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25
Phylogenetic Trees
• Systematics, the scientific study of the diversity
of organisms, reveals the evolutionary
relationships between organisms.
• Taxonomy, a subdivision of systematics, is the
theory and practice of classifying organisms.
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Phylogenetic Trees
• A phylogeny is a hypothesis which describes the
history of descent of a group of organisms from
their common ancestor.
• A phylogenetic tree represents that history.
• A lineage is represented as a branching tree, in
which each split or node represents a speciation
event.
Figure 25.1 How to Read a Phylogenetic Tree
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Phylogenetic Trees
• Systematists expect traits inherited from an
ancestor in the distant past to be shared by a
large number of species.
• Traits that first appeared in a more recent
ancestor should be shared by fewer species.
• These shared traits, inherited from a common
ancestor, are called ancestral traits.
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Phylogenetic Trees
• Any features (DNA sequences, behavior, or
anatomical feature) shared by two or more
species that descended from a common ancestor
are said to be homologous.
• For example, the vertebral column is homologous
in all vertebrates.
• A trait that differs from its ancestral form is called
a derived trait.
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Phylogenetic Trees
 Convergent evolution occurs when
independently evolved features subjected to
similar selective pressures become superficially
similar.
 Evolutionary reversal occurs when a character
reverts from a derived state back to an ancestral
state.
Figure 25.2 The Bones Are Homologous, but the Wings Are Not
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Steps in Reconstructing Phylogenies
• Systematists use many characters to reconstruct
phylogenies, including physiological, behavioral,
molecular, and structural characters of both living
and fossil organisms.
• The more traits that are measured, the more
inferred phylogenies should converge on one
another and on the actual evolutionary pattern.
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Steps in Reconstructing Phylogenies
• An important source of information for systematists
is morphology, which describes the sizes and
shapes of body parts.
• Early developmental stages of many organisms
reveal similarities to other organisms, but these
similarities may be lost in adulthood.
 Human gill slits during development.
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Steps in Reconstructing Phylogenies
• Molecular traits are also useful for constructing
phylogenies.
• The molecular traits most often used in the
construction of phylogenies are the structures of
nucleic acids (DNA and RNA) and proteins.
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Steps in Reconstructing Phylogenies
• Comparing the primary structure of proteins:
 Homologous proteins are obtained and the
number of amino acids that have changed since
the lineages diverged from a common ancestor
are determined.
• DNA base sequences:
 Chloroplast DNA (cpDNA) and mitochondrial
DNA (mtDNA) have been used extensively to
study evolutionary relationships.
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Steps in Reconstructing Phylogenies
• Relationships between apes and humans were
investigated by sequencing a hemoglobin
pseudogene.
• Chimpanzees and humans share a more recent
common ancestor with each other than they do
with gorillas.
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Reconstructing a Simple Phylogeny
• A simple phylogeny can be constructed using
eight vertebrates species: lamprey, perch, pigeon,
chimpanzee, salamander, lizard, mouse, and
crocodile.
• Traits that are either present (+) or absent (–) are
used in the phylogeny.
Table 25.1 Eight Vertebrates Ordered According to Unique Shared Derived Traits (Part 1)
Table 25.1 Eight Vertebrates Ordered According to Unique Shared Derived Traits (Part 2)
Figure 25.5 A Probable Phylogeny of Eight Vertebrates
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Biological Classification and
Evolutionary Relationships
• The system of biological classification used today
was developed by Carolus Linnaeus in 1758.
• His two-name system is referred to as binomial
nomenclature.
• The first name identifies the genus; the other name
identifies the species.
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Biological Classification and
Evolutionary Relationships
• Homo sapiens is the name of the modern human
species.
• The generic name is always capitalized, whereas
the specific name is not, and both names are
italicized.
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Phylogenetic Trees Have Many Uses
• Biologists determined that the hundreds of diverse
cichlids could not have evolved in such a short time.
• A new phylogeny of the cichlids of Lake Victoria and
other lakes in the region was developed using 300
mtDNA sequences.
• This phylogeny suggested that the ancestors of the
Lake Victoria cichlids came from the much older lake
Kivu.
• The phylogeny also indicated that some of the
cichlid lineages found only in Lake Victoria split at
least 100,000 years ago, suggesting that the lake did
not completely dry up about 15,000 years ago.