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Chap 18- Classification
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
Estimate that anywhere between 2 and 100
million additional species have yet to be
discovered
Chap 18- Classification
Why Classify?
For study purposes, biologists must give
each organism a name
Must also attempt to organize living things
into groups that have biological meaning
Biologists use classification system to name
organisms & group them in a logical
manner
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In taxonomy, scientists classify organisms
and assign each organism a universally
accepted name
Using scientific name reduces confusion
about which organism is being discussed
When taxonomists classify organisms, place
them into biologically significant groups
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Science often requires smaller categories as
well as larger, more general categories
Good system of classification – organisms
placed into particular group more similar to
each other than to organisms in another
group
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Assigning Scientific Names
By 18th century, scientists recognized that
referring to organisms by common names
was confusing
To eliminate confusion, scientists agreed to
a single name for each species
Latin & Greek were used for scientific
names
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Early Efforts
1st attempts often described physical
characteristics in great detail
Some names twenty words long
Also difficult to standardize names of
organisms since different scientists
described different characteristics
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Binomial Nomenclature
Carolus Linnaeus- Swedish botanist, 18th
century
Developed two-word naming system called
binomial nomenclature
Each species assigned a two-part scientific
name
Always written in italics; 1st word capitalized
Homo sapiens
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Linnaeus’ System of Classification
Hierarchical; consists of levels
Seven levels (smallest to largest): species,
genus, family, order, class, phylum, and
kingdom
Each level called a taxon, or taxonomic
category
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Species- group of similar organisms that
can breed and produce fertile offspring
Genus- group of closely related species
Family- group of genera that share many
characteristics
Order- broad taxonomic category composed
of similar families
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Class- composed of similar orders
Ex.- Order Carnivora placed in class
Mammalia (animals that are warmblooded, have body hair, and produce milk
for offspring)
Phylum- Several different classes; members
may be different from one another, but
share important characteristics
Kingdom- most inclusive of all taxons
Grizzly bear
Black bear
Giant panda
Red fox
KINGDOM Animalia
PHYLUM Chordata
CLASS Mammalia
ORDER Carnivora
FAMILY Ursidae
GENUS Ursus
SPECIES Ursus arctos
Albert squirrel Coral snake Sea star
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Organisms determine who belongs to their
species by choosing who to mate with
Taxons above the species level are
“invented” by taxonomists who decide how
to distinguish between one genus, family,
or phylum
Always tried to group according to
biologically important characteristics
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Linnaean system has limitations & problems
System uses homologies to group species
into larger and more general categories
Since Darwin, classification is a way of
describing evolutionary relationships
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Which Similarities Are Most Important?
Linnaeus grouped species into larger taxa
mainly according to visible similarities &
differences
Which are most important?
Ex.- Dolphins
Fish due to fin-like limbs and live in water?
Mammals due to milk and breathe air?
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Evolutionary Classification
Darwin’s ideas gave rise to study of
phylogeny (evolutionary relationships
between organisms)
Now group organisms into categories that
represent lines of evolutionary descent,
not just physical similarities
Systematics- classification based on
evolutionary relationships, not just physical
similarities
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Species within a genus more closely related
to each other than to species within
another genus
Because all members of a genus share a
recent common ancestor
So, all genera in a family share a common
ancestor; much farther in past
Higher the taxon level, further back the RCA
of all organisms in the taxon
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Evidence used in Systematic Taxonomy:
•
•
•
•
Fossil record
Morphology
Embryological Patterns of Development
Similarities in Macromolecules
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Organisms that appear very similar (like
barnacles and limpets) may not share a RCA
Natural selection has often caused convergent
evolution
Barnacles & limpets used to be classified
together based on superficial similarities
Evolutionary classification now shows
barnacles more closely related to crabs than
limpets
Appendages
Crab
Conical Shells
Barnacle
Limpet
CLASSIFICATION BASED ON
VISIBLE SIMILARITIES
Crustaceans
Crab
Gastropod
Barnacle
Evolutionary
Classification
Limpet
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Classification Using Cladograms
Refining the process, many biologists now
prefer a method called cladistic analysis
Identifies and considers only those
characteristics of organisms that are
evolutionary innovations
Characteristics that appear in recent parts of
a lineage but not in older membersderived characters
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Derived characters used to construct a
cladogram
Diagram that shows evolutionary
relationships among a group of organisms
Useful tools that help to show how one
lineage branched from another in the
course of evolution
Cladogram represents a type of evolutionary
tree
Appendages
Crab
Conical Shells
Barnacle
Limpet
Crustaceans
Crab
Gastropod
Limpet
Barnacle
Molted
exoskeleton
Segmentation
Tiny free-swimming larva
CLASSIFICATION BASED ON
VISIBLE SIMILARITIES
CLADOGRAM
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Similarities in DNA & RNA
All classification methods discussed so far
based primarily on physical similarities &
differences
Organisms with very different anatomies can
have common traits
Since DNA & RNA so similar across all
forms of life, provide method of comparing
organisms at genetic level
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Genes of many organisms show important
similarities at molecular level
Similarities in DNA can be used to help
determine classification & evolutionary
relationships
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Even genes of diverse organisms such as
humans and yeasts show many similarities
Humans have gene for muscle protein
myosin
Yeasts also have gene for protein myosin
Indicator that humans and yeasts share a
common ancestry
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Can also help show evolutionary
relationships of species and how species
have changed
More similar DNA sequences of two species,
more recently they shared a common
ancestor
More two species have diverged, less
similar their DNA will be
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Tree of Life Evolves
In Linnaeus’ time, scientific view of life was
simpler
Only known differences among living things
were traits that separated animals & plants
As biologists learned more about natural
world, realized Linnaeus’ two kingdoms did
not represent full diversity of life
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Microorganisms were first given a new
kingdom based on significant differences
from animals & plants (Protista)
Next, mushrooms, yeasts, & molds were
separated from the plants & placed into
own kingdom (Fungi)
Later on, bacteria were realized to be
missing membrane-bound organelles and
placed into own kingdom (Monera)
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Recently, as new data about bacteria
accumulated, Monera were recognized as
being two separate groups
As a result, Monera has been split into two
new kingdoms: Eubacteria &
Archaebacteria
Gives us six kingdom system: Eubacteria,
Archaebacteria, Protista, Fungi,
Plantae, & Animalia
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Three-Domain System
Some most recent evolutionary trees have
been produced using comparative studies
of rRNA found in all living things
These molecular analyses given rise to a
new taxonomic category, the domain
The domain is more inclusive categorylarger than a kingdom
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Three domains:
-Eukarya: composed of protists, fungi,
plants, & animals
-Bacteria: corresponds to kingdom
Eubacteria
-Archaea: corresponds to kingdom
Archaebacteria
Classification of Living Things
DOMAIN
Bacteria
Archaea
KINGDOM
Eubacteria
Archaebacteria
CELL TYPE
CELL
STRUCTURES
NUMBER OF
CELLS
MODE OF
NUTRITION
EXAMPLES
Eukarya
Protista
Prokaryote
Prokaryote
Eukaryote
Cell walls with
peptidoglycan
Cell walls without
peptidoglycan
Unicellular
Autotroph or
heterotroph
Streptococcus,
Escherichia coli
Fungi
Eukaryote
Plantae
Animalia
Eukaryote
Eukaryote
Cell walls of
Cell walls of
cellulose in some; chitin
some have
chloroplasts
Cell walls of
cellulose;
chloroplasts
No cell walls or
chloroplasts
Unicellular
Most unicellular;
Most
some colonial;
multicellular;
some multicellular some unicellular
Multicellular
Multicellular
Autotroph or
heterotroph
Autotroph or
heterotroph
Autotroph
Heterotroph
Methanogens,
halophiles
Mushrooms,
Amoeba,
yeasts
Paramecium,
slime molds, giant
kelp
Mosses, ferns,
flowering plants
Sponges,
worms, insects,
fishes,
mammals
Heterotroph
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Domain Bacteria
Unicellular and prokaryotic
Cells have thick, rigid cell walls (contain
peptidoglycan) around cell membrane
Ecologically diverse
Some photosynthesize, others do not
Some are aerobic, some anaerobic
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Domain Archaea
Unicellular and prokaryotic
Live in very extreme environments
Most are anaerobic
Cell walls lack peptidoglycan
Cell membranes contain unusual lipids not
found in other organisms
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Domain Eukarya
Consists of all organisms that have a
nucleus
Protista
Composed of eukaryotic organisms that
cannot be classified as plants, animals, or
fungi
Members display great variety
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Most are single-celled, some are multi-cellular
Some are photosynthetic, some are
heterotrophic
Some share characteristics with fungi, some
with plants, others with animals
Fungi
Heterotrophs
Feed on dead or decaying organic matter
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Secrete digestive enzymes into their food
source (unlike other heterotrophs)
Most are multicellular, some (yeasts) are
unicellular
Plantae
Multicellular, photosynthetic autotrophs
Nonmotile
Have cell walls that contain cellulose
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Includes cone-bearing & flowering plants as
well as mosses and ferns
Multicellular algae are found in Protista, not
Plantae
Animalia
Multicellular & heterotrophic
No cell walls
Most animals are motile, for at least part of
life cycle
DOMAIN
ARCHAEA
DOMAIN
EUKARYA
Kingdoms
DOMAIN
BACTERIA
Eubacteria
Archaebacteria
Protista
Plantae
Fungi
Animalia