18-1 Finding Order in Diversity
Natural selection and other processes have
led to a staggering diversity of organisms.
Biologists have identified and named about
1.5 million species so far.
They estimate that 2–100 million additional
species have yet to be discovered.
To study the diversity of life, biologists
use a classification system to name
organisms and group them in a logical
In the discipline of taxonomy, scientists
classify organisms and assign each
organism a universally accepted name.
When taxonomists classify organisms,
they organize them into groups that have
Assigning Scientific Names
Common names of organisms vary, so
scientists assign one name for each
Because 18th century scientists
understood Latin and Greek, they used
those languages for scientific names.
This practice is still followed in naming
Early Efforts at Naming Organisms
The first attempts at standard scientific
names described the physical
characteristics of a species in great detail.
These names were not standardized
because different scientists described
Carolus Linneaus developed a naming
system called binomial nomenclature.
In binomial nomenclature, each
species is assigned a two-part
The scientific name is italicized.
The first part of the name is the genus
A genus is a group of closely related species.
The second part of the name is the
species name (lowercase).
The species name often describes an important
trait or where the organism lives.
Linnaeus's System of Classification
Linnaeus not only named species, he also
grouped them into categories.
Linnaeus’s seven levels of
classification are—from smallest to
Each level is called a taxon, or
Species and genus are the two
Genera that share many characteristics are
grouped in a larger category, the family.
An order is a broad category composed of
The next larger category, the class, is
composed of similar orders.
Several different classes make up a phylum.
The kingdom is the largest and most
inclusive of Linnaeus's taxonomic
Linnaeus grouped species into larger taxa
mainly according to visible similarities and
•Phylogeny is the study of
Biologists currently group organisms into
categories that represent lines of
evolutionary descent, or phylogeny, not
just physical similarities.
Grouping organisms based on
evolutionary history is called
The higher the level of the taxon, the further
back in time is the common ancestor of all
the organisms in the taxon.
Organisms that appear very similar may not
share a recent common ancestor.
•Different Methods of Classification
Appendages Conical Shells Crustaceans Mollusk
Crab Barnacle Limpet Crab Barnacle Limpet
CLASSIFICATION BASED ON
VISIBLE SIMILARITY CLADOGRAM
Superficial similarities once led barnacles
and limpets to be grouped together.
Appendages Conical Shells
Crab Barnacle Limpet
However, barnacles and crabs share an
evolutionary ancestor that is more recent
than the ancestor that barnacles and
Barnacles and crabs are classified as
crustaceans, and limpets are mollusks.
Many biologists now use a method called
•Cladistic analysis considers only
new characteristics that arise as
lineages evolve ( called derived
•Characteristics that appear in recent parts of a
lineage but not in its older members are called
Derived characters can be used to
construct a cladogram, a diagram
that shows the evolutionary
relationships among a group of
Cladograms help scientists understand how
one lineage branched from another in the
course of evolution.
A cladogram shows the evolutionary
relationships between crabs, barnacles, and
Crab Barnacle Limpet
Tiny free-swimming larva
Molted external skeleton
The genes of many organisms show
important similarities at the molecular level.
Similarities in DNA can be used to help
determine classification and evolutionary
•DNA evidence shows evolutionary relationships of
•The more similar the DNA of two
species, the more recently they shared a
common ancestor, and the more closely
they are related in evolutionary terms.
•The more two species have diverged from each
other, the less similar their DNA is.
•Comparisons of DNA are used to mark
the passage of evolutionary time.
•A molecular clock uses DNA
comparisons to estimate the length of
time that two species have been evolving
A B C
A gene in an ancestral
Species Species Species
A molecular clock relies on mutations to mark
Simple mutations in DNA structure occur
Neutral mutations accumulate in different
species at about the same rate.
Comparing sequences in two species shows
how dissimilar the genes are, and shows
when they shared a common ancestor.
The Tree of Life Evolves
•Systems of classification adapt to new
•Linnaeus classified organisms into two
kingdoms—animals and plants.
•The only known differences among living
things were the fundamental traits that
separated animals from plants.
There are enough differences among
organisms to make 5 kingdoms:
•Recently, biologists recognized that
Monera were composed of two distinct
groups: Eubacteria and Archaebacteria.
The six-kingdom system of
Changing Number of Kingdoms
Introduced Names of Kingdoms
Monera Protista Fungi Plantae
AnimaliaProtista Fungi Plantae
The Three-Domain System
•Molecular analyses have given
rise to the three-domain system
of taxonomy that is now recognized
by many scientists.
•The domain is a more inclusive category
than any other—larger than a kingdom.
The three domains are:
•Eukarya, which is composed of
protists, fungi, plants, and animals.
•Bacteria, which corresponds to
the kingdom Eubacteria (true
•Archaea, which corresponds to
the kingdom Archaebacteria.
Modern classification is a rapidly changing
As new information is gained about
organisms in the domains Bacteria and
Archaea, they may be subdivided into
•Members of the domain Bacteria are
•Their cells have thick, rigid cell walls that
surround a cell membrane.
•Their cell walls contain peptidoglycan.
•Members of the domain Archaea are
•Archaea live in extreme
•Their cell walls lack peptidoglycan, and
their cell membranes contain unusual
lipids not found in any other organism.
•The domain Eukarya consists of
organisms that have a nucleus.
•Eukarya includes the kingdoms
•The kingdom Protista is composed of
eukaryotic organisms that cannot be
classified as animals, plants, or fungi.
•Its members display the greatest variety.
•They can be unicellular or multicellular;
photosynthetic or heterotrophic; and can
share characteristics with plants, fungi,
•Members of the kingdom Fungi are
heterotrophs with cell walls that
•Most fungi feed on dead or decaying organic
matter by secreting digestive enzymes into it
and absorbing small food molecules into their
•They can be either multicellular (mushrooms) or
•Members of the kingdom Plantae are
multicellular, photosynthetic autotrophs.
•Plants are nonmotile—they cannot move
from place to place.
•Plants have cell walls that contain cellulose.
•The plant kingdom includes cone-bearing
and flowering plants as well as mosses and
•Members of the kingdom Animalia are
multicellular and heterotrophic.
•The cells of animals do not have cell walls.
•Most animals can move about.
•There is great diversity within the animal
kingdom, and many species exist in nearly
every part of the planet.
Which statement about classification is true?
a. Biologists use regional names for organism
b. Biologists use a common classification
system based on similarities that have
c. Biologists have identified and named most
species found on Earth
d. Taxonomy uses a combination of common
and scientific names to make the system
Linnaeus's two-word naming system is
a. binomial nomenclature.
c. trinomial nomenclature.
Several different classes make up a(an)
A group of closely related species is
Which of the following lists the terms in
order from the group with the most
species to the group with the least?
a. order, phylum, family, genus
b. family, genus, order, phylum
c. phylum, class, order, family
d. genus, family, order, phylum
Grouping organisms together based on
their evolutionary history is called
a. evolutionary classification.
b. traditional classification.
c. cladogram classification.
d. taxonomic classification.
Traditional classification groups
organisms together based on
a. derived characters.
b. similarities in appearance.
c. DNA and RNA similarities.
d. molecular clocks.
In an evolutionary classification system,
the higher the taxon level,
a. the more similar the members of the
b. the more common ancestors would be
found in recent time.
c. the fewer the number of species in the
d. the farther back in time the common
ancestors would be.
Classifying organisms using a
cladogram depends on identifying
a. external and internal structural
b. new characteristics that have
appeared most recently as lineages
c. characteristics that have been
present in the group for the longest
d. individual variations within the group.
To compare traits of very different
organisms, you would use
a. anatomical similarities.
b. anatomical differences.
c. DNA and RNA.
d. proteins and carbohydrates.
Organisms whose cell walls contain
peptidoglycan belong in the kingdom
Multicellular organisms with no cell
walls or chloroplasts are members of
Organisms that have cell walls
containing cellulose are found in
a. Eubacteria and Plantae.
b. Fungi and Plantae.
c. Plantae and Protista.
d. Plantae only.
Molecular analyses have given rise to a
new taxonomic classification that
a. three domains.
b. seven kingdoms.
c. two domains.
d. five kingdoms.
Which of the following contain more
than one kingdom?
a. only Archaea
b. only Bacteria
c. only Eukarya
d. both Eukarya and Archaea