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.