The Tree of Life

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The Tree of Life
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Why Classify?

• We classify to make organisms easier to study.
• To study the diversity of life, biologists use a
  classification system to name organisms and group
  them in a logical manner.
• The field of taxonomy allows scientists to
  classify organisms and assign each organism a
  universally accepted name in order to study the
  diversity of life.

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Linnaeuss System of Classification

• During the eighteenth century, Swedish botanist
  Carolus Linnaeus developed a two-word naming
  system called binomial nomenclature
• Linnaeus often classified organisms based on
  similarities in structures and details of anatomy

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Binomial Nomenclature

• In binomial nomenclature, each species is
  assigned a 2-part Latin name
• The name is always written in italics with the
  1st word is always capitilized
• Example Homo sapiens (humans)
• The 1st part of the name is the genus and the
  second part of the name is unique to each species
  within that genus

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Linnaeuss System of Classification

• Linnaeuss system of classification uses 7
  taxonomic categories (from largest to smallest)
• Kingdom largest most inclusive
• Phylum includes many different organisms that
  share important characteristics
• Class composed of similar orders
• Order composed of similar families
• Family genera that share many characteristics
• Genus a group of closely related species
• Species a group of individuals so similar that
  they can breed and produce fit offspring

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Human Classification

• Kingdom Anamalia
• Phylum Chordata
• Subphylum Vertebrata
• Class Mammalia
• Order Primates
• Family Hominidae
• Genus Homo
• Species Homo sapiens

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KINGDOM Animalia
PHYLUM Chordata
CLASS Mammalia
ORDER Carnivora
FAMILY Ursidae
GENUS Ursus
SPECIES Ursus arctos
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Problems with Traditional Classification

• During Linnaeuss time, scientists classified
  organisms based on their physical appearancebut
  todaywe know that doesnt always work.
• Ex Dolphins ? Fish or Mammals?
• Remember Convergent Evolution ? sometimes
  organisms that are different from each other
  evolve similar body structures, due to the change
  in the environment.
• This does not mean they need to be classified in
  the same groupmaybe they only evolved similar
  body structures!
• These situations make it very difficult for
  scientists to classify.

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Evolutionary Classification

• Biologists now group organisms into categories
  that represent lines of evolutionary descent, not
  just physical features
• Evolutionary classification (phylogeny) is the
  strategy of grouping organisms together based on
  their evolutionary history

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Traditional v/s Evolutionary Classification
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Classification Using Cladograms

• To refine evolutionary classification, biologists
  now prefer a method called cladistics
• Cladistics considers only those characteristics
  that are new characteristics that arise as
  lineages evolve over time
• Characteristics that appear in recent parts of a
  lineage but not in its older members are called
  derived characters
• Characteristics that are shared by all members of
  a lineage are ancestral/primitive characters and
  should not be used when building cladograms.
• Cladogram a diagram that shows the evolutionary
  relationships among a group of organisms
  includes new characteristics that arise as
  lineages evolve.

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Derived Characteristics
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Anatomy of the Cladogram
Monophyletic group clade
J
Taxon
I
F G H
C D E
Paraphyletic group
Lineage (represents a sequence of
ancestor-descendent populations)
A B
MONOPHYLETICGROUPS include ancestor all
descendents PARAPHYLETIC GROUPS includes
ancestor and some, but not all descendent
Node (represents most recent common ancestor of
two or more taxa)
Synapomorphy (shared derived character)
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Monophyletic Clades

• A valid clade is monophyletic signifying that it
  consists of the ancestor species and all its
  descendants.

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Outgroups

• Systematists use a method called outgroup
  comparison
• To differentiate between shared derived and
  shared primitive characteristics
• As a basis of comparison we need to designate an
  outgroup
• which is a species or group of species that is
  closely related to the ingroup, the various
  species we are studying
• Outgroup comparison
• Is based on the assumption that homologies
  present in both the outgroup and ingroup must be
  primitive characters that predate the divergence
  of both groups from a common ancestor

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Performing Outgroup Comparison
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Outgroup Comparisons

• First, look at the animals we are studying and
  establish which characteristics that they share
  which are unique to each individual species.
• Then use these derived characters to build your
  cladogram.

CELLS BACKBONE LEGS HAIR OPPOSABLE THUMB
Slug
Catfish
Frog
Tiger
Human
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Opposable Thumbs
Hair
Legs
Backbone
Cells
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The Three-Domain System

• Molecular analyses have given rise to the most
  current classification system the Three Domain
  System
• The 3 Domain System is the most recent
  classification system and includes
• Bacteria
• Archaea
• Eukarya

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Domains are BIGGER/MORE INCLUSIVE than kingdoms
Domains Kingdoms
Bacteria Eubacteria
Archaea Archaebacteria
Eukarya Protista Fungi Plantae Animalia
These 2 kingdoms used to be combined into one
called Monera
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Remember - There are 3 Methods of Classifying
Organisms

• The 3 Domain System (most current)
• Bacteria, Archae, Eukarya
• The 5 Kingdom System (Whittaker)
• Monera, Protista, Fungi, Plantae, Animalia
• The 6 Kingdom System still used in conjunction
  with the 3 domain system
• Archaebacteria, Eubacteria, Protista, Fungi,
  Plantae, Animalia

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Section 18-3
Classification of Living Things
Go to Section
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Domain Bacteria

• Kingdom Eubacteria
• Unicellular
• Prokaryotic
• Cell wall of peptidoglycan
• Autotrophic or heterotrophic
• Examples Streptococcus, E. Coli

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Domain Archaea

• Kingdom Archaebacteria
• Unicellular
• Prokaryotic
• Cell walls w/out peptidoglycan
• Autotrophic or heterotrophic
• Methanogens halophiles

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Domain Eukarya
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Whittakers five-kingdom system
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Our changing view of biological diversity
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The Three Domain System

• Describes classification as
• Not all prokaryotes are closely related (not
  monophyletic)
• Prokaryotes split early in the history of living
  things (not all in one lineage)
• Archaea are more closely related to Eukarya than
  to Bacteria
• Eukarya are not directly related to Eubacteria
• There was a common ancestor for all extant
  organisms (monophyletic)
• Eukaryotes are more closely related to each other
  (than prokaryotes are to each other)