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Teresa Audesirk • Gerald Audesirk • Bruce E. Byers
Biology: Life on Earth
Eighth Edition
Lecture for Chapter 18
Systematics: Seeking
Order Amidst Diversity
Copyright © 2008 Pearson Prentice Hall, Inc.
Chapter 18 Outline
• 18.1 How Are Organisms Named and
Classified? p. 358
• 18.2 What Are the Domains of Life? p. 360
• 18.3 Why Do Classifications Change? p. 364
• 18.4 How Many Species Exist? p. 366
Section 18.1 Outline
• 18.1 How Are Organisms Named and
Classified?
– Classification Originated as a Hierarchy of
Categories
– Systematists Identify Features That Reveal
Evolutionary Relationships
– Anatomy Plays a Key Role in Systematics
– Molecular Similarities Are Also Useful for
Reconstructing Phylogeny
Systematics
• Systematics is the branch of biology
concerned with
– Reconstructing phylogeny (evolutionary
history)
– Naming organisms and placing them into
hierarchical categories based upon their
evolutionary relationships
Major Categories of Classification
• The eight major categories of classification,
in order of decreasing inclusiveness are
– Domain
– Kingdom
– Phylum
– Class
– Order
– Family
– Genus
– Species
Scientific Names
• The scientific name of an organism is
formed from the genus and species
– The genus Sialia (bluebirds) includes three
species:
• Sialia sialis (the eastern bluebird)
• Sialia mexicana (the western bluebird)
• Sialia currucoides (the mountain bluebird)
Scientific Names
• Each two-part scientific name is unique
and recognized worldwide
Scientific Names
• Scientific names are always underlined or
italicized
– The first letter of the genus name is always
capitalized
– The first letter of the species name is always
lower case
• The species name is always paired with its
genus name
The Origin of Classification
• Aristotle (384-322 B.C.)
– Was among first to develop a standardized
language for naming organisms
– Classified about 500 organisms into 11
hierarchical categories based on various
characteristics
The Origin of Classification
• Carolus Linnaeus (1707-1778)
– Laid the groundwork for the modern
classification system
– Placed organisms into hierarchical categories
based on their resemblance to other
organisms
– Introduced the scientific name composed of
genus and species
The Origin of Classification
• Charles Darwin (1809-1882)
– Published On the Origin of Species, which
demonstrated that all life is related by
common ancestry
Evolutionary Relationships
• Biologists realized that taxonomic
categories should reflect evolutionary
relatedness
– The more categories two organisms share,
the closer their evolutionary relationship
Evolutionary Relationships
• All organisms share certain similarities
• Similarities result from common ancestry
or convergent evolution
Present-Day Classification
• Systematists determine evolutionary
relationships based on similarities due to
common ancestry
– Similarities may be anatomical or molecular
Anatomical Similarities
• Systematists examine similarities in
external body structure
Anatomical Similarities
• Systematists examine similarities in
internal body structures, such as skeletons
and muscles
Anatomical Similarities
• Systematists examine microscopic
similarities to discern finer details
– Number and shape of the “teeth” on the
tongue-like radula of a snail
– Shape and position of the bristles on a marine
worm
– External structure of pollen grains of a
flowering plant
Molecular Similarities
• Systematists examine genetic similarities
between:
– DNA nucleotide sequences
– Chromosome structure
• It has been estimated that 99% of the chimpanzee
genome is identical to that of humans
Section 18.2 Outline
• 18.2 What Are the Domains of Life?
– The Five-Kingdom System Improved
Classification
– A Three-Domain System More Accurately
Reflects Life’s History
– Kingdom-Level Classification Remains
Unsettled
The Two-Kingdom System
• Before 1969, all forms of life were
classified into two kingdoms
– Animalia
– Plantae (included plants, bacteria, fungi and
photosynthetic eukaryotes)
The Five-Kingdom System
• Proposed by Robert H. Whittaker (1969)
• Kingdoms include
– Monera (all prokaryotes)
– Plantae
– Fungi
– Animalia
– Protista (eukaryotes that are not plants, fungi,
or animals)
The Three-Domain System
• Introduced by Carl Woese (1990)
• Discovered that kingdom Monera included
two very distinct groups (Bacteria and
Archaea) based on nucleotide sequences
of ribosomal RNA
The Three-Domain System
• Domains include
– Bacteria (prokaryotic)
– Archaea (prokaryotic)
– Eukarya (eukaryotic)
Kingdom-Level Classification
• Systematists have yet to reach a
consensus about the precise definitions of
new prokaryotic and eukaryotic kingdoms
• Figure 18-6, p. 364, shows the
evolutionary relationships among some
members of the domain Eukarya…
Section 18.3 Outline
• 18.3 Why Do Classifications Change?
– Species Designations Change When New
Information Is Discovered
– The Biological Species Definition Can Be
Difficult or Impossible to Apply
New Information Is Discovered
• Systematists regularly propose changes in
species-level classification
New Information Is Discovered
• African elephant species has been divided
into two species, the savannah elephant
and the forest elephant
– Discovered that the two groups have little
gene flow between them
New Information Is Discovered
• Red wolves may not be a distinct species
– DNA evidence suggests that red wolves are
hybrids between gray wolves and coyotes
Species Definitions Change
• The biological species concept defines
species as “groups of interbreeding natural
populations, which are reproductively
isolated from other such groups”
– Cannot be applied to asexually reproducing
organisms
Species Definitions Change
• Alternative species definitions have been
proposed, one of which is the phylogenetic
species concept
Phylogenetic Species Concept
• The phylogenetic species concept defines
a species as “the smallest diagnosable
group that contains all the descendants of
a single common ancestor”
– Can be applied to sexually and asexually
reproducing organisms
– May eventually replace the biological species
concept
Section 18.4 Outline
• 18.4 How Many Species Exist?
How Many Species Exist?
• Biodiversity is the total number of
species in an ecosystem
How Many Species Exist?
• Number of named species is currently
about 1.5 million (biased toward large
organisms in temperate regions)
– 5% prokaryotes and protists
– 22% plants and fungi
– 73% animals
How Many Species Exist?
• Estimated that 7 million to 10 million
species may exist
How Many Species Exist?
• Between 7,000 and 10,000 new species are
identified annually, mostly in the tropics
• Tropical rain forests are believed to be
home to two-thirds of the world’s existing
species, most of which have yet to be
named
How Many Species Exist?
• Because tropical rain forests are being
destroyed so rapidly, species may become
extinct before we ever knew they existed