Classification
Chapter 18
History of Taxonomy
Section 18-1
History of Taxonomy
• Taxonomy is the
branch of biology that
names and groups
organisms according
to their
characteristics and
evolutionary history.
Aristotle’s Classification
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Organisms were first classified more than 2,000 years ago
by the Greek philosopher Aristotle.
Aristotle classified living things as either plants or animals.
He grouped animals into land dwellers, water dwellers, and
air dwellers.
He also grouped plants into three categories, based on
differences in their stems.
Linnaeus’s System
• Carolus Linnaeus
devised a system of
grouping organisms
into hierarchical
categories.
• Linnaeus used an
organism’s
morphology, its form
and structure, to
categorize it.
Levels of Classification
• Linnaeus devised a nested
hierarchy of seven different
levels of organization.
• Linnaeus’s largest category is
called a kingdom.
• There are two kingdoms, plant
and animal, which are the same
as Aristotle’s main categories.
Levels of Classification
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Each subset within a kingdom is
known as a phylum, in the animal
kingdom, or a division, in the plant
kingdom.
Within a phylum or division, each
subset is called a class, and each
subset within a class is called an
order.
Still smaller groupings are the
family and then genus.
The smallest grouping of all, which
contains only a single organism
type, is known as the species.
Binomial Nomenclature
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In Linnaeus’s system, the
species name (also called
the scientific name) of an
organism has two parts.
The first part of the name
is the genus, and the
second part is the species
identifier, usually a
descriptive word.
This system of two-part
names is known as
binomial nomenclature.
Binomial Nomenclature
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Linnaeus’s choice of seven
levels of classification was
arbitrary.
Other levels have been added.
Botanists sometimes split
species into subsets known as
varieties
Zoologists refer to variations of
a species that occur in different
geographic area as subspecies.
Phylogeny
• To classify
organisms, modern
taxonomists consider
the phylogeny or
evolutionary history,
of the organism.
• Much of Linnaeus’s
work in classification
is relevant today,
even in this
phylogenetic context.
Modern Phylogenetic Taxonomy
Section 18-2
Modern Taxonomy
•
When placing an
organism into a
taxonomic category,
modern taxonomists may
consider
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Morphology
Chromosomal
characteristics
Nucleotide and amino
acid sequences
Embryological
development
Systematics (1)
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Modern taxonomists
agree that the
classification of
organisms should reflect
their phylogeny.
The application of
phylogeny is a
cornerstone of a branch
of biology called
systematics
Systematics (2)
• Systematics organizes
the diversity of living
things in the context
of evolution.
• Systematic
taxonomists use
several lines of
evidence to construct
a phylogenetic tree.
Systematics (3)
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A phylogenetic tree is a
family tree that shows the
evolutionary
relationships thought to
exist among groups of
organisms
A phylogenetic tree
represents a hypothesis,
and it is generally based
on several lines of
evidence.
Cladistics (1)
• One relatively new
system of
phylogenetic
classification is
called cladistics.
• Cladistics uses
shared derived
characters to
establish evolutionary
relationships.
Cladistics (2)
• A shared derived
character is a
feature that
apparently evolved
only within the
group under
consideration.
Creation of a Cladogram
Modern Classification
•
Another feature that is
considered in
classification are
homologous features,
features that have similar
structure and come from
similar embryonic layers
but have completely
different functions
Modern Classification
•
Analogous features are
ones that come from
different embryological
development but look
similar and perform
similar functions, like the
wings of bats and the
wings of insects.
Modern Classification
•
A phylogenetic tree is
subject to change as new
information rises from
different lines of
evidence
(a) Fossil Record
(b) Morphology
(c) Embryological
Development
(d) Chromosomes &
Macromolecules
(a) Fossil Record
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•
Fossil record often
provides clues to
evolutionary
relationships.
The fossil record may
provide the framework of
a phylogenetic tree, but a
systematic taxonomist
would seek to confirm the
information it provided
with other lines of
evidence.
(b) Morphology
• Taxonomists
study an
organism’s
morphology
and compare it
with the
morphology of
other living
organisms.
(c) Embryological Patterns of Development
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•
Early patterns in
embryological
development provide
evidence of phylogenetic
relationships
They also provide a
means of testing
hypotheses about
relationships that have
been developed from
other lines of evidence.
(c) Embryological Patterns of Development
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The fertilized egg is
known as a zygote which
undergoes several cell
divisions to become a
hollow ball of cells called
a blastula
A small indent on the
blastula develops, this is
the blastopore.
(c) Embryological Patterns of Development
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The blastopore will
develop into an opening
of the digestive tract.
In echinoderms and
chordates the blastopore
becomes the anus.
In all other animals the
blastopore becomes the
mouth.
This leads to the
conclusion that
vertebrates and
echinoderms are
more closely
related.
(d) Chromosomes & Macromolecules
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Taxonomists use comparisons of macromolecules such as DNA, RNA,
and proteins as a kind of “molecular clock”
Scientists compare amino acid sequences for homologous protein
molecules of different species.
The number of differences is a clue to how long ago two species
diverged from a shared evolutionary ancestor.
Two Modern Systems of
Classification
Section 18-3
Six-Kingdom System
A classification
system that
recognizes two
broad types of
bacteria has lead
to the
development of a
classification
system that
utilizes six
kingdoms.
Prokaryotic Cells
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Biologists have
discovered cells that only
have DNA as a single
strand floating in the
cytoplasm of the cell –
these are known as
prokaryotic cells.
Bacteria are prokaryotic
organisms
Kingdom Archaebacteria
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Unicellular prokaryotes
Unique cell membranes
Unique biochemical and
genetic properties
Some species are
autotrophic, producing
food by chemosynthesis
Many live in harsh
environments
The prefix archae comes
from the Greek word for
“ancient”
Reproduce by binary
fission
Kingdom Eubacteria
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The eu part of eubacteria
means “true.”
Unicellular prokaryotes
Most species of
eubacteria use oxygen,
but a few species cannot
live in the presence of
oxygen.
Reproduce by binary
fission.
Eukaryotic Cells
• Cells that have
their DNA
surrounded by a
membrane are
known as
eukaryotic cells.
• Plant and animal
cells are
eukaryotic cells.
Kingdom Protista
• Made up of a variety of
eukaryotic, mostly
single-celled organisms
• They have a membranebound true nucleus with
linear chromosomes,
and they have
membrane-bound
organelles.
Kingdom Fungi
• Made up of
heterotrophic
unicellular and mostly
multicellular, eukaryotic
organisms
• Absorb nutrients rather
than ingesting them
Kingdom Plantae
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Consists of multicellular
eukaryotics
All except for a few
parasitic forms are
autotrophic and use
photosynthesis as a source
of energy.
Most live on land.
Most have a sexual cycle
based on meiosis.
Kingdom Animalia
• Made up of eukaryotic ,
multicellular ,
heterotrophic organisms
• They ingest their food
• Most all animals have a
standard sexual cycle
that employs meiosis.
Three Domain System
• The young science of molecular biology has led
to an alternative to the six-kingdom system
• By comparing sequences of ribosomal RNA in
different organisms has been used to estimate
how long ago pairs of different organisms shared
a common ancestor.
• The phylogenetic tree from these data shows that
lving things fall into three broad groups, or
domains.
Three-Domain System
Three-Domain System
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Domain Archaea is known as the kingdom
Archaebacteria in the six-kingdom system.
Domain Bacteria is known as the kingdom Eubacteria in
the six-kingdom system.
Domain Eukarya consists of the protists, the fungi, and
the plants and animals.
Taxonomic Kingdoms of Life