OLE 6.12D Taxonomic Classification: Classifying Organisms
How do Biologists Classify Organisms?
Classification is the process of organizing and grouping things. The scientific study of how organisms are
classified is called taxonomy. Taxonomy is useful because once a scientist knows the classification of an
organism, he or she/the scientist already knows a lot of information about that organism. Biologists initially
grouped organisms based on their similarities. Modern classification systems group organisms according to
how they are related to each other.
Naming Organisms Taxonomy involves naming organisms. In the 1730s, the Swedish botanist Carolus
Linnaeus devised a system of naming organisms that is still used today. Linnaeus placed organisms in groups
based on similarities in their observable features. Each organism was given a unique, two-part scientific name.
This system is called binomial nomenclature.
The first word in an organism’s scientific name is its genus. A genus (plural genera) is a classification grouping
that contains similar, closely related organisms. For example, pumas and house cats are both classified in the
genus Felis. Organisms in the genus Felis share characteristics such as sharp, retractable claws, and behaviors
such as hunting other animals.
The second word in a scientific name often describes a distinctive feature of an organism, such as where it
lives or its appearance. Together, the two words form the scientific name of an organism. All organisms with
the same scientific name belong to the same species. A species (SPEE sheez) is a group of similar organisms that
can mate with each other and produce offspring that can also mate and reproduce.
What Are the Levels of Classification?
The classification system that scientists use today is based on
the contributions of Linnaeus. But today’s classification
system uses a series of many levels to classify organisms.
The Major Levels of Classification Organisms are classified
by their characteristics and relatedness. The diagram below
shows the levels most biologists use. First, an organism is
placed in a broad group, which in turn is divided into more
specific groups.
A domain is the highest level of organization. Within a
domain, there are kingdoms. Within kingdoms, there are
phyla (singular phylum). Within phyla are classes. Within
classes are orders. Within orders are families. Each family
contains one or more genera. Finally, each genus contains
one or more species. The more classification levels two
organisms share, the more closely related they are.
How are Organisms Classified into Domains?
Today, biologists commonly use a three-domain system to classify organisms. The three domains are Bacteria,
Archaea, and Eukarya. Organisms are placed into domains based on their cell type and their chemical
composition.
Domain Bacteria Members of the domain Bacteria are prokaryotes. Prokaryotic cells have no nuclei. All
prokaryotes are single-celled organisms. Some bacteria are autotrophs (they make their own food), while
others are heterotrophs (they eat other organisms or the remains of organisms). Bacteria usually reproduce
asexually through a process called binary fission, in which one cell divides to form two identical cells.
Domain Archaea Members of the domain Archaea, like bacteria, are unicellular prokaryotes that often
reproduce through binary fission. And like bacteria, some archaea are autotrophs and others are
heterotrophs. Archaea are classified in their own domain because their chemical makeup differs from that of
bacteria. Bacteria and archaea also differ in the structure of their cells. Some archaea are found in harsh,
extreme environments. These harsh places include vents deep under the ocean, oil wells, and hot springs,
where the temperature can be above 80°C.
Domain Eukarya Organisms in the domain Eukarya are eukaryotes, organisms with cells that contain nuclei.
Within this domain, organisms have many different characteristics.
How are Organisms Classified into Kingdoms?
The domains Bacteria and Archaea represent two kingdoms, Bacteria and Archaea. The domain Eukarya,
however, contains four kingdoms: protists, fungi, plants, and animals. Organisms are placed into kingdoms
based on characteristics such as their ability to make food, the number of cells in their bodies, and how they
reproduce.
Protists A protist is any eukaryotic organism that cannot be classified as a fungus, plant, or animal. Because its
members are so different from one another, the protist kingdom is sometimes called the “odds and ends”
kingdom. For example, some protists are autotrophs, while others are heterotrophs. Most protists are
unicellular, but some, such as seaweeds, are multicellular. Protists can reproduce sexually, but most
reproduce asexually.
Fungi Mushrooms, molds, and mildew are all fungi. The majority of fungi are multicellular, but a
few, such as the yeast used in baking, are unicellular. Fungi are found almost everywhere on
land, but only a few live in fresh water. All fungi are heterotrophs. Most fungi feed by absorbing
nutrients from dead or decaying organisms. Fungi reproduce both asexually and sexually.
Plants Plants are all multicellular eukaryotes, and most live on land. Also, plants are autotrophs that make
their own food. Plants do not move from place to place on their own. Their life cycles include both asexual and
sexual reproduction.
Animals All animals are multicellular eukaryotes. In addition, all animals are heterotrophs. Animals have
different adaptations that allow them to locate, capture, eat, and digest food. Members of the animal
kingdom live in diverse environments throughout Earth. Most animals can move from place to place for at
least part of their life cycle. Most animals reproduce sexually, though some, such as sponges, jellyfish, and
worms, can reproduce asexually.
The table below summarizes the characteristics of the major groups of organisms.
DOMAIN
Bacteria
Archaea
Eukarya
KINGDOM
Bacteria
Archaea
Protists
Fungi
Plants
Animals
CELL TYPE
Prokaryotic
Prokaryotic
Eukaryotic
Eukaryotic
Eukaryotic
Eukaryotic
NUMBER OF
Unicellular
Unicellular
Mostly
Mostly
Multicellular Multicellular
CELLS
unicellular
multicellular
FOOD SOURCE Autotroph or Autotroph or Autotroph or Heterotroph Autotroph
Heterotroph
heterotroph heterotroph heterotroph
REPRODUCTION Asexual
Asexual
Most asexual Asexual
Asexual and Most sexual,
and sexual
sexual
some asexual
1. What are the two parts of the scientific name of an organism? ____________________________________
2. What is the broadest taxonomic classification of living organisms? _____________________
3. What is the primary characteristic that divides the domain Eukarya from the other two domains? ________
_______________________________________________________________________________________
4. How are plants and animals alike? ___________________________________________________________
5. List four characteristics that are used to classify organisms into kingdoms. ___________________________
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6. How are bacteria and archaea similar? How are they different? ____________________________________
_______________________________________________________________________________________
7. Identify the mode of reproduction of organisms that are classified in the following kingdoms or domains:
Bacteria - _____________
Protists - ______________
Plants - _______________
Archaea - _____________
Fungi - ________________
Animals - ______________
8. Complete the chart to identify which kingdoms contain prokaryotic organisms and which contain
eukaryotic organisms.
Prokaryotes
Eukaryotes
9. List the levels of classification of living things, starting with the broadest taxonomic classification.
_______________________________________________________________ ________
10. Complete the Venn diagram to identify which kingdoms contain autotrophic and heterotrophic
organisms.
11. Identify which kingdom or kingdoms include unicellular organisms. Which kingdom or kingdoms include
multicellular organisms?
Unicellular kingdoms: ____________________________________________________________________
Multicellular kingdoms: ___________________________________________________________________
12. Your friend finds a new organism and lists its basic characteristics. Use your friend’s list to classify the
organism in a currently recognized kingdom. _______________________________

It has nuclei in its cells.

It has more than one cell.

It cannot make its own food.

It can reproduce both asexually and sexually.

It absorbs nutrients from dead organisms.
13. Many biologists think that protists do not really form a kingdom of their own, but rather represent
members of several different groups. Why might biologists think this? Do you agree or disagree? Why?
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