Chapter 18: Classification

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Chapter 18 Classification
18-1 History of Taxonomy
18-2 Modern Phylogenetic Taxonomy
18-3 Two Modern Systems of Classification
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18-1 History of Taxonomy
I. Early Systems of Classification (Aristotle,
Greece 2,100 years ago)

• Classified organisms as plants or animals BASED
  on HABITAT ? LAND, WATER, and AIR dwellers, used
  COMMON names.

• 17th Century DRAWBACKS Common names began to
  VARY from one locale to next Common names did
  NOT describe accurately AND stated nothing about
  INTERRELATEDNESS between organisms.

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(1) Taxonomy

• NAMES and GROUPS organisms according to
  MORPHOLOGY and EVOLUTIONARY history.

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II. Carolus Linnaeuss System (Swedish
botanist1700s)

• Grouped organisms INTO hierarchical categories,
  based on MORPHOLOGY. Father of
  Binomial-Nomenclature

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Critical Thinking
(1) Linnaeuss work was done many years before
Darwin and Mendel. Explain why many of
Linnaeuss categories are still relevant in light
of genetic and evolutionary relationships among
organisms.
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(A) Levels of Classification

• A nested hierarchy of 7 levels, allowing species
  to be GROUPED with increasingly RELATED organisms.

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(1) Kingdom Animalia
(2) Phylum Arthropoda
(3) Class Insecta
(4) Order Mantodea
(5) Family Mantidae
(6) Genus Mantus
(7) Species religiosa
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(B) Binomial Nomenclature (written in Latin and
italicized)

• Species name (scientific name) of an organism
  consists of 2 parts

(1) Genus (gets capitalized)
(2) Species Identifier (usually a descriptive
word, lowercase)
Exs Homo sapiens, Pisum sativum, Drosophila
melanogaster
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(1) Varieties

• Plants belong to SAME species BUT with slightly
  different MORPHOLOGY.
• Ex Peaches and nectarines are FRUITS of TWO
  slightly different varieties of the peach tree,
  Prunus persica.

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(2) Subspecies

• Variations of a species that occur in DIFFERENT
  geographic areas.

Ex Terrapene carolina triungui is a SUBSPECIES
of the common eastern box turtle, Terrapene
carolina, and gets its NAME from having THREE
(rather than four) toes on its HIND feet.
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(3) Phylogeny

• Evolutionary HISTORY of a species.

NOTE By using morphology, Linnaeus focused on
PHENOTYPES that were largely influenced by GENES
and are STILL clues of common ANCESTRY.
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18-2 Modern Phylogenetic Taxonomy
I. Systematics (Systematic Taxonomy)

• Organizes the DIVERSITY of species in context of
  EVOLUTION (i.e., uses PHYLOGENETIC
  approach to construct an evolutionary TREE)

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Critical Thinking
(2) Biologists believe that there are probably
millions of undescribed and unclassified species
on Earth. Why might so many species still be
undescribed or unclassified today?
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(1) Phylogenetic Tree

• Shows the relationships (or phylogeny) among
  groups of organisms.
• NOTE Although ONLY a hypothesis, a phylogenetic
  tree is BASED on several lines of EVIDENCE

(a) Fossil Record
(b) Morphology
(c) Embryology
(d) Biochemical Comparison (amino acid, DNA, and
chromosomes)
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(A) The Fossil Record

• MAY provide evidence, BUT a SYSTEMATIC
  TAXONOMIST would require OTHER sources of
  evidence.

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(B) Morphology

• The GREATER the number of HOMOLOGOUS structures
  between 2 species, the MORE closely-related
  phylogeny.

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(C) Embryological Patterns of Development

• EARLY development ? EVIDENCE of phylogeny
  BETWEEN species. (from ZYGOTE to BIRTH)

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(1) Blastula (in Kingdom Animalia)

• A ball of dividing cells arising from ZYGOTE
  only HOURS after fertilization (conception) a
  blastula is formed.

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(2) Blastopore (in the blastula)

• Small indentation becomes POSTERIOR end of
  digestive system in vertebrates AND echinoderms.
  
  (BUTin invertebratesit becomes
  the ANTERIOR end of the system)

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NOTE Each cell in an embryo of a VERTEBRATE or
ECHINODERM has the POTENTIAL to form an entire
organism (TWINS), WHEREAS in invertebrates, if
ANY cell is removed, it will DIE (pre-determined
parts).
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(3) Echinoderm (from embryological evidence)

• MORE closely related to VERTEBRATES than to
  other INVERTEBRATE species (including mollusks).

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(D) Chromosomes and Macromolecules (amino acids,
DNA)

• Biochemical comparisons ? changes in SEQUENCE
  are GREATER in species with MORE-DISTANT
  ancestors.

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II. Cladistics (MOST RECENT system of taxonomy)

• System of classification using SHARED DERIVED
  CHARACTERS to show relationships BETWEEN species.

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(1) Derived Characters (CRITERIA for a CLADOGRAM)

• A feature evolving ONLY within the GROUP under
  consideration.
• (Ex With birds, FEATHERS are an example of a
  DERIVED character that OTHER animals do NOT
  possess)

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Critical Thinking
(3) Legs are an example of a shared derived
character in vertebrates. Arthropods, such as
lobsters and crickets, also have legs, but they
are NOT accepted as a character shared with
vertebrates. Why might this be?
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Critical Thinking
(4) Why does the cladistic approach to
classification suggest that the class Reptila
(reptiles) is not a phylogenetic classification?
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(2) Cladogram (result of cladistic analysis)

• Shared DERIVED characters are evidence of COMMON
  ANCESTRY between SPECIES that share them.

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18-3 Two Modern Systems of Classification
I. Six-Kingdom System (replacing old system of 5)

• Research warrants TWO kingdoms of bacteria (not
  one), updating TO a SIX-kingdom system of
  LIFEFORMS.

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(A) Kingdom Archaebacteria (ANCIENT bacteria)

• Unicellular PROKARYOTIC ?many species are
  CHEMOAUTOTROPHIC while others are HETEROTROPHIC.

NOTE Adapted to living in HARSH, hostile
environments including sulfurous HOT Springs in
Yellowstone National Park and SALT Lakes.
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(B) Kingdom Eubacteria (modern bacteria)

• Unicellular PROKARYOTIC, most are AEROBIC,
  (others anaerobic), and either heterotrophic OR
  autotrophic (cyanobacteria).

NOTE Due do exponential growth AND a very short
generation time, bacteria have a RAPID
evolutionary RATE which may support their
RESISTANCE to environmental change.
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(C) Kingdom Protista

• Unicellular AND multicellular, EUKARYOTIC
  organisms that can be autotrophic OR
  heterotrophic.

NOTE Most unicellular AND aquatic, however can
be plant-like, animal-like, or fungus-like
protists.
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(D) Kingdom Fungi

• Unicellular AND multicellular, EUKARYOTIC
  organisms that ALL are heterotrophic.

NOTE Fungi species secrete ENZYMES to break down
organic materials into nutrients rather than
ingesting them. (i.e., EXTRA-cellular digestion)
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(E) Kingdom Plantae

• Multicellular EUKARYOTIC ? AUTOTROPHIC (and in
  RARE instances, heterotrophic) ? terrestrial AND
  aquatic species.

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(F) Kingdom Animalia

• Multicellular EUKARYOTIC ?heterotrophic use
  LOCOMOTION to obtain nutrition, mate, AND evade
  predation.

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II. Three-Domain System (NEW to classification,
ABOVE KINGDOMS)

• Biochemical comparison of rRNA ? shows HOW LONG
  since different species SHARED a common ancestor.

NOTE Since ALL organisms (even archaebacteria)
have ribosomes, the rRNA is used to study the
PHYLOGENY between ANY two SPECIES.
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Critical Thinking
(5) In the five-kingdom system, which is still
used by some scientists, all species of bacteria
are grouped into Kingdom Monera. Why might there
have been only one bacterial kingdom in the past?
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(1) Domains (THREE DOMAINS feed into SIX KINGDOMS)

• PHYLOGENETIC tree drawn from rRNA shows SPECIES
  fall into THREE broad groups or DOMAINS.

• Ex Plants, animals, and fungi are ONE SMALL
  TWIG of a large branch (a domain) that includes
  ALL eukaryotes.

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(2) Domain Archaea (OLDEST, ANCIENT domain)

• Represents species belonging to Kingdom of
  Archaebacteria.

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(3) Domain Bacteria (RECENTLY EVOLVED domain)

• Represents species belonging to the Kingdom of
  Eubacteria.

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(4) Domain Eukarya

• Represents ALL species belonging to Kingdoms of
  Protista, Fungi, Plantae, and Animalia.

• NOTE Remember ALL eukaryotes have a TRUE
  NUCLEUS with linear (non-plasmid) chromosomes AND
  membrane-bound organelles.

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Extra Slides AND Answers for Critical Thinking
Questions
(1) Linnaeus based his system on morphological
traits of an organism. Morphological traits are
heavily influenced by genes, which are clues to
common ancestry.
(2) Members of the class Reptilia appear to have
evolved from several different ancestral lines.
Reptiles consist of several groups that arose in
parallel.
(3) Answers will vary, but students might suggest
that because bacteria are small and fairly
simple, they have few characters on which to base
taxonomic decisions. Alternatively, they might
suggest that differences between eubacteria and
archaebacteria are mainly molecular and the
technology for identifying such differences has
only recently been developed.
(4) Many of the unclassified species may be few
in number or may be located in remote habitats.
(5) Arthropod legs are structurally very
different from the legs of vertebrates.
Arthropod legs are covered by an exoskeleton and
do not have an internal skeleton of bone.
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Revisiting Evolution

• All organisms on Earth are thought to have
  descended from a common ancestor.

• Evolutionary biologists recognize six major
  evolutionary pathways and classify organisms into
  six kingdoms.

Assessing Prior Knowledge

• What molecule in organisms is passed from one
  generation to the next? Why would similarities
  in this molecule between different organisms be a
  good basis for classification?

• Identify the first type of organisms that are
  believed to have appeared on Earth and explain
  how these organisms obtained nutrition.