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Chapter 18
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Classification
Summary
18 –1 Finding Order in Diversity
There are millions of species on Earth. To study the diversity of
life, biologists use a classification system to name organisms
and group them in a logical manner. A good classification system
groups together organisms that are more similar to one another
than they are to organisms in other groups.
To name organisms, scientists needed to use names that
could be understood and used worldwide. By the 1700s, scientists
tried to solve this problem by agreeing to use one name for
each species. They began using Latin or Greek names that
described the physical characteristics of organisms. At first, the
names were very long. Then, Carolus Linnaeus developed a
naming system called binomial nomenclature. This system is
still in use. In binomial nomenclature, each species is assigned
a two-part scientific name.
• The first part of the name refers to a genus (plural: genera).
A genus is a group of closely related species. For example,
the genus Ursus refers to bears.
• The second part of the name refers to one species. A species
is a group of individuals that can interbreed. The name Ursus
maritimus, for example, refers to the polar bear species.
The first letter of the genus name is always capitalized, while
the species name is always in lowercase. Both names are printed in
italics to make the names easy to recognize.
Linnaeus also developed a system for grouping organisms.
Organisms that shared important traits were classified in the
same taxon, or group. Linnaeus’s classification system has
seven groups, or taxon. From smallest to largest, the taxon
are as follows:
• Species, a group of individuals that can interbreed
• Genus, a group of closely related species
• Family, a group of similar genera
• Order, a group of similar families
• Class, a group of similar orders
• Phylum, a group of similar classes
• Kingdom, a group of similar phyla
Linnaeus named two kingdoms of living things: Animalia (animal)
and Plantae (plant).
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18–2 Modern Evolutionary Classification
Early classification was based on visible similarities. Biologists
now group organisms according to evolutionary relationships.
This is called evolutionary classification. Species within one
genus are more closely related to one another than to species
in another genus. All members of a genus share a recent
common ancestor.
All genera in a family also share a common ancestor. However,
this ancestor is farther in the past than the common ancestor of
the species within a genus. In fact, the higher the taxon level, the
farther back in time is the common ancestor of the organisms in
that taxon.
To decide evolutionary relationships, many biologists use
cladistic analysis. Cladistic analysis is a classification method that
is based on derived characters. Derived characters are new traits
that arise as a group evolves. Derived traits are found in closely
related organisms but not in their distant ancestors. Derived
characters are used to make a cladogram. A cladogram is a
diagram that shows the evolutionary relationships among a group
of organisms. A cladogram is a type of evolutionary tree, much
like a family tree. One type of evidence that can be used to
determine evolutionary relationships is genetic evidence.
The genes of many organisms show important similarities at
the molecular level. Similarities in DNA can be used to help
determine classification and evolutionary relationships.
• The more similar the molecules in species, the more recently
the species shared a common ancestor. Thus, the more
closely related they are.
• Comparisons of DNA also are used to estimate how long
two species have been evolving independently. This is done
using a model called a molecular clock. The model assumes
that neutral mutations—those not affecting phenotype—
accumulate in gene pools. Two species evolving
independently from each other will accumulate different
neutral mutations over time. The more different neutral
mutations that are present, the longer the two species have
been evolving independently.
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18–3 Kingdoms and Domains
Biologists realized that Linnaeus’s two kingdoms did not include
all life-forms. This led to a system with additional kingdoms.
• First, microorganisms such as bacteria were discovered.
Microorganisms did not fit into the plant or animal kingdom.
They were placed in their own kingdom, called Protista.
• Next, mushrooms, yeast, and molds were separated from
plants. They were placed in the kingdom Fungi.
• Later, bacteria were separated from other Protista. They were
placed in a new kingdom, called Monera.
• Most recently, the Monera were divided into two kingdoms:
Eubacteria and Archaebacteria.
In the 1990s, a six-kingdom system of classification was proposed.
The six-kingdom system of classification includes the kingdoms
Eubacteria, Archaebacteria, Protista, Fungi, Plantae, and
Animalia.
Many biologists now use a new taxon—the domain. The
domain is one level higher than the kingdom. There are three
domains: Bacteria, Archaea, and Eukarya.
• The domain Bacteria includes only the kingdom Eubacteria.
All members of this domain are unicellular organisms
without a nucleus. They have cell walls that contain
peptidoglycan.
• The domain Archaea includes only the kingdom
Archaebacteria. All members of the domain Archaea are
unicellular organisms without a nucleus. They have cell
walls that do not contain peptidoglycan.
• The domain Eukarya includes the kingdoms Protista, Fungi,
Plantae, and Animalia. All members of this domain have cells
with a nucleus.
▪ Most members of the kingdom Protista are unicellular.
Some Protista are autotrophs, and others are heterotrophs.
▪ Most members of the kingdom Fungi are multicellular.
All are heterotrophs.
▪ All members of the kingdom Plantae are multicellular
autotrophs. Most plants cannot move about, and their cells
have cell walls.
▪ All members of the kingdom Animalia are multicellular
heterotrophs. Most animals can move about. Their cells lack
cell walls.
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