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Biological
Classification
BioEd Online
Why Do We Classify Organisms?
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Biologists group organisms to organize
and communicate information about
their diversity, similarities and proposed
relationships.
Classification systems change with
expanding knowledge about new and
well-known organisms.
Approximately 1.75 million species have
been classified so far.
Tacitus bellus
History of Classification
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Aristotle (2000 + y.a.) classified organisms as either
plants (by size) or animals (red-blooded or not).
History of Classification
In the 1700s and 1800s:
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Carolus Linnaeus, a Swedish botanist, used similarities and differences in
morphology and behavior to classify birds.
Linnaeus developed the first taxonomy system and made it possible to include
evolutionary principles in classification in the 1800s.
Jean-Baptiste Lamarck, Charles Darwin, and Ernest Haeckel introduced
classification systems based on evolutionary relationships to organize biological
diversity.
Scientific Names
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Carolus von Linnaeus devised the currently used
binomial nomeclature.
Two-word naming system:
 Genus
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Noun, Capitalized,
Underlined or Italicized
Species
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Descriptive, Lower Case,
Underlined or Italicized
Ex.:
Genus: Homo or Homo
Species: sapiens or sapiens
Complete scientific name of species:
Homo sapiens or Homo sapiens
Abbreviated form: H. sapiens or H. sapiens
Carolus von Linnaeus
(1707-1778)
Swedish scientist who laid
foundation for modern
taxonomy
Hierarchical Classification into Taxa
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Taxonomists classify organisms by dividing them into smaller groups
based on more specific criteria. A named groups of organisms is a
taxon.
Taxonomic categories (taxa):
 Kingdom
King
 Phylum
Philip
 Class
Came
 Order
Over
 Family
For
 Genus
Green
 Species
Spaghetti
Beginning with species, each category becomes progressively more
comprehensive. Ex.: while the leopard, tiger and domestic cat all
belong to different genera, they are grouped together in the same
family.
Modern Classification - Species Concepts
Modern Classification Systems /
Disciplines of Biology
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Systematics is the branch of biology
concerned with the study of biological
diversity.
Taxonomy is the part of systematics
concerned with identifying, naming, and
classifying species based on
morphological and behavioral similarities
and differences.
Binomial Nomeclature is used to name
species using two words, genus and
species.
Populus tremuloides
Quaking Aspen
Systematics
Evolutionary Classification of Biodiversity
Systematics is the study of the evolution of biological
diversity, and combines data from the following areas:
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Fossil record
Comparative homologies, morphological characters
(traits) derived from common ancestry
Cladistics, the study of evolutionary relationships
between species based on shared characters
(inherited traits)
Biochemical characters, such as comparative
sequencing of DNA/RNA among organisms
Molecular clocks
Comparing Morphological Characters
A Molecular Clock
The rate at which mutations
occur varies depending on:
•Type of mutation
•Location of mutation in the
genome
•Type of protein affected
•Population in which the
mutation occurs
This inconsistency makes
molecular clocks difficult to read.
Molecular clocks are often used
along with the fossil record and
help determine when a species
evolved.
Taxonomic Diagrams based on Phylogeny
Mammals
Turtles
Lizards and
Snakes
Phylogenetic Tree:
a form of
cladogram; nodes
represent common
ancestors
Crocodiles
Birds
Mammals
Turtles
Lizards and
Snakes
Crocodiles
Birds
Cladogram:
a branched diagram that
shows the proposed
phylogeny (evolutionary
history) of a species; nodes
represent common ancestors
Lily Cladogram
The closer two groups
are, the larger the
number of characters
they share.
Dichotomous Keys Identify Organisms
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Dichotomous keys are tools used to identify
organisms; field guides contain dichotomous keys.
Dichotomous keys contain pairs of contrasting
descriptions - choices between two options.
After each description, the key directs the user to
another pair of descriptions or identifies the organism.
Example (identifying a plant using its leaf):
1. a) Is the leaf simple? Go to 2
b) Is the leaf compound? Go to 3
2. a) Are margins of the leaf jagged? Go to 4
b) Are margins of the leaf smooth? Go to 5
Kingdoms and Domains
The three-domain system
Bacteria
Archaea
Eukarya
The six-kingdom system
EuBacteria
Archaeabact
eria
Protista
Plantae
Fungi
Animalia
Fungi
Animalia
The traditional (now outdated) five-kingdom system
Monera
Protista
Plantae
Kingdom Eubacteria (Domain Bacteria)
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Prokaryotes whose cell walls contain peptidoglycan, a protein-sugar
polymer that is porous and strong
All unicellular, mostly heterotrophic, some autotrophic
Disease-causing bacteria and many harmless bacteria found in the
environment are in this kingdom
Kingdom Archaebacteria (Domain Archaea)
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Prokaryotes, thought to be more ancient than bacteria, no
peptidoglycan in cell walls
All unicellular, mostly heterotrophic, some autotrophic (including
chemotrophic)
Called “extremophiles” because they can live in the most extreme
environments on earth, such as hot springs, salty lakes, thermal
vents on the ocean floor, and the mud of marshes
* All bacteria reproduce asexually (binary fission),
many also sexually (conjugation) *
Kingdom Protista
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Eukaryotes that differ
significantly from each
other but do not fit into
any other kingdom
Unicellular, colonial, or
multicellular organisms,
but their cells cannot form
true tissues/organs
Autotrophic or
heterotrophic
Reproduce sexually and
asexually using spores,
cysts, gametes
Classified into three
groups by trophism:
fungus-like, plant-like,
and animal-like
Kingdom Fungi
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Eukaryotes whose cells can form true tissues / organs
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Cell walls contain chitin, a rigid polymer that gives structural support
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All heterotrophic / decomposers – secrete digestive enzymes onto their
food and absorb the nutrients (extracellular digestion)
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Most multicellular, a few unicellular (yeasts); all sessile (cannot move)
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Reproduce sexually and asexually by spores, fragmentation/regeneration
A mushroom is
a fungus.
Kingdom Plantae
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Eukaryotes whose cells can form true tissues / organs
Autotrophs - form the basis of all land ecosystems – and a few
heterotrophs (yes, some plants are parasites!)
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All multicellular and sessile (cannot move)
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Reproduce sexually and asexually (spores, seeds, cuttings, etc.)
A leaf is a
plant organ.
Kingdom Animalia
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Eukaryotes whose cells form true tissues / organs
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All multicellular
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All heterotrophic
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Most motile (can move)
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Most reproduce sexually,
some asexually