Transcript Taxonomy and Systematics

Systematics and the Phylogenetic Revolution
Chapter 25
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The Classification of Organisms
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Classification organisms first classified by
Aristotle over 2,000 years ago
Classification scheme of the Middle Ages
was replaced with a binomial system by
Linnaeus about 250 years ago.
binomial - two-part name for each species
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Species Names
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Taxa - group of organisms at a particular
level in a classification system (taxonomy)
By convention:
– first word of binomial name is genus and is
always capitalized
– second word refers to particular species
and is not capitalized
 together form scientific name, written in
italics
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The Taxonomic Hierarchy
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Species
Genus
Family
Order
Class
Phylum
Kingdom
Domain
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Evolutionary Relationships
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Systematics - reconstruction and study of
evolutionary relationships
– construct phylogeny by looking at
similarities and differences between
species
– Cladistics - distinguishes ancestral from
shared characters
 only shared derived characters are
considered in determining evolutionary
relationships
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Cladistics
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A cladogram is constructed that depicts
hypothesis of evolutionary relationships
– Species that share derived characters
belong to a clade.
– Each node represents a hypothetical
ancestral species.
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Cladogram
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Cladistics
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Outgroup comparison A
species or group of
species closely related to,
but not a member of, the
group under study is
designated an outgroup.
 Character states
exhibited by the
outgroup are
assumed ancestral,
and other states are
considered derived.
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Principle of parsimony favors the hypothesis
that requires the fewest assumptions
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Cladogram
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Systematics and classification
– A monophyletic group includes the most
recent common ancestor of the group and all
of its descendants.
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paraphyletic group includes the most recent common
ancestor of the group, but not all of its descendents.
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A polyphyletic group does not include the most recent common
ancestor of all the members.
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Analogy Versus Homology
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Examination of the characters on a
cladogram can provide insight on how they
evolved, how many times they evolved, and
how complex characters evolved.
Wings in birds and insects
Eyes in mammals and squid
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The Kingdoms of Life
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Most biologists use a sixkingdom system.
– Animalia
– Plantae
– Fungi
– Protista
– Archaebacteria
– Bacteria
Domains - taxonomic level
above kingdoms
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Impact of Molecular Cladistics
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Traditional classification systems are based
on similar traits, but do not take into account
evolutionary relationships.
– no clear basis for determining relative
importance of traits
Systematic phylogenetics is based on
evolutionary relationships using cladistics.
– Each clade has a single common
ancestor.
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Tree of Life
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Domain Archaea
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Share key characteristics:
– cells wall lack peptidoglycan
– possess unusual lipids and ribosomal RNA
Three general categories
– methanogens - obtain energy using
hydrogen gas to reduce carbon dioxide to
methane gas
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Domain Archaea
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extremophiles - grow under extreme
conditions
– thermophiles (heat)
– halophiles (salt)
– pH tolerant
– pressure tolerant
nonextreme archaebacteria - grow in same
environment as eubacteria
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Domain Bacteria
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most abundant organisms on earth
most taxonomists recognize 12-15 major
groups
as different from archaebacteria as from
eukaryotes
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Domain Eukarya
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Appear in fossil record only about 1.5 billion
years ago.
– complex cellular organization
– Fungi, Plants, and Animals are welldefined evolutionary groups.
 largely multicellular
– Diversity among protists is much greater
than within or between Fungi, Plants, and
Animals.
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Domain Eukarya
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With few exceptions, all modern eukaryotic
cells possess energy-producing organelles
(mitochondria).
– Some protist phyla have also acquired
chloroplasts and are photosynthetic.
 Mitochondria and chloroplasts are both
believed to have entered early
eukaryotic cells by endosymbiosis.
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Domain Eukarya
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Key characteristics
– Compartmentalization
 Discrete compartments provide
evolutionary opportunities for increased
specialization within the cell.
– Multicellularity
 Activities of individual cells are
coordinated and the cells themselves
are in contact.
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Domain Eukarya
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Key characteristics
– Sexuality
 alternate between syngamy and meiosis
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Impact of Molecular Cladistics
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Molecular systematics labels
protists as paraphyletic.
Insects may be a sister group of crustaceans.
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Major Mammalian Groups
Eutherian mammals may be reclassified.
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