1.4
prokaryote a single-celled organism
that does not contain membrane-bound
organelles
eukaryote any organism whose cells
contain organelles; some eukaryotic
organisms are single-celled, while others
are multicellular
BooKMArK
kingdoms and Domains
The Linnaean classification system recognized only two kingdoms of living things—
Animals and Plants. When biologists began to use microscopes, they discovered
many new micro-organisms and gained a better understanding of the cellular structure of living things. Biologists discovered prokaryotes—unicellular organisms, such
as bacteria, that do not have membrane-bound organelles. They also discovered
many new eukaryotes—unicellular and small multicellular organisms with organelles.
These organisms were called protists—their cells had a nucleus and other organelles
but showed an extraordinary diversity of shapes, sizes, and life cycles. Biologists also
began to see evidence of significant differences between plants and fungi.
Today, most biologists classify the more than 1.7 million known living species
into six kingdoms. As you will learn, these six kingdoms are grouped within a higher
taxonomic rank.
Kingdoms of Life
LEARNiNg TIP
Name Change
Archaea used to be called
Archaebacteria, but we now know
that they are distinct from Eubacteria,
the “true bacteria.”
Figure 1 shows a typical representation of the six kingdoms of life. The most easily recognizable kingdoms are the Animals, Plants, and Fungi. These kingdoms include many
large multicellular organisms. The other kingdoms include mostly or entirely microscopic
single-celled, or unicellular, organisms. The eukaryotic Protista is by far the most diverse
group of organisms. Many of these are tiny, single-celled organisms such as amoeba and
paramecium, while others, such as the kelps, are among the largest living organisms.
The Eubacteria, which are commonly called “bacteria,” and Archaea are prokaryotic and
are often extremely difficult to distinguish due to their extremely small size and lack of
internal structure. Most prokaryotes consist of individual cells, but some others consist
of many cells that have joined to form long strands or clustered arrangements.
arthropoda
echinodermata
basidiomycetes
angiosperms
pteridophyta
chordata
mollusca
Kingdom
Animals
ascomycetes
Kingdom
Plants
gymnosperms
cnidaria
Kingdom
Fungi
rhodophyta
sporozoa
platyhelminthes
zygomycetes
porifera
chrysophyta
pyrrophyta
chlorophyta
sarcodina
mastigophora
Kingdom Protista
cyanobacteria
26
Chapter 1 • Understanding Biodiversity
other bacteria
Kingdom Eubacteria
ciliophora
extreme
halophiles
proteobacteria
spirochetes
annelida
rotifera
bryophyta
phaeophyta
Figure 1 The six kingdoms of life
nematoda
extreme
thermophiles
Kingdom Archaea
methanogens
NEL
You should note that the relative size of each kingdom in most diagrams, including
Figure 1, does not represent the diversity, sheer numbers, or evolutionary relationships of the various kingdoms. The Animal, Plant, and Fungi kingdoms are usually
greatly exaggerated in size. This reflects our own interest in these groups as well as
our familiarity with them in our everyday life.
Table 1 gives an overview of key characteristics of the six kingdoms. Keep in
mind that there are exceptions to these general descriptions and that the number of
different species of micro-organisms is likely grossly underestimated.
UNiT TASK
Which kingdom or kingdoms do the
organisms in your chosen group
belong to?
Table 1 Common Characteristics of the Six Kingdoms of Life
Kingdom
Cell characteristics
Other characteristics
Examples (Organisms shown in the photos are bolded.)
Eubacteria
> 10 000
described
species
• Prokaryotic
• Cell walls contain a
unique compound.
• Cells are variable in
shape and size.
• Diverse means of obtaining energy
and nutrients—photosynthetic,
chemotropic, or heterotrophic
• All can reproduce asexually
Escherichia coli,
Salmonella,
nitrogen-fixing bacteria
Archaea
> 400
described
species
• Prokaryotic
• Cell walls and cell
membranes have a
unique structure.
• Most are extremely
small.
• Some colonize extreme
environments.
• Live in the digestive tracts
of mammals and marine
environments
• All can reproduce asexually.
extreme thermophiles
(organisms that thrive at
high temperatures),
methanogens (microorganisms that produce
methane as a byproduct)
Protista
> 100 000
described
species
• Eukaryotic
• Cells have extreme
diversity of cellular
structure.
• Some have chloroplasts
and cell walls.
• May be heterotrophic or
photosynthetic, or both
• Have variable forms of movement
• Usually live in aquatic or other
moist environments
• Reproduce sexually and
asexually
Amoeba, kelps, green
algae
Fungi
> 100 000
described
species
• Eukaryotic
• The cell wall is
composed of chitin.
• Most are multicellular.
• Cells have no
chloroplasts.
• All are heterotrophic
• Most are terrestrial
• Reproduce sexually and asexually
mushrooms, yeasts,
moulds
Plants
> 250 000
described
species
• Eukaryotic
• All are multicellular
• Cell walls are
composed of cellulose.
• Possess chloroplasts
• Autotrophic and photosynthetic
• Most are terrestrial
• Reproduce sexually and asexually
mosses, ferns, conifers,
flowering plants
Animals
> 1.2
million
described
species
• Eukaryotic
• All are multicellular.
• Cells have no cell walls
or chloroplasts.
• All are heterotrophic
• Most reproduce sexually.
• Live in terrestrial and aquatic
environments
elephants, sponges,
corals, insects, snails,
birds, humans
NEL
1.4 Kingdoms and Domains 27
A simplified phylogenetic tree (Figure 2) shows key evolutionary relationships
among the six kingdoms. Note that the Protist Kingdom does not represent a single
clade. The brown, red, and green algae are more closely related to plants, while the
amoeboid protists are more closely related to fungi and animals.
thermophiles
halophites
Archaea
methanogens
ancestral
eukaryotic
cell
original
cell
Animals
nonphotosynthetic
eukaryotes
Fungi
amoebozoa
brown algae
photosynthetic
eukaryotes
Protists
red algae
green algae
Plants
purple bacteria
photosynthetic bacteria
Eubacteria
other bacteria
past
present
Figure 2 A simplified phylogenetic tree of the six kingdoms
domains of Life
In 1996, Carl Woese conducted a detailed analysis of living organisms. He revealed
that all organisms could be classified into three distinct groups. These groups, called
domains, showed significant differences in their genetic makeup, which was very compelling evidence of their distinct evolutionary kinships (Figure 3).
Archaea
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domain the highest taxonomic level;
there are three domains of life
gi
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trich
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dipl
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flagella
omon
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Most recent common ancestor
of all living organisms
Figure 3 The three domains of life based on the work of Carl Woese. This detailed arrangement of
the branches is continually being refined as new data become available.
28
Chapter 1 • Understanding Biodiversity
NEL
The Domain Eubacteria contains only the Kingdom Eubacteria. Similarly, the
Domain Archaea contains only the kingdom Archaea. The third domain, Eukaryotes,
contains the other four kingdoms: Protista, Animals, Plants, and Fungi. The branches
of the tree and the distances between the branches show an estimate of how closely
related the groups are. You may be surprised to see that, while there are very dramatic
differences in the physical appearances of animals, plants, and fungi, they were all
placed very close together on this tree. The proximity is determined largely by comparing the genetic makeup of different species and groups.
Investigation
1.4.1
Sampling the Diversity of Life (page 32)
After reading about Kingdoms
and Domains, you can complete
Investigation 1.4.1.
In this field study, you will sample
and then classify organisms from a
chosen ecosystem.
research This
who Is in My Clade?
SKILLS
HANDBOOK
Skills: Researching, Analyzing, Communicating
Taxonomists have studied the phylogenetic relationships of all wellknown groups of organisms. Their findings are often presented
in the form of phylogenetic trees. In this activity you will examine
the phylogenetic relationships of a species of your choosing. You
should choose a relatively well known mammal or bird, since you
are more likely to be familiar with its related groups.
1. Choose a species and describe its distinguishing
characteristics. Using the Internet and other resources, find
out its genus name.
2. Research the names and characteristics of all other species
that are in the same genus.
3. Find out which taxonomic family and order your species
belongs to. List the names of several other species that are
in the same order.
A2.1
4. Find two or three simple phylogenetic trees that illustrate
the relationships among these species and clades that your
chosen species belongs to. Draw your own version of the
tree based on the information you have found. Be creative,
but present your findings accurately.
A. How similar were members of the same genus? What
features do they share? What features can be used to tell
them apart? K/u T/I
B. List and describe a number of other species to highlight the
diversity within the same order. K/u T/I
C. Based on your research, which groups within the order are
most closely related? K/u
go To N ELS oN S C i EN C E
1.4 Summary
UNiT TASK BooKMArK
• Mostbiologistsclassifylivingthingsusingasystemofsixkingdomsandthree
domains.
• Th
emosteasilyrecognizedkingdomsareAnimals,Plants,andFungi.
• Protistaistheonlykingdomthatdoesnotrepresentaclade.
• EubacteriaandArchaeaarediffi
culttodistinguishbecauseoftheirextremely
small size and lack of internal structure.
• Th
esix-kingdom,three-domainsystemisbasedonphylogeneticrelationships
revealed by genetic evidence.
To which domains do the organisms in
your chosen group belong?
1.4 Questions
1. List the six kingdoms and three characteristics of each.
K/u
2. Why do you think biologists originally placed fungi within
the plant kingdom? K/u T/I
3. Use the simplified phylogenetic tree in Figure 2 (page 28) to
answer the following questions: K/u T/I C
(a) State the four groups that gave rise to the protists.
(b) To which other group(s) are the Amoebozoa most
closely related?
(c) Suggest some possible reasons to account for why
biologists have grouped the Amoebozoa with brown,
red, and green algae in the kingdom Protista.
NEL
(d) Are animals more closely related to the Archaea or to
the Eubacteria? Explain your reasoning.
4. Make a simple sketch of a “tree of life” that includes six
kingdoms. Label the kingdoms but colour the branches
according to domain. Use a different colour for each
domain. K/u C
5. Describe the most significant differences between the
three domains of life. Why are the Eubacteria and Archaea
difficult to distinguish? K/u C
1.4 Kingdoms and Domains
29