Chapter 15:
Biological Classification
What is this?
These are some of the common
names for this animal. Scientists
have assigned it a single scientific
name: Porcellio scaber
Sow bug
Wood louse
Potato bug
Roly-poly
Pill bug
The Importance of Scientific Names
• Each kind of organism on Earth is assigned a
unique two-word scientific name
– Binomial nomenclature
• All biologists, regardless of their native language,
use scientific names when speaking or writing
about organisms
• Most organisms also have common names
• Using scientific names enables scientists to
exchange information about an organism and to be
certain that they are referring to the same living
thing
What’s in a Scientific Name?
• First word describes the organism in a general way
• The second word identifies the exact kind of living
thing
• The first word of a scientific name is the name of the
genus to which the organism belongs
– Group of animals that share major characteristics
• The second word in a scientific name identifies one
particular kind of organism within the genus
• Scientists call each different kind of organism a
species
• The correct name for an organism must include BOTH
parts of its scientific name
Comparison of Red Oak and
Willow Oak
Red Oak
Genus Name
Willow Oak
Quercus
Quercus
Scientific Name
Quercus rubra
Quercus phellos
Traits
Acorns about
25mm long
Acorns about
15mm long
Common in open
Northeastern
forests; tolerant of
city soot and cold
temperatures
Popular shade tree
found in the South;
grows well in rich,
moist soil
Lobed leaves
Unlobed, narrow
leaves
Scientific Names Must Conform to a
Set of Rules
• All scientific names must consist of Latin
words
• Two different organisms cannot be assigned
the same name
• Organisms in different genera cannot have the
same genus name
• When choosing a name for a species, biologists
often pick a name that describes the
appearance or distribution of an organism
The second word of a scientific name is often
descriptive of an organism or its distribution. The
green anole lizard Anolis carolinensis and the
chickadee Parus carolinensis are both found in
North Carolina and South Carolina.
Tyrannosaurus rex, which means
“tyrant-lizard-king,” was named
for its enormous teeth and
tremendous size. This dinosaur
measured about 50 feet in length.
The frog Rhinoderma
darwinii was named to
honor Charles Darwin.
Why are Scientific Names in Latin?
• In the Middle Ages, when scientists began to
name organisms, Latin was used in academic
circles
• Scientists and other scholars found it easier to
communicate with each other in Latin
• Latin was the language of the scholar and was
used for all spoken and written communication
• Easier to still use Latin than to rename all 1.4
million known organisms
• Latin is a universal language
Linnaeus Devised the Two-Name
System
• The modern system of naming organisms was
developed by Swedish botanist Carl Linnaeus
• In Linnaeus’s day, organisms were given very
long Latin names (sometimes more than 15
words), which were often changed according
to the whims of particular scientists
• Linnaeus assigned a standard, two-word Latin
name to each organism known in his time
Writing a Scientific Name is Simple
• When you write a scientific name, always
capitalize the genus name
• Begin the second word with a lowercase letter
• Both parts of a scientific name are underlined or
written in italics
– Homo sapiens
– Homo sapiens
• After the first use of the full scientific name, the
genus name can be abbreviated as a single letter if
the meaning is clear
– H. sapiens
Classification of Living Things
• The Greek philosopher Aristotle grouped
animals by their physical similarities
• Today biologists classify organisms based
on their physical, genetic, biochemical, and
behavioral similarities
• The classification of organisms is based on
decisions made by many scientists using
available information
Classification of Living Things
• The science of classifying living things is
called taxonomy
• Taxonomists are scientists who examine,
classify, and argue about where organisms fit
in a group
• In a hierarchal system of classification, species
are assigned to genera, genera are assigned to
families, and families are assigned to groups of
increasing size
Organisms are Classified by
Similarity
• In biological classification, organisms are
assigned to a group because they share
distinctive characteristics with other members
of that group
• The biological hierarchy of classification has
seven different levels
– Kingdom, phylum, class, order, family,
genus, and species
Organisms are Classified by Similarity
• The smallest group in biological classification is the
species
• Similar species are collected into a genus
• Similar genera are united into a family
• Families that are alike are combined into an order
• Similar orders are collected into a class
• Classes are united into a phylum
• Finally, similar phyla are collected into a kingdom
• The more classification categories two species
share, the more traits they have in common
Kingdom
Phylum
Class
Order
Family
Genus
Species
Biological Classification
•
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Kristy
Poured
Coffee
On
Fred’s
Green
Shirt
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Kingdom
Phylum
Class
Order
Family
Genus
Species
Classification and Evolution
• The biological hierarchy of classification is based
on the fact that different degrees of similarity exist
among organisms
• For Darwin, classification provided strong
evidence supporting evolution
• Organisms are similar because they descended
from a common ancestor
• The more similarities two organisms share, the
more recently they shared a common ancestor
• Thus, the more classification categories two
organisms share, the more closely related they are
Similarity Does not Guarantee Close
Relationship
• Compare the two oceandwelling animals seen
here
• Both have stream-lined
bodies, paddle-like fins,
and flattened tails
• Would you say these
organisms are closely
related?
*gills obtain
oxygen from
water
*belongs to the
class
Chondrichthyes
*skin has
placoid
(toothlike)
scales
*skeleton of
cartilage
*vertical tail fin
*breathes air
through lungs
*belongs to the
class Mammalia
*skin has hair
*skeleton of
bone
*horizontal tail
fin
Similarity Does not Guarantee Close
Relationship
• Similar appearance does not guarantee
common ancestry
• Because the number of differences between
sharks and dolphins far exceeds the number
of similarities, it is easy to reject the
hypothesis that these animals are close
relatives
Methods of Taxonomy
• The example of the shark and dolphin
illustrates the difficulty in determining
which similarities will be useful when
classifying an organism
• There are two alternative methods of
choosing which similarities are important
• The first method is cladistics
Taxonomy and Technology
• Biologists have traditionally compared the
appearances of organisms in order to discover
the relationships among them
• Biologists also consider the behavioral
patterns, methods of reproduction, life cycles,
and development from fertilization to
adulthood
• Technological advances have enabled
biologists to study the genes that produce the
traits used to classify organisms
Taxonomy and Technology
• Taxonomists use techniques of molecular
biology to compare the DNA nucleotide
sequences of different organisms
• Comparisons of DNA sequences are especially
important for the taxonomist because
mutations are random events
• As time passes, more mutations tend to occur
in the DNA of a particular species
• Thus, DNA acts as a “molecular clock”
What Is a Species?
• A species is just a level in the classification
system to which scientists assign very similar
organisms
• Over time, species change and give rise to new
species in a process known as speciation
• Biologists have traditionally defined a species
as organisms that are able to interbreed with
each other to produce fertile offspring and that
usually do not reproduce with members of
other groups
What Is a Species?
• This definition works well for most animals
• For example, the horse and the zebra belong to
different species
• Although they can mate, the resulting offspring,
the “zebroid”, is sterile
• Reproductive barriers between species are not
always perfect
• Hybrids are offspring that result from
interbreeding by individuals of different species
– Coyotes, dogs, and wolves
A Species is a Unique Kind of
Organism
• A species is basically a unique kind of
organism
• Members of a species share at least one
inherited characteristic not found in other
similar organisms
• In sexually reproducing species, this
distinctive characteristic is maintained from
generation to generation because members of
different species do not interbreed
Six – Kingdom System
• Biologists used to classify every living thing
into either kingdom Plantae or kingdom
Animalia
• However, numerous living things do not quite
fit either description
• For example, where would a mushroom fit?
• Since Linnaeus’s time, biologists have learned
a great deal about the structure and function of
living things
Six – Kingdom System
• This information has enabled them to make
increasingly precise distinctions among the major
groups of organisms
• Most biologists now use a six-kingdom system of
classification
– Archaebacteria
– Eubacteria
– Protista
– Fungi
– Plantae
– Animalia
Bacteria
• All prokaryotes, also called bacteria, are in
the kingdoms Archaebacteria or Eubacteria
• The bacteria represent the most ancient
groups on earth
• They have adapted to almost every
environment
• All bacteria lack cell nuclei
Kingdom Archaebacteria
• The archaebacteria evolved before oxygen
filled our atmosphere and now are found in
extreme environments
• Fewer than 100 species have been
recognized so far
• Archaebacteria are believed to be the
ancestors of the protists
Kingdom Eubacteria
• Contains most of the common bacteria that
share our world
• They are an extremely diverse group,
containing both autotrophic and heterotrophic
forms
• Approximately 5,000 species have been
characterized so far, but many more exist
• Eubacteria are believed to be the ancestors of
mitochondria and chloroplasts, organelles
within eukaryotic cells
Kingdom Protista
• All the multicellular eukaryotes not
classified as plants, animals, or fungi are
assigned to this kingdom
• Protists include protozoa, such as Amoeba
and Paramecium, and algae, such as
seaweeds and kelps
• Slime molds and water molds also belong to
this kingdom
Kingdom Fungi
• Mushrooms, yeast, and molds are members of
this kingdom
• Instead of roots, stems, and leaves, fungi are
made of thin filaments that penetrate the soil or
decaying organisms, absorbing nutrients from
them
• Fungi do not contain chloroplasts and cannot
make their own food by photosynthesis
Kingdom Plantae
• This kingdom includes only terrestrial
multicellular organisms that use photosynthesis
to obtain their nutrients
• Nearly all plants occur on dry land, but a few
grow submerged in fresh water, and a very few
grow at the edges of the sea
• Plants cells have cell walls
• Because some green algae are so similar to
plants they have been identified as the
ancestral groups for this kingdom
Kingdom Animalia
• The first members of this kingdom evolved in
the ocean
• The largest number of animal phyla are still
found only in the sea
• Organisms in kingdom Animalia are
multicellular
• Animals do not photosynthesize
• Their cells do not have cell walls
• Nearly all animals have some sort of nervous
system