Lesson Explainer: Systems of Classification Biology

In this explainer, we will learn how to describe the classification systems proposed by Linnaeus and Whittaker and recall organisms that are difficult to classify.

As early as the 4th century BCE, Aristotle published works separating living things into two groups: plants and animals. Scientists have been thinking about classifying organisms for a long time as this makes them easier to study. When organisms are separated into groups based on meaningful similarities, we are able to easily compare and contrast their characteristics.

Definition: Biological Classification

Biological classification is the grouping of organisms based on meaningful similarities.

The basis for our current system of classification was later cemented in the 18th century by the work of Carl Linnaeus, a Swedish botanist. He divided life into the same two groups: vegetables, which we now call “plants,” and animals. Linnaeus called these groups “kingdoms.” In Linnaeus’s system, a third kingdom, called “Minerals,” encompasses all nonliving things. Figure 1 shows the kingdoms proposed by Linnaeus.

Fact: Carl Linnaeus (1707–1778)

Carl Linnaeus was a Swedish botanist who is credited with the development of the system of binomial nomenclature as well as popularizing the system of taxonomic hierarchy.

Key Term: Kingdom Plantae (Plant Kingdom)

The plant kingdom is one of the two kingdoms of life proposed by Linnaeus. The plant kingdom included immobile organisms that grew from the ground.

Key Term: Kingdom Animalia (Animal Kingdom)

The animal kingdom is one of the two kingdoms of life proposed by Linnaeus. The animal kingdom included mobile organisms that moved around to seek food.

Example 1: Recalling Linnaeus’s Classification of Organisms

Carl Linnaeus classified organisms into two distinct groups. What were these groups?

Answer

Carl Linnaeus is often referred to as “the father of modern taxonomy.” In the 1700s, this Swedish botanist both developed the system of binomial nomenclature and popularized the taxonomic hierarchy that is still widely used today. However, at the time of these advancements, the microscope was still not widely used and not nearly as powerful as today’s models. Linnaeus based his kingdoms on observable characteristics and the work of popular thinkers before him, such as Aristotle. Linnaeus classified everything on Earth into three kingdoms: animal, vegetable, and mineral. Of those, only two groups contain living organisms.

The two groups are the animal kingdom and the plant kingdom.

Besides defining his two kingdoms of life, Linnaeus also developed the hierarchical system of classification in which organisms are divided into increasingly specific groups. These groups are called “taxa,” and they belong to taxonomic levels that are ranked from large and general to small and specific. The kingdom is the most general of these levels in the Linnaean system. The rest of the taxonomic ranks, from largest to smallest, are phylum, class, order, family, genus, and species. Figure 2 illustrates the taxonomic ranks and their order.

Around this time, improvements in the technology of the microscope, along with its increasing use, began opening a new world of discovery. As microscopic organisms were studied and understood, the line between animal and plant became blurred. A third kingdom, Protista, was proposed to encompass all the various microscopic, unicellular organisms that did not seem to be either plants or animals. The micrograph below shows some Euglena, which are unicellular organisms and members of the protist kingdom.

Euglena spirogyra

Figure3

Key Term: Kingdom Protista (Protist Kingdom)

The protist kingdom was originally proposed to encompass all microscopic, unicellular life that was neither plant nor animal.

Advances in microscopy also led to differences between organisms at the cellular level becoming apparent. Scientists were getting their first view of subcellular structures and organelles, as well as smaller unicellular organisms like bacteria. These tiny organisms do not possess a nucleus, unlike the cells of plants, animals, and protists. As a result, a separate kingdom, Monera, was created to account for these organisms, which are called “prokaryotes.” The micrograph below shows some bacteria viewed through an electron microscope, which are examples of prokaryotic organisms. Organisms made of cells that possess a nucleus are called “eukaryotes.”

Streptococcus pyogenes infection

Figure4

Definition: Eukaryotic Organism

A eukaryotic organism is an organism whose cells possess a nucleus. Eukaryotic organisms may be unicellular or multicellular.

Definition: Prokaryotic Organism

A prokaryotic organism is an organism that does not possess a nucleus. Prokaryotic organisms are all unicellular.

Key Term: Kingdom Monera (Bacteria/Prokaryotic Kingdom)

Kingdom Monera is one of the 5 kingdoms proposed by Whittaker. It includes all prokaryotic organisms.

Almost two centuries later, in 1975, another botanist named Robert Whittaker noticed a division in the plant kingdom. At the time, anything that grew from the ground was considered a plant. But some of these organisms are green and make their own nutrients from sunlight, and some are not green and absorb their nutrients from dead and decaying organic matter. Whittaker separated stationary autotrophic plants from stationary heterotrophic fungi, increasing the number of kingdoms to five. Figure 7 shows the 5-kingdom classification system that was put forward by Whittaker.

Fact: Robert Whittaker (1920–1980)

Robert Whittaker was an American botanist who is famous for proposing the five kingdoms of life that are still commonly referred to today. These kingdoms are Monera, Protista, Fungi, Plantae, and Animalia.

Key Term: Kingdom Fungi (Fungus Kingdom)

The fungi kingdom includes immobile, heterotrophic organisms like mushrooms, mold, and yeast.

Definition: Autotroph

An autotroph is an organism that is able to synthesize its own food from inorganic materials.

Definition: Heterotroph

A heterotroph is an organism that obtains food from consuming other organisms or organic matter.

Example 2: Recalling Whittaker’s 5 Kingdoms

Robert H. Whittaker classified organisms into 5 kingdoms. What were these kingdoms?

Answer

Robert Whittaker was an American botanist of the 20th century. In comparison to Carl Linnaeus, who divided life into two kingdoms, plant and animal, in the 1700s, Whittaker had access to much more technology and biological knowledge. By the 1900s, microscopes had become much more advanced and widely used. Using microscopes, scientists had discovered the protist kingdom of microscopic organisms that seemed to be neither plant nor animal. More microscopic power revealed tiny bacteria that did not possess a nucleus surrounding their DNA, so the monera kingdom was devised to encompass all these prokaryotic organisms. Finally, Whittaker observed that the cells of plants contained chloroplasts and were autotrophic, while the cells of fungi did not. Fungi are heterotrophs that must absorb nutrients from their surroundings. The separation of the fungi from the plant kingdom gave rise to Whittaker’s 5 kingdoms.

The 5 kingdoms that Whittaker classified organisms into are Monera, Protista, Fungi, Plantae, Animalia.

Example 3: Contrasting Whittaker’s and Linnaeus’s Systems of Classification

Which of the following correctly compares Linnaeus’s and Whittaker’s systems of classification?

  1. Whittaker had more biological and technological knowledge when creating his system of classification.
  2. Linnaeus had more biological and technological knowledge when creating his system of classification.

Answer

Carl Linnaeus and Robert Whittaker are both botanists famous for their contributions to the study of taxonomy. Linnaeus lived in Sweden in the 1700s. He helped popularize the taxonomic hierarchy and developed the system of binomial nomenclature, both of which are still widely used today. At the time of Linnaeus, there were only two commonly accepted kingdoms of life: plant and animal. Robert Whittaker was a 20th-century American scientist who proposed and famously popularized the use of the 5 kingdoms: plant, animal, monera, fungi, and protist. As microscopes became more powerful and more common, scientists were able to distinguish differences between organisms at the cellular level. The advancement from two kingdoms to five kingdoms was made possible by the widespread use of more advanced microscopes.

That means that Whittaker had more biological and technological knowledge when creating his system of classification.

These five kingdoms were developed by separating organisms based on the variations in their observable, physical characteristics. Even though what we were able to observe has improved over time, this method of classification is considered to be obsolete. We call classification based on physical traits “artificial classification.”

Definition: Artificial Classification

Artificial classification is the classification of organisms based only on their physical characteristics.

Just like the microscope changed our definitions of the kingdoms of life centuries ago, genetic technology continues to improve systems of classification today. Genetic analysis has allowed us to group organisms based on their evolutionary relationships, not just physical traits. We call this type of classification “phylogenetic classification”, or “natural classification.”

Definition: Natural (Phylogenetic) Classification

Natural classification is the classification of organisms based on genetic and evolutionary relationships.

It is important to remember that classification is an evolving discipline. Taxonomists are always looking for ways to improve it and make groupings more accurate.

Today, most scientists agree that there is a need for a level larger than kingdom. We call it “empire” or “domain.” Many modern classification systems involve two or three domains that are based on cell type. There is no universally agreed-upon number of kingdoms. Some scientists support removing this taxonomic rank altogether, some have proposed as few as three, and others have proposed more than eight different kingdoms. The 5-kingdom system is still referred to widely throughout the scientific community.

An interesting question of classification has to do with what we sometimes call noncellular life, or acellular life. How do we classify items that possess some of the characteristics of living things but are not made of cells?

A common example is a virus, which is made of proteins and genetic material, but cannot reproduce on its own. Another example is a viroid, which consists of genetic material without a protein coating. Since viruses and viroids both possess genetic material and the ability to make new viruses or viroids in living cells, they are sometimes referred to as acellular life-forms. Figure 7 shows the life cycle of a virus. A prion is an abnormally formed protein that can cause other proteins to adopt the same abnormal shape. Prions are sometimes considered difficult to classify because they seem to possess one of the characteristics common to living things, having the ability to reproduce in a fashion, as shown in Figure 6. However, prions are not considered to be acellular life-forms. Viruses, viroids, and prions are known for causing disease. In humans, viruses cause the common cold, flu, and many other illnesses. Creutzfeldt–Jakob disease (CJD) is a neurological disorder caused by prions. Viroids are only known to cause diseases in plants.

Definition: Virus

A virus is an acellular life-form that uses a living host cell in order to replicate and produce more viruses.

Definition: Prion

A prion is a misfolded protein that has the ability to cause properly folded proteins to become misfolded as well, thus replicating itself.

“Noncellular life” may seem like a contradiction. Are not all living things made of one or more cells? Since these acellular “life-forms” are made of biological molecules and exist by interacting with cellular life, there is a strong case to expand the taxonomic system to include them in some way, even though they are not generally considered to be “living.” If you were a taxonomist, how would you do it? The answer to this question may be the next big breakthrough in the study of biological classification.

Example 4: Explaining Why It Is Difficult to Classify Viruses

Why is it difficult to classify viruses into Whittaker’s system of classification?

Answer

Whittaker’s 5 kingdoms are protist, plant, fungus, animal, and monera. These kingdoms only include cellular life. Organisms in the Monera kingdom are made of prokaryotic cells and organisms in the other four kingdoms are made of eukaryotic cells. Scientists generally consider something to be alive only if it consists of one or more cells. A virus is basically a sac of proteins that surround a collection of genetic material. While organisms, which we consider to be living things, are made of cells, have a metabolism, and possess other characteristics, viruses possess none of these things. A virus replicates by inserting its genetic material into a living cell, which forces the cell to make more viruses. Viruses need cells to replicate, although they are not made of cells themselves.

It is difficult to classify viruses into Whittaker’s system of classification because they are generally considered to be nonliving.

Let’s review what we have covered in this explainer.

Key Points

  • The two kingdoms of life established by Carl Linnaeus are animal and plant.
  • The five kingdoms of life established by Robert Whittaker are animal, plant, protist, monera, and fungus.
  • Taxonomy is a constantly evolving and changing field shaped by advances in knowledge and technology.
  • The current systems of classification do not include acellular life-forms such as viruses and viroids.

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