Biological Classification Systems: Understanding Life’s Organization

Posted on Sep 6, 2025 in Biology

The Science of Taxonomy and Biological Classification

As you have discovered, living things are put into groups according to their characteristics. This is called classification. The characteristics used to classify living things determine the groups they are placed into. The chosen characteristics form a classification system.

About half a million different kinds of living things have been discovered so far, with more organisms added to this list each year. Scientists use a classification system to keep track of these different types of living things. The science of classifying living things is called taxonomy. Scientists who classify living things are called taxonomists. Classification allows scientists to organize information about living things and to help identify newly discovered organisms.

Linnaeus’s Classification System

The classification system scientists use today was created in the 1700s by a scientist named Linnaeus. Linnaeus was the first taxonomist. His system has been changed and refined through the years by other taxonomists. This system begins by dividing all living things into large groups called kingdoms. Each kingdom is then divided into smaller and smaller groups. Altogether, there are seven major classification groups. Kingdoms are the first of these groups.

The number of different kinds of organisms in each group decreases when moving from the kingdom group to each of the next levels. The organisms in each group share more and more characteristics in common as they are classified into each level in the classification system. From largest to smallest, the classification groups are:

• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species

A species is the smallest classification group. All organisms in a species share the same physical characteristics and can reproduce among themselves.

Binomial Nomenclature: Naming Organisms

Linnaeus used the descriptions of organisms he recorded to place them in groups based on their observable features. He also used his observations to devise a naming system for organisms. In Linnaeus’s system, called binomial nomenclature, each organism is given a two-part name.

Genus and Species Names

The first part of an organism’s scientific name is its genus. For example, pumas, ocelots, and house cats are all classified in the genus Felis. Organisms classified in the genus Felis share characteristics such as sharp, retractable claws and behaviors such as hunting other animals.

The second part of an organism’s scientific name is its species name. A species name sets one species in a genus apart from another. The species name often describes a particular characteristic or feature of an organism, such as where it lives or its color. For example, the scientific name for many pumas is Felis concolor. Concolor means “the same color” in Latin. The scientific name for some ocelots is Felis pardalis. The word pardalis means “spotted like a panther” in Latin. The scientific name for house cats is Felis domesticus. The species domesticus means “of the house” in Latin.

Notice that both the genus and species names are Latin words. Linnaeus used Latin words in his naming because Latin was the language scientists communicated in during that time. Notice also that the complete scientific name is written in italics. The genus is always capitalized, while the species begins with a lowercase letter.

Below you will find the scientific names for some common organisms:

The Six Kingdoms of Life

When Linnaeus developed his system of classification, there were two kingdoms: plant and animal. However, the use of the microscope led to the discovery of new organisms and the identification of differences among cells. A two-kingdom system was no longer useful. Today, the system of classification includes six kingdoms: Archaebacteria, Eubacteria, Protists, Fungi, Plants, and Animals.

Archaebacteria (ahr-kee-bak-TEER-ee-uh)

Archaebacteria, which means “ancient bacteria,” are unicellular organisms. Unicellular means being comprised of only one cell. Archaebacteria can be either autotrophic (capable of making their own food) or heterotrophic (not capable of making their own food). Archaebacteria are prokaryotes (prō-kăr‘ē-ōts’), which are organisms whose cells lack a nucleus. Until recently, archaebacteria were classified in the kingdom Monera.

Archaebacteria existed on Earth billions of years before the arrival of dinosaurs. Many archaebacteria live in extreme conditions. They can be found in places like hot springs in water temperatures as hot as 110°C. Others can be found in environments that are as acidic as lemon juice. Some archaebacteria can be found in salty waters such as Utah’s Great Salt Lake. Archaebacteria can also live in the intestines of animals, the mud at the bottom of swamps, and in sewage. It is these bacteria that produce the foul odors you may associate with these places.

Eubacteria (yoo-bak-TEER-ee-uh)

Eubacteria, until recently, were also part of the kingdom Monera. However, too many differences were found between the archaebacteria and the eubacteria, so they were each given their own kingdom. Eubacteria, like archaebacteria, are unicellular prokaryotes. And like archaebacteria, some are autotrophs and some are heterotrophs. The chemical makeup of eubacteria is different from archaebacteria, thus classifying them into their own kingdom.

Unlike archaebacteria, eubacteria do not live in extreme environments. However, they do live everywhere else. For example, millions of eubacteria live on and in your body. Eubacteria coat your skin and swarm in your nose. Most of them are either useful or harmless to you. Eubacteria are responsible for yogurt, as well as strep throat. Most eubacteria, however, are helpful. Some produce foods like yogurt, some produce vitamins, and some recycle essential chemicals such as nitrogen.

Protista (pro-TEE-stuh)

The Protist kingdom is sometimes called the “odds and ends” kingdom because its members are so different from one another. For example, some protists are autotrophs, while others are heterotrophs. Also, though most protists are unicellular, some, such as the organisms commonly called seaweeds, are multicellular.

Unlike bacteria, however, protists are eukaryotes (yū-kăr‘ē-ōts), organisms with cells that contain nuclei. In addition to being eukaryotes, all protists live in moist surroundings.

Because of the great variety of protists, they are generally grouped into three categories:

• Animal-like protists (Protozoans): These are heterotrophs, and most are able to move around and obtain food. Unlike animals, protozoans are unicellular. Many scientists distinguish different types of protozoans based on their movement and lifestyle.
  • Some move and feed by forming pseudopods – temporary bulges of the cell membrane that fill with cytoplasm. The word pseudopod means “false foot.” Organisms in this group include the amoeba.
  • The second type has structures called cilia. These hair-like projections move in a wavelike pattern that propels the organism. Paramecia belong in this cilia group.
  • The third group is protozoans with flagella. Flagella are long and whip-like and help in movement and obtaining food.
  • The last group, called sporozoans, is classified more by their lifestyle than movement. These organisms are parasites that feed on the cells and body fluids of their hosts.
• Fungus-like protists: These are like animals in the sense that they are heterotrophs. However, like fungi, they have a cell wall and use spores to reproduce.
• Plant-like protists: These are like plants in the sense that they are autotrophs. Plant-like protists are commonly called algae and are even more varied than animal-like or fungus-like protists.

Fungi (FUN-ji)

Most fungi share three important characteristics: they are eukaryotes, use spores to reproduce, and are heterotrophs that feed in a similar way. Except for yeasts, which are unicellular, fungi cells are arranged in structures called hyphae. Hyphae are the branching, threadlike tubes that make up the bodies of multicellular fungi. The appearance of a fungus depends on the arrangement of its hyphae. Fungi include common bread mold, mushrooms, yeast, and even Penicillium notatum, which produces the penicillin antibiotic.

Plantae (PLAN-tay)

Plants are autotrophs that produce their own food. In addition, all plants are eukaryotes that contain many cells. Unlike other eukaryotes, however, plant cells have a cell wall. The cell wall is a boundary that surrounds the cell membrane and separates it from the environment. Plants are multicellular, with the same type of cells grouped into tissues.

Animalia (an-uh-MALE-ee-uh)

Animals are multicelled organisms. The cells of most animals are grouped together to form different kinds of tissue. Tissue may then combine to form an organ, which is a group of different tissues that work together to perform a specific job that is more complex than the function of each tissue by itself. Many organs then combine to form an organ system, such as the skeletal system. Most animals have highly developed brains and body systems. All animals are heterotrophs and typically reproduce sexually. All animals require food, water, and oxygen to stay alive.

The Importance of Classification

All living things can be classified into one of these six kingdoms. Modern biologists classify organisms into the seven levels of the classification system, starting with kingdom, the broadest. Scientists use taxonomic keys to identify organisms. A taxonomic key is a series of paired statements that describe the physical characteristics of different organisms.

You can probably tell that taxonomy can be very complicated. It is not easy to classify every living thing. However, it is very helpful and important to scientists who study the living world. Classification helps scientists learn all there is to know about plants and animals. It helps us understand the world around us and allows us to discover and identify new organisms.