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TAXONOMY
reflect
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
Class
Order
Family
Animalia Chordata Mammalia Carnivora Canidae
Genus Species
Canis Familiaris
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TAXONOMY
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.
what do you think?
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.
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TAXONOMY
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.
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.
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TAXONOMY
what do you think?
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.
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.
A mushroom is actually the
fruiting body, or reproductive
organ, of a fungus.
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TAXONOMY
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.
This sea sponge is in the
same kingdom as humans.
look out!
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.
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TAXONOMY
what do you think?
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
Yes
the fish’s head?
No
4. Does the fish have a
Long tail
long 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.
what do you think?
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?
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TAXONOMY
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
Kingdom
Cell type
Number of
Cells
Presence
of cell wall
Mode
of food
intake
Bacteria
Eubacteria
Archaebacteria Protista
Plantae
Fungi
Eukaryote Eukaryote Eukaryote Eukaryote
Unicellular
Multicellular
Multicellular
Yes
Autotroph & Autotroph &
heterotroph heterotroph
Yes
Heterotroph
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TAXONOMY
connecting with your child
axonomy 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.
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?
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.
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