CHAPTER 26
PHYLOGENY AND SYSTEMATICS
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What are the evolutionary
relationships between a
human, a mushroom, and
a flower?
A phylogeny based on
DNA data reveals that
animals (including
humans) and fungi
(including mushrooms)
are more closely related
to each other than to
plants.
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OVERVIEW
Phylogeny is the evolutionary history of a species or group
of species.
The discipline of systematics classifies organisms and
determines evolutionary relationships of living and extinct
organisms.
Systematists use fossil, molecular, and genetic data to infer
evolutionary relationships
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I. Concept 26.1: Classification
A. Taxonomy
The ordered division of organisms into categories
based similarities and differences
-useful component of systematics
B. Binomial Nomenclature (two word name)
1. First proposed by Carolus Linnaeus in 18th century
2. Two main characteristics:
a. Each species has a two-part name.
b. Species are organized hierarchically into broader
and broader groups of organisms.
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3. The first part of the name is the genus.
4. The second part, called the specific epithet, is
unique for each species within the genus.
5. Ex: Human—Homo sapiens which means wise man
C. Hierarchical Classification
1. Groups species into increasingly broad taxonomic
categories
2. Taxonomic groups from broad to narrow are:
domain, kingdom, phylum, class, order, family,
genus, and species
3. A taxonomic unit at any level of hierarchy is called a
taxon
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D. Phylogenetic Tree
1. Diagram used to show
the evolutionary
history of a group of
organisms
2. Can provide important
information about
similar characteristics
in closely related
species
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II. Concept 26. 2: Morphological and
Molecular Data
A. To infer relationships, systematists gather information
about morphologies, genes, and biochemistry of living
organisms
B. Organisms with similar morphologies or DNA sequences
are likely to be more closely related than organisms with
different structures or sequences
C. When constructing a phylogeny, systematists need to
distinguish whether a similarity is the result of homology
or analogy
1. Homology is similarity due to shared ancestry
Ex: bones of a whale’s flipper and a tiger’s paw
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2. Analogy is similarity due to convergent evolution
Convergent evolution occurs when two organisms developed
similarities as they adapted to similar environmental
challenges—not because they evolved from a common
ancestor
Ex: wing of a butterfly is analogous to wing of a bat because
both are adaptations for flight
3. Analogous structures or molecular sequences that evolved
independently are also called homoplasies
4. Homology can be distinguished from analogy by comparing
fossil evidence and the degree of complexity
5. The more complex two similar structures are, the more likely it
is that they are homologous
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D. Molecular systematics uses DNA and other molecular
data to determine evolutionary relationships.
The more alike the DNA sequences of two organisms,
the more closely related they are evolutionarily.
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III. Concept 26.3: Constructing
Phylogenetic Trees
A. Cladistics
1. Cladistics groups organisms by common descent
2. Once homologous characters have been identified,
they can be used to infer a phylogeny
3. A cladogram depicts patterns of shared characteristics
among taxa and forms the basis of a phylogenetic tree
4. A clade is a group of species that includes an ancestral
species and all its descendants
Not all groupings of organisms qualify as clades
5. A valid clade is monophyletic, signifying that it consists
of the ancestor species and all its descendants
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MONOPHYLETIC GROUP (CLADE)
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Group I, consisting of 3
species (A, B, C) and their
common ancestor (1), is a
clade, also called a
monophyletic group.
A monophyletic group
consists of an ancestral
species and all of its
descendants
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PARAPHYLETIC GROUP
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Group II is paraphyletic.
A paraphyletic grouping
consists of an ancestral
species and some, but not
all, of the descendants.
In this case Group II
consists of an ancestral
species (2) and some of its
descendants (species D, E,
F) but not all of them
(missing species G).
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POLYPHYLETIC GROUP
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Group III is polyphyletic.
A polyphyletic grouping
consists of various species
that lack a common
ancestor
In this case, species D, E, F,
and G share common
ancestor (2), but species C
has a different ancestor
(1).
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B. Shared Ancestral and Shared Derived Characters
1. A shared ancestral character is found not only in the
clade being analyzed but also in older clades
Ex: Backbone in vertebrates
2. A shared derived character is unique to a particular
clade.
 Ex: Hair in mammals
3. A character can be both ancestral and derived,
depending on the context
4. When inferring evolutionary relationships, it is useful
to know in which clade a shared derived character
first appeared
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Phylogenetic Tree
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IV. Concept 26.4: Genome
Documentation(Not on exam)
A. Comparing nucleic acids or other molecules to infer
relatedness is a valuable tool for tracing organisms’
evolutionary history
B. DNA that codes for ribosomal RNA changes relatively
slowly and is useful for investigating branching
points hundreds of millions of years ago
C. Mitochondrial (mt)DNA evolves rapidly and can be
used to explore recent evolutionary events
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V. Concept 26.5: Molecular Clocks(not
on exam)
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Molecular clocks are methods used to measure the
absolute time of evolutionary change based on the
observation that some genes and other regions of the
genome appear to evolve at constant rates.
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VI. Concept 26.6: Three-Domain
System
A. Supported by data from many sequenced genomes
B. A Comparison of the Three Domains of Life
Characteristics
Bacteria
Archaea
Eukarya
Nuclear Envelope
No
No
Yes
Membrane-enclosed organelles
No
No
Yes
Introns
No
Yes
Yes
Histone proteins associated with DNA
No
Yes
Yes
Circular Chromosome
Yes
Yes
No
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You should now be able to:
1.
2.
3.
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The taxonomic categories and how they indicate
relatedness
How systematics is used to develop phylogenetic
trees
The three domains of life including their similarities
and their differences