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CLASSIFICATION
Q: WHAT IS THE GOAL OF
BIOLOGISTS WHO CLASSIFY LIVING
THINGS?
Why classify organisms?
How do evolutionary relationships affect the way
scientists classify organisms?
What are the major groups within which all
organisms are currently classified?
A: TO FIND ORDER IN DIVERSITY
Q: What is one thing we can do to “find order in a
diverse world”?
A: Give it a name and describe it
This is one of the most basic skills a scientist does,
but exactly how do scientists name things?
COMMON NAMES
Depending where you live, common names are
assigned to living things—but this presents a
problem because common names may vary from
place to place and among languages
 Examples:

bobcat, cougar, puma, mountain lion all refer to the
same type of cat
 In the UK a buzzard is a hawk, while in the US it is a
vulture
 Maryland’s state flower—black eyed Susan, yellow
cornflower

ASSIGNING SCIENTIFIC NAMES
To be useful, each scientific name must refer to
one and only one species, and everyone must use
that same name for that species.
 It must be written in Latin

DICHOTOMOUS KEYS
One of the earliest classification systems
 Consists of a series of paired statements or
questions that describe alternative possible
characteristics of an organism.
 The paired statements usually describe the
presence or absence of a certain visible
characteristics or structures.
 Each set of choices is arranged so that each step
produces a smaller subset.

USING A DICHOTOMOUS KEY
How would you describe this leaf?
Step
Leaf Characteristics
Tree
1a
1b
Compound leaf (leaf divided into leaflets)…go to
step 2
Simple leaf (leaf not divided into leaflets)…go to
step 4
2a
2b
Leaflets all attached at a central point 
Leaflets attached at several points…go to step 3
Buckeye
3a
3b
Leaflets tapered with pointed tips 
Leaflets oval with rounded tips 
Pecan
Locust
4a
4b
Veins branched out from one central point…go to
step 5
Veins branched off main vein in middle of the
leaf…go to step 6
5a
5b
Heart-shaped leaf 
Star-shaped leaf 
Redbud
Sweet gum
6a
6b
Leaf with jagged edges 
Leaf with smooth edges 
Birch
Magnolia
BINOMIAL NOMENCLATURE
Definition: having a two-part scientific name
 Written in italic if typing or underlined if handwritten
 First word (of name) is capitalized and the second
word (of name) is lower-cased
 Example: Felis concolor or Homo saphien
 Created by a Swedish botanist named Carolus
Linnaeus

BINOMIAL NOMENCLATURE CONT.
Polar bear’s scientific name is Ursus maritimus
 Ursus is the genus (plural: genera) to which the
polar bear belongs
 A genus is a group of similar species
 The genus Ursus contains 4 species of bears
including Ursus arctos—the brown bear or
“grizzly”
 The second part of the name is the species
 A species is defined as a group of individuals
capable of interbreeding and producing fertile
offspring
 The species name is often a description of an
important trait or the organism’s habitat.

Species and subspecies of Ursus
American Black Bear, Ursus americanus
Cinnamon Bear, Ursus americanus
cinnamomum
Kermode Bear, Ursus americanus
kermodei
Brown Bear, Ursus arctos
Grizzly Bear, Ursus arctos horribilis
Kodiak Bear, Ursus arctos middendorffi
Polar Bear, Ursus maritimus (earlier
Thalarctos maritimus)
Asiatic Black Bear, Ursus thibetanus, or
Selenarctos thibetanus
A hybrid between grizzly bears and polar
bears has also been recorded (known
commonly as a pizzly bear). The official
name is a grizzly-polar bear hybrid.
CLASSIFYING SPECIES INTO LARGER
GROUPS
The science of naming and grouping organisms is
called systematics
 The goal of systematics is to organize living
things into groups (called taxa; singular: taxon)
that have biological meaning

HOW DID LINNAEUS GROUP SPECIES INTO
LARGER TAXA?

Classification System based on a hierarchy or set
of ordered ranks (smallest to largest):







Species
Genus
Family
Order
Class
Phylum (Division)
Kingdom
PROBLEMS WITH TRADITIONAL
(LINNAEAN) CLASSIFICATION
Linnaean classification emphasized the overall
similarities and differences to classify an
organism
 Modern systematists apply Darwin’s ideas
(descent with modification) to classification and
try to look beyond simple similarities and
differences to ask questions about evolutionary
relationships (i.e. who is more closely related to
each other)

LESSON ASSESSMENT
1.
2.
3.
4.
5.
6.
Identify two goals of systematics.
Why do the common names of organisms—like
daisy or mountain lion—often cause problems for
scientists?
The scientific name of the sugar maple is Acer
saccharum. What does each part of the name
designate?
List the ranks of the Linnaean system of
classification from largest to smallest.
In which group of organisms are the members more
closely associated—kingdom or order. Explain your
answer.
Why might modern biologists have a problem with
the term “species”?
TODAY’S MAIN QUESTION:
HOW DO EVOLUTIONARY RELATIONSHIPS AFFECT THE WAY
SCIENTISTS CLASSIFY ORGANISMS?

Guiding Questions:
What is the goal of evolutionary classification?
 What is a cladogram and how do you interpret a
cladogram?
 How are DNA sequences used in classification?

MODERN EVOLUTIONARY CLASSIFICATION
Remember: Linnaean classification system is
based on similarities and differences
 Remember: Darwin’s “Tree of Life” suggests that
all living things are related to each other through
evolutionary relationships back to a common
ancestor
 This presents some problems in how we classify
living things.
 Example: Birds and Reptiles

WHAT IS THE GOAL OF EVOLUTIONARY
CLASSIFICATION?
The concept of descent with modification (by
Darwin) led to the study of phylogeny—the
evolutionary history of lineages—which led to
phylogenetic systematics (aka evolutionary
classification).
 The goal of phylogenetic systematics is to group
species into larger categories that reflect lines of
evolutionary descent, rather than overall
similarities and differences.

EVOLUTIONARY CLASSIFICATION
Places organisms into higher taxa whose
members are more closely related to one another
than they are to members of any other group.
The larger the taxon is, the farther back in time
all of its members shared a common ancestor.
 Classifying organisms according to these above
rules places them into groups called clades.
 A monophyletic clade is a group of species that
includes a single common ancestor and all
descendants of that ancestor—living and extinct.
 Paraphyletic clades include a common
ancestor but are missing one or more
descendants.

WHAT IS A CLADOGRAM?

A diagram that links groups of organisms by
showing how evolutionary lines or lineages
branched off from common ancestors.
BUILDING CLADOGRAMS

Speciation
The process by which one ancestral species splits into
two new ones
 This is the basis by which each branching point or
node is made in a cladogram

The node represents the last point at which the
two new lineages above the node shared a
common ancestor
 The bottom or “root” of a cladogram represents
the common ancestors shared by all organisms in
the cladogram

Derived Character:
A trait that arose in
the most recent
common ancestor or a
particular lineage
and was passed along
to its descendants.