Lesson Video: Systems of Classification Biology

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


Video Transcript

In this video, we’ll learn about the science of classifications and how it’s changed over time. We’ll specifically look at the kingdoms of life and the influential work of two scientists. We will describe the five kingdoms outlined by Robert Whittaker. Then, we’ll talk about the limitations of the system of kingdoms and how that’s being addressed.

Scientists have long thought about classifying the objects in the world around them. Classifying things makes them easier to study. When organisms are separated into groups based on meaningful similarities, we’re then able to more easily compare and contrast their characteristics. As early as 300 years before the common era, Aristotle published works separating living things into two groups; those groups were plants and animals. This work was later cemented in the 18th century by a scientist named Carl Linnaeus, a Swedish botanist who called these groups kingdoms.

Linnaeus is famous for developing binomial nomenclature. Binomial nomenclature is the scientific system that we use for naming organisms by their genus and their species. He is also credited with standardizing the hierarchical system of classifications, or taxonomy, in which organisms are divided into increasingly specific groups. These groups belong to taxonomic levels, which are ranked from largest to smallest. The most general of these levels in the Linnael system is called kingdom. Linnaeus divided all of life into two kingdoms, which he called animal and vegetable. Everything nonliving he placed on a separate branch of the same tree and called mineral.

At around the same time, improvements and widespread use of the microscope were opening a whole new world of discovery. As more microscopic organisms were studied and understood, the line between the plants and the animal kingdom became blurred. Eventually, a third kingdom called protist was proposed to encompass all the various microscopic unicellular organisms. Advances in microscopy also led to differences between organisms at the cellular level, becoming a parent. A separate kingdom, monera, was created to include all unicellular organisms that did not possess a true nucleus.

Finally, in 1969 another botanist named Robert Whittaker separated stationary autotrophic plants from stationary heterotrophic fungi, increasing the number of kingdoms to five. At the time of the development of the five-kingdom system, scientists were still classifying organisms based on observable characteristics, a method that we now refer to as artificial classification. Advances in genetic technology allow us to classify organisms based on evolutionary relationships, a system that we call natural or phylogenetic classification. One notable shortcoming of the five-kingdom method is that it doesn’t possess a category for the classifications of noncellular life.

The term noncellular life generally refers to objects that are not made of cells but possess some of the characteristics of living things. These items include viruses, thyroids, and prions. These are objects that are made of biomolecules and are able to reproduce, but they’re not considered to be alive because they’re not made of cells. Where do they belong on our tree of life? And do they belong there at all? It’s a question that scientists are still answering today. Next, let’s try a practice question.

Which of the following cannot be classified into a kingdom in Whittaker’s system of classification? (A) Humans, (B) bacteria, (C) yeast, (D) prions, or (E) archaea.

This question is asking us to recall Whittaker’s system of classification, which included five distinct kingdoms, and to choose the answer option that is not able to be placed into one of those five kingdoms. The five kingdoms described in Whittaker’s system are monera, protist, plant, fungi, and animal. All five of these kingdoms have in common that they’re made of living organisms that consists of at least one cell. Members of the monera kingdom are prokaryotes; it’s single-celled organisms that do not possess a nucleus. The protist kingdom was devised to encompass microscopic life that is neither plant fungus nor animal.

The plant kingdom generally includes all multicellular autotrophs, which are separate from heterotrophic fungi that do not photosynthesize. And multicellular heterotrophs that generally move about are considered animals. Now let’s take a closer look at our five answer choices. Humans are complex multicellular organisms that have the ability to move about. We are considered animals. Both archaea and bacteria are single-celled organisms that do not possess a nucleus. They possess some physical differences and live in different environments, but they’re both classified as monera.

Yeast are unicellular organisms that do not move around on their own and absorb their nutrition from their surroundings. They also have a cell wall which contains a material called chitin. These traits classify yeast as a member of the kingdom fungi. Since each of the organisms we’ve identified already can be classified in one of the five kingdoms and we’re looking for the one organism which cannot, we could conclude at this point that the correct answer is prions. But let’s take a closer look at what a prion is. A prion is an abnormally folded protein which has the ability to interact with normal proteins and cause them to become abnormally folded as well. Since prions are proteins, they’re not made of cells, which means that prions cannot be classified in Whittaker’s five-kingdom system.

Let’s try one more practice question.

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

In order to answer this question, we’ll first recall that the classification of organisms is a science that’s existed since ancient times. Scientists today use genetic information to classify organisms based on evolutionary relationships. But as early as Aristotle, scientists were grouping organisms together based on observable characteristics. Today, we often refer to the five-kingdom system of classification developed by Robert Whittaker. His kingdoms of life included monera, protist, plant, fungi, and animal.

But Carl Linnaeus is often credited with coining the term kingdoms for these types of groups. He classified all things, living and nonliving, into three kingdoms: animal, vegetable, and mineral. But our question is asking us specifically about how Linnaeus classified organisms or living things. Linnaeus’s two kingdoms for organisms were the animal kingdom and the plant kingdom.

Next, we’ll wrap up our lesson by reviewing what we’ve learned. In this video, we learned about the classification of life into kingdoms by two scientists, Carl Linnaeus and Robert Whittaker. We learned about Linnaeus’s earlier system of two kingdoms of life, which include animal and vegetable or plant, and then about how technological advances gave rise to three more kingdoms and Whittaker’s five-kingdom system, which includes animal, plant, protists, fungi, and monera.

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