Classifying Living Things
If you look around a kitchen, you might notice that items are organized based on their function. Forks, for example, may be separated from knives and spoons. This categorization helps organize utensils based on their function. For example, you may use a spoon to eat soup but not to cut a vegetable.

 
In a similar way, there have been many different ideas about how to organize, or classify, living things. A Greek philosopher named Aristotle (384-322 B.G.E.) was one of the first people to classify organisms. Aristotle placed all organisms into two large groups: plants and animals. He classified animals based on the presence of “red blood,” the animal’s environment, and the shape and size of the animal. He classified plants according to the plant’s structure, its size and whether it was a tree, a shrub, or an herb. 

Curious individuals have spent centuries examining the similarities and differences among organisms to classify them into different groups. Taxonomy is a branch of biology that identifies, names, and classifies species based on their natural relationships. The study of taxonomy developed as scientists looked more closely at organisms, with the first taxonomic system developed by Carolus Linnaeus.

Evidence for Classification
In the 1700s, Carolus Linnaeus, a Swedish physician and botanist, proposed a system to classify organisms based on similar structures. Linnaeus placed all organisms into two main groups called kingdoms. Over the next 200 years, people learned more about and discovered new organisms. In 1969, American biologist Robert H. Whittaker proposed a five-kingdom system for classifying organisms. His system included the kingdoms of Monera, Protists, Plants, Fungi, and Animals. 

The classification system of living things is still changing today. The current classification method is called systematics. Systematics uses all the evidence that is known about organisms to classify them. This evidence includes an organism’s cell type, its habitat, the way an organism obtains food and energy, structure and function of its features, and the common ancestry of organisms. Systematics also includes molecular analysis—the study of genetic material within organisms. 

Using systematics, scientists identified two distinct groups in Kingdom Monera: Bacteria and Archaea (ar KEE uh). This led to the development of another level of classification called domains. All organisms are now classified into one of three domains—Bacteria, Archaea, or Eukarya (yew KER ee uh)—and then into one of six kingdoms.

Molecular analysis technology has changed taxonomy. Instead of just looking at the features of different organisms, scientists are also looking at genetic material. This helps to see more differences among organisms. Observe the images of the giraffes. Giraffes were originally classified as one species with nine sub-groups. However, molecular analysis technology offers more evidence of differences in genetic material to categorize organisms. Now, scientists classify the common giraffe as four different species.

The Taxonomic Classification System

The modern taxonomic classification system has eight levels. It is arranged using hierarchical organization with the domain level having the most differences in traits. The species level has organisms with the most similarities. When an organism is discovered, it is classified into each taxonomic level by comparing the similarities and differences of its traits to other organisms. Let's explore how the fennec fox, Vulpes zerda, is taxonomically classified

Domain
The highest level of the hierarchy of classification is the domain. The domain is the taxonomic group of one or more kingdoms. This is the broadest group that is the most complex with the largest species diversity. The organisms in this group have the most differences in their traits in comparison to other levels. Since the fennec fox is made of eukaryotic cells, it is categorized into the domain Eukarya which contains both plants and animals.

Kingdom
The next level of classification is kingdom. A kingdom is a taxonomic group of related phyla or divisions. Classification is narrowed down to include organisms with certain traits. The fox is grouped into kingdom Animalia because it is a multicellular heterotroph made of animal cells that do not contain cell walls. Plants are in a different kingdom

Phylum
The next level of classification is phylum. The phylum is a taxonomic group of related classes. Foxes are grouped into the same phylum, Chordata, since they have a structure called a nerve cord.

Class
The next level of classification is class. The class is a taxonomic group that contains one or more related orders. Foxes are grouped into the same class, Mammalia, since they have fur and are fed with milk. They are also warm-blooded.

Order
The next level of classification is order. The order is the taxonomic group that contains related families. Foxes are grouped into the same order, Carnivora, for their predaceous diet. They have anatomy specific to this diet, including sharp teeth and claws used to rip meat apart. Foxes are different from other mammals that are not predaceous, and do not have these traits.

Family
The next level of classification is family. The famlly.is  taxonomic group of similar, related genera. Foxes are grouped into the same family, Canidae. Some of the traits described in this family include non-retractable claws and full sets of teeth that are large and unspecialized. Some also form packs or groups for hunting or living, like dogs and Wolves.

Genus
The next level of classification is genus. The genus is a group of similar species. Foxes are grouped into the same genus, Vulpes, which includes several fox species. Individuals in this genus have long dense fur and a rounded tail. Their legs are shorter than other animals in the Canidae family, and they have pointed snouts and upright pointed ears.

Species
The last level of classification is species. A species is a group of organisms that have similar traits and are able to produce fertile offspring. The fennec fox has a species name of Vulpes zerda. Zerda means “blonde” or “gold”, for the color of its fur. Its most defining features are its large ears and very small size (35- to 40-cm in length). Its small size and large ears release heat, which is helpful since it lives in the desert. Its large ears also help it to listen for prey underground.

Scientific Names
The system Linnaeus developed for naming organisms, binomial nomenclature is  naming system that gives each organism a two-word scientific name. This two-word scientific name is the name of the organism’s species, such as Vulpes zerda for the fennec fox. The first word is the organism’s genus, such as Vulpes. The second word might describe the organism’s appearance or its behavior.

Using Scientific Names

When you discuss organisms, you might use names such as bird, tree, or mushroom However, these are common names for several different species. Sometimes there are several common names for one organism. Other times, a common name might refer to several different types of organisms. The common bear might be called a brown bear or a grizzly bear, but it has only one scientific name, Ursus arctos. The scientific name is the same worldwide, so everyone uses the same name for the same species. This ensures

that scientists across the world are studying the same organism.

Classification Tools
Suppose you go out into the forest and see several trees. You want to learn more about them, but how do you figure out which trees you are observing? There are several tools that you can use to identify organisms. A dichotomous key is used to identify unknown organisms while phylogenetic trees, including cladograms, are used to understand the evolutionary history of an organism. These tools help us understand how organisms are similar or different.

Dichotomous Keys
A dichotomous key is a series of descriptions arranged in pairs that leads the user to the identification of an unknown organism. The chosen description leads to either another pair of statements or the identification of the organism. Choices continue until the organism is identified.

Phylogenetic Trees
Recall that different organisms are grouped into different categories based on similarities or differences in their characteristics. Scientists show these relationships using a phylogenetic (fy loh juh NEH tihk) tree, also called a phylogeny (fy LAH juh nee). A phylogenetic tree is a branched diagram that shows the relationships among organisms, including a common ancestor. 

Parts of a Phylogeny
Learning to read a phylogenetic tree will allow you to understand how organisms are related to one another through evolutionary time. The first step is learning the parts of the phylogenetic tree.

Understanding Phylogeny
After learning the parts of the phylogenetic tree, you can observe how the branches of the tree relate. Patterns of the branching indicate whether organisms are closely related or not. For example, organisms that are separated by only one node, or common ancestor, are more closely related than those separated by several nodes.

Reading a Phylogeny

Notice that the snow leopard is on its own branch in the phylogenetic tree. This means it is the most distantly related to the other organisms in order Carnivora. The vertical branch connecting the Mustalidae and Canidae families indicates that these families are related. The next level on the hierarchy of organization is the genus. Since the red and gray wolves are part of the same genus, Canis, these species share the most similarities in traits. The ancestry on a tree can start from any direction.

Cladograms
A family tree shows the relationships among family members, including common ancestors. Biologists use a similar diagram, called a cladogram. A cladogram is a type of phylogenetic diagram that shows the relationships among organisms based on similar characteristics. A cladogram has a series of branches. Each branch follows a new characteristic or trait. Each trait is observed in all the species that come after it.

Reading a Cladogram
All the animals observed in the cladogram have vertebrae, which appeared first in fish. All have four limbs, except for the fish. Fur appears later in evolutionary time for just the rodents and primates. Those with more similar traits are more closely related. Therefore, rodents are more closely related to primates than to a bird o fish.