups of organisms since Linnaeus. The main categories that biologists use are listed here from the most specific to the least specific category (Figure below). All organisms can be classified into one of three domains, the least specific grouping. The three domains are Bacteria, Archaea, and Eukarya. The Kingdom is the next category after the Domain. All life is divided among six kingdoms: Kingdom Bacteria, Kingdom Archaea, Kingdom Protista, Kingdom Plantae, Kingdom Fungi, and Kingdom Animalia.

Diagram of the classification categories for organisms

This diagram illustrates the classification categories for organisms, with the broadest category (kingdom) at the bottom, and the most specific category (species) at the top. We are Homo sapiens. Homo is the genus of great apes that includes modern humans and closely related species, and sapiens is the only living species of the genus.

Defining a Species

Even though naming species is straightforward, deciding if two organisms are the same species can sometimes be difficult. Linnaeus defined each species by the distinctive physical characteristics shared by these organisms. But two members of the same species may look quite different. For example, people from different parts of the world sometimes look very different, but we are all the same species (Figure below).

So how is a species defined? A species is defined as a group of similar individuals that can interbreed with one another and produce fertile offspring. A species does not produce fertile offspring with other species.

This group of children are all members of the same species

These children are all members of the same species, Homo sapiens.


  • Scientists have defined several major categories for classifying organisms: domain, kingdom, phylum, class, order, family, genus, and species.
  • The scientific name of an organism consists of its genus and species.

Explore More

Use the resources below to answer the following questions.

Explore More I

  1. What do taxonomists study? How does their work help other scientists?
  2. Who was the first person we know of who developed a system to categorize things? How was this done? Is his system still used today?
  3. What contribution to taxonomy did Carolus Linnaeus make?

Explore More II

Use the below activity to see specific examples of how organisms are categorized. Make sure you go through all three types of organisms so you can gain a good understanding of the level at which different types of organisms separate from each other.


  1. Who is the inventor of the modern classification system?
  2. List the classification categories for organisms from the broadest category to the most specific.
  3. What is meant by binomial nomenclature?
  4. Define a species.

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    binomial nomenclature System of naming organisms using two terms, the first one indicating the genus and the second the species.
    classify To place things into groups based on shared qualities.
    domain Least specific category of classification.
    evolution Process in which something passes to a different stage, such as a living organism turning into a more advanced or mature organism; the change of the inherited traits of a group of organisms over many generations.
    genus Group of related species.
    species Group of similar individuals that can interbreed with one another and produce fertile offspring.
    taxonomy Science of naming and grouping organisms.

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