SECTION
17.4 Domains and Kingdoms
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KEY CONCEPT The current tree of life has three domains.
Classification is always a work in progress.
The tree of life is a model that shows the most current understanding of
how all living things are related. Over time, the organization of the tree
of life has changed. With new findings and new discoveries, scientists
Connecting CONCEPTS
changed how organisms are classified.
Classification Recall from
Until the mid-1800s, all living things were divided into two kingSection 17.1 that the kingdom
doms: Plantae (plants) and Animalia (animals). Then, a third kingis the largest category in the
Linnaean classification system.
dom, called Protista, was added for single-celled organisms. A fourth
kingdom, called Monera, was added in the early 1900s to give prokaryotes a kingdom of their own. In the mid-1900s, kingdom Fungi was
added for organisms such as mushrooms and molds. In 1977, kingdom
Monera was divided into two groups of prokaryotes—Bacteria and
Archaea. Each of these changes was based on new scientific findings.
HISTORY OF THE KINGDOM SYSTEM
1753 Two kingdoms
Plantae
Animalia
1866 Three kingdoms
1938 Four kingdoms
Plantae
Animalia
1959 Five kingdoms
Protista
Plantae
Plantae
Plantae
Animalia
Animalia
Animalia
1977 Six kingdoms
Protista
Protista
Protista
Archaea
Monera
Monera
Fungi
Fungi
Bacteria
Genetic studies show that the two groups of prokaryotes, bacteria and
archaea, are very different from each other. In terms of genes, they are
more different from each other than animals are from plants.
In which of the six kingdoms are humans
categorized?
Prokaryote
Eukaryote
The three domains in the tree of life are
Bacteria, Archaea, and Eukarya.
The six kingdoms can be divided into three domains. A
domain is a grouping larger than a kingdom. Prokaryotes
are divided into two domains: Bacteria and Archaea. All
eukaryotes are placed into a third domain, called Eukarya.
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A prokaryotic cell does not have a
membrane-bound nucleus or organelles. A eukaryotic cell has a membrane-bound nucleus and organelles.
TREE OF LIFE
The most recent classification system divides life into
three domains, which include six kingdoms.
Domain: Bacteria
Domain: Archaea
Kingdom:
Plantae
Domain: Eukarya
Kingdom:
Fungi
Kingdom: Protista
Kingdom: Archaea
Kingdom:
Animalia
Kingdom: Bacteria
Scientists constructed this evolutionary tree by comparing rRNA sequences from
species in each of the six kingdoms. The distances between branches represent the
number of differences in rRNA sequences among these species.
Source: C. Woese, PNAS 97:15.
• Bacteria The domain Bacteria includes the single-celled prokaryotes
in the kingdom Bacteria. This domain is one of the largest groups of
organisms on Earth. In fact, there are more bacteria in your mouth
than there are people that have ever lived! Bacteria are often classified
by their shape or whether they cause disease.
• Archaea The domain Archaea (ahr-KEE-uh) contains single-celled
prokaryotes in the kingdom Archaea. Archaea and bacteria are both
single-celled prokaryotes, but the two groups of organisms have
many differences. For example, they have very different cell wall
chemistry. Archaea are often classified for the extreme environments
they can live in, such as deep sea vents and hot geysers.
• Eukarya The domain Eukarya (yoo-KAR-ee-uh) is made up of all
organisms with eukaryotic cells. Some eukaryotes are single-celled,
like most protists. Other eukaryotes are multicellular, like you. The
domain Eukarya includes the kingdoms Protista, Plantae, Fungi,
and Animalia.
Interactive Reader
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Notice that protists, plants, fungi, and animals are all grouped in the
same domain. But bacteria and archaea are in different domains. Scientists have agreed on this grouping because it clearly represents the diversity of prokaryotes. This grouping reflects the fact that protists, plants,
fungi, and animals are more closely related to each other than are bacteria and archaea.
on domains
Earth would
you find the greatest variety
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What are Where
the three
of life?
8=:8@
of species?
17.4
Bacteria
Archaea
Vocabulary Check
Eukarya
Mark It Up
Go back and highlight
each sentence that
has a vocabulary
word in bold.
1. List the kingdom(s) that belong to each domain in the chart below.
DOMAIN
BACTERIA
ARCHAEA
EUKARYA
Kingdom(s)
17.4
The Big Picture
2. Why has the classification of different groups of living things changed
over the years?
3. How is a kingdom different from a domain?
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