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Taxonomy
Classifying and naming organisms is a practice that dates back to
ancient Greece. Aristotle was one of the first to group and
categorize living things based on their characteristics. After
Aristotle, scientists and academics continued his work by adding to
classification systems and creating new ones as discoveries were
made. By the 18th century, there were multiple classification
systems in place, each with its own way of categorizing and
naming species. In order for collaboration in the scientific
community to advance, changes had to be made. What changes to
classification were necessary? What tools could scientists use to
organize organisms consistently? What characteristics represent
different groups of organisms?
A Standardized System of Classification and Naming
In the past, scientists were unable to properly communicate about living organisms.
Newly discovered species were randomly named. In fact, some may have been discovered
multiple times due to the lack of ability to distinguish the classification systems. A standardized
system of grouping and naming life was necessary in order to allow scientists to communicate and
maintain organization of the wide diversity of life on Earth.
Carolus Linnaeus was an 18th century scientist who focused his studies on plants. However, he is
known best as the father of taxonomy. Taxonomy is a systematic process of classifying living
organisms into different groups based on their physical traits and genetic relationships. Over the
years, Linnaeus’ original system has been modified as new discoveries were made, but the basic
system is still intact.
The groupings of living things begin as broad classifications and become narrower and more
specific as they continue. The highest and broadest level of classification is called the domain. It is
followed by kingdom, phylum, class, order, family, genus, and species. The table below shows the
classification of the domestic dog from domain to species.
Domain
Eukarya
Kingdom Phylum
Animalia Chordata
Class
Order
Family
Mammalia Carnivora Canidae
Genus Species
Canis Familiaris
Organisms are commonly referred to according to the two most specific taxonomic levels: genus
and species, which are often Latin. This is called binomial nomenclature. The taxonomic name of
modern humans is Homo sapiens. The genus is always capitalized, the species is lowercase, and
the whole name is written in italics. By using this same system, scientists around the globe can
freely communicate with certainty that they are referring to the same organisms.
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Taxonomy
Imagine that, before the establishment of taxonomy, a scientist working in
Africa writes a letter to a biologist in England claiming that he has discovered
a new organism. It is unique, and he has never seen one before. He has
named the animal a glotchbot. The British biologist spreads the word about
the glotchbot through the scientific community, and everyone in the
community becomes excited about this new discovery. The scientist returns
from Africa with a picture of the glotchbot shown on the right. Suddenly, the
scientific community loses all interest and both the scientist and the biologist
are disrespected. How would a standard system of taxonomy have changed
the outcome of this scenario?
Career Corner: Taxonomist
Scientists who study taxonomy and use the classification system to identify and name organisms
are taxonomists. Taxonomists are first and foremost scientists. They have a fundamental
knowledge of biology or other related fields. They often have advanced degrees in zoology, animal
physiology, botany, or other life sciences. Museums, zoos, aquariums, and universities are
common places of employment for taxonomists. Here they can study DNA, environments, and
other influences that have contributed to characteristics of life. Taxonomists’ knowledge is often
used to educate others through lectures and publications about conservation of endangered or
threatened species.
Classification into Domains: Bacteria, Archaea, and Eukarya
The three domains differ fundamentally in their cellular structures and
genetic makeup. The domains are so broad that all life can be
separated into just three different categories. Let’s examine the basic
differences between these three categories of life.
Domain Bacteria: This domain consists of unicellular prokaryotes.
They lack a cell nucleus and membrane-bound organelles, but they are
surrounded by a thick cell wall.
Bacteria can be found nearly everywhere on Earth, including living inside human beings’ mouths
and stomachs. Bacteria are incredibly diverse. Some are free-living, while others rely on a host to
survive. Many use oxygen, while others are killed by the presence of oxygen. Like plants, some
bacteria are photosynthetic. Many bacteria cause infections, such as strep throat (Streptococcal
pharyngitis), food poisoning (Escherichia coli and Salmonella enterica), and plant wilt in sweet
corn (Erwinia stewartii). Most bacteria are beneficial and serve a necessary role in their
environment. There are a wide variety of characteristics and functions among the members of
Domain Bacteria.
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Taxonomy
Domain Archaea: Like Domain Bacteria, the members of Archaea
are unicellular prokaryotes. They also have a cell wall, but it differs
in composition from those of bacteria. Archaean cell walls lack the
substance peptidoglycan found in bacteria. Their cell membranes
also differ, containing unusual lipids that are not found in any other
organisms on Earth. (A lipid is a type of biomolecule; fats, oils, and
waxes are examples of lipids.) One of the most distinct features of
Domain Archaea is that they are able to survive in some of the
most extreme environments on Earth. Archaea have been found in
the hot springs of Yellowstone National Park in Wyoming and in
deep oceanic hydrothermal vents measuring over 100°C (212°F).
Others live in environments with extremely high salinity and acidity.
Archaea are thermophiles
because they thrive in
hot environments like this
geothermal pool.
Domain Eukarya: This domain differs from the others because its members‘ cells contain a
nucleus and membrane-bound organelles. Most eukaryotic species are multicellular, but some are
unicellular. Domain Eukarya is quite diverse and contains the most well known organisms.
Eukaryotes are found all over the world in a variety of environments. The domain is so diverse that
it is best to study the organisms of Eukarya in their narrower classification groups.
Suppose a group of scientists discovered a new prokaryotic organism in a highly acidic sulfur vent
in Antarctica. Which domain does the organism most likely belong to?
The Four Kingdoms of Eukaryotes
Domain Eukarya is incredibly diverse. It includes organisms from daffodils to dragonflies and
orangutans to oak trees. It is divided into four kingdoms based on the most general characteristics.
The kingdoms are Protista, Plantae, Fungi, and Animalia.
Each kingdom is further divided into phyla, then classes, orders, and so on. The members of each
kingdom have distinct enough characteristics to allow us to begin identifying organisms.
Protista: These ancient eukaryotes have some characteristics not shared by many other
members of the domain, including the fact that many are unicellular. Even within the kingdom
there is great diversity. In fact, many protists are classified in this kingdom just because they do
not fit in any of the others. They vary greatly in their appearance, mobility, reproduction, and
methods for obtaining food. Some protists are even photosynthetic. Examples include many
phytoplankton, red and brown algae, and dinoflagellates.
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Taxonomy
Plantae: Plants are very common eukaryotes. They include a wide variety of organisms with
unique characteristics and functions, as well. But there are some properties of Kingdom Plantae
that they all share. Plants are multicellular organisms that are able to photosynthesize. Since
plants can use energy from the Sun to produce food, they are considered autotrophs. Plants lack
mobility and often must rely on the wind or animals to help them reproduce through cross
pollination. All plants have the same basic parts, including roots, stems, and leaves. Their cells are
unique from other eukaryotes because they are surrounded by a rigid cell wall made mostly
of cellulose. The cell wall gives plants structure and support, allowing them to grow tall and expose
their green leaves to the Sun for photosynthesis.
Fungi: Fungi, such as mushrooms, are often confused for
plants. They do share some similarities. Most, for example,
are multicellular, although yeasts, a type of fungi, are
unicellular. Like plants, the cells of fungi have a cell wall, which
is usually made of chitin instead of cellulose. Fungi, however,
cannot produce their own food through photosynthesis, so
they are called heterotrophs. This kingdom has some
characteristics that differ from any of the other eukaryotes. A
primary difference is that fungi grow long filaments called
hyphae.
Many fungi feed by releasing enzymes outside of their bodies.
The enzymes break down and digest nearby leaves, fruits,
and other substances. Once digested, the molecules of food
are absorbed into the fungal body. These enzymes are also
important to decomposition. Fungi break down dead, organic
matter and return nutrients to the soil. They help maintain the
balance with organisms like plants that take nutrients from the
soil.
Animalia: The animal kingdom is undoubtedly the most well
known because it includes humans. Like all the kingdoms,
Animalia is quite diverse. In addition to humans, it includes
birds, fish, insects, and a wealth of other animals. What they
all have in common is that animals are multicellular, are
heterotrophic, and have cells lacking a cell wall. Also, animals
are motile at least at some point in their lives. Beyond these
characteristics, animals vary greatly in their body plans,
reproduction, methods for obtaining food, and many other
factors.
A mushroom is actually the
fruiting body, or reproductive
organ, of a fungus.
This sea sponge is in the
same kingdom as humans.
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Taxonomy
Anyone who has visited tide pools or snorkeled in the ocean knows that some animals, such as
sponges, barnacles, and coral are fixed in place. However, these organisms all belong to Kingdom
Animalia, which is characterized by motility. The caveat is that animals are motile at some point in
their lives, but not necessarily their entire lives. Adult sponges and coral, for example, are sessile—
that is, immobile or fixed in one place. However, as zygotes they have cilia allowing movement
through the water to find a preferred location. Barnacles are somewhat similar. They have two
larval stages in which they are able to swim through the water using setae, which are hair-like
bristles used for movement. In the second larval stage, barnacles cannot take in any food, so they
have a limited amount of time to find the best place for their adult form to become fixed in place.
Tools for Classifying Organisms
Two methods scientists rely on to identify and classify organisms are dichotomous keys and
cladograms. These tools help scientists determine how organisms are related through common
ancestry. A dichotomous key is a type of flow chart made up of questions or paired statements
about an organism. Following each of the steps of a dichotomous key helps scientists identify
organisms based on their traits.
Use the dichotomous key below to identify the fish shown on the right.
1. Is the fish’s body long
and thin?
Yes
No
2. Does the fish have
Pointed
pointed or rounded fins? Rounded
3. Are the eyes on top of the Yes
fish’s head?
No
4. Does the fish have a long Long tail
tail or a short tail?
Short tail
5. Does the fish have
Yes
spots?
No
6. Does the fish have
Yes
whiskers?
No
Go to step 2
Go to step 3
Trumpet fish
Moray eel
Go to step 4
Go to step 5
Spotted eagle ray
Witch flounder
Go to step 6
Glassy sweeper
Spotted goat fish
Band-tail puffer
A cladogram is a branched diagram resembling a tree that shows the evolutionary relationship
among organisms. It is often used to show how similarities are derived from common ancestry.
Places where a lineage branches off in a cladogram are called nodes. They represent speciation
events. The fewer the number of nodes between organisms, the more closely they are related.
Cladograms provide scientists with a visual summary of how organisms in any taxonomic grouping
are related.
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Taxonomy
Look at the cladogram below. Which two organisms are more closely related, a hagfish and a
lizard or a pigeon and a chimp? How do you know?
What Do You Know?
The characteristics used to classify organisms into taxonomic groups help scientists identify and
organize living things. With the information gathered, they can construct tools such as cladograms
that show the evolutionary relationships, including common ancestry, of living things.
Fill in the table with information about the three domains.
Domain
Bacteria
Kingdom
Eubacteria Archaebacteria Protista
Plantae
Fungi
Cell type
Eukaryote Eukaryote Eukaryote Eukaryote
Number of
Unicellular
Multicellular
Multicellular
Cells
Presence of
Yes
Yes
cell wall
Mode of Autotroph & Autotroph &
Heterotroph
food intake heterotroph heterotroph
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Taxonomy
Taxonomy in the Real World
To help students learn more about taxonomy, have them visit a zoo, botanical garden, plant
nursery, or even a local pond or stream where they can observe the characteristics of many
different organisms. Have students collect the binomial nomenclature of the species they observe
where possible. These are commonly posted at zoos, botanical gardens, and some nurseries.
Students should gather as much information about the different species’ physical appearances
they encounter as possible. Encourage students to use a camera to take pictures of the different
species or draw pictures of what they see.
At home, have students identify the domain for each of the organisms they observed. Most likely
every organism will belong to Domain Eukarya. Then, have students classify the organisms into
kingdoms, phyla, and so on, as far as they can go with the information they gathered. Next, have
students research the genus and species names for the organisms. Using this information and
online resources, students can check their own classifications against the true taxonomy. Once
complete, have students build either a cladogram or a dichotomous key for organisms they
observed. Encourage students to analyze the cladogram to determine which characteristics
evolved latest and which organisms are most closely related.
Here are some questions to discuss with students:
•  How does a standard system of classification help you do research?
•  What characteristics can you use to classify organisms into domains? Into kingdoms?
•  What role would a cladogram play for a scientist who discovers a fossil of an extinct organism?
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