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CAMPBELL BIOLOGY IN FOCUS
2014 Pearson Education, Inc.
Urry Cain Wasserman Minorsky Jackson Reece
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
20 Phylogeny
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2014 Pearson Education, Inc.
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2014 Pearson Education, Inc.
From Two Kingdoms to Three Domains
Early taxonomists classified all species as either plants or animals
Later, five kingdoms were recognized: Monera (prokaryotes), Protista, Plantae, Fungi, and Animalia
More recently, the three-domain system has been adopted: Bacteria, Archaea, and Eukarya
The three-domain system is supported by data from many sequenced genomes
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2014 Pearson Education, Inc.
The three-domain system highlights the importance of single-celled organisms in the history of life
Domains Bacteria and Archaea are single-celled prokaryotes
Only three lineages in the domain Eukarya are dominated by multicellular organisms, kingdoms
Plantae, Fungi, and Animalia
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2014 Pearson Education, Inc.
The Important Role of Horizontal Gene Transfer
The tree of life suggests that eukaryotes and archaea are more closely related to each other than to
bacteria
The tree of life is based largely on rRNA genes, which have evolved slowly, allowing detection of
homologies between distantly related organisms
Other genes indicate different evolutionary relationships
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2014 Pearson Education, Inc.
There have been substantial interchanges of genes between organisms in different domains
Horizontal gene transfer is the movement of genes from one genome to another
Horizontal gene transfer occurs by exchange of transposable elements and plasmids, viral infection,
and fusion of organisms
Horizontal gene transfer complicates efforts to build a tree of life
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2014 Pearson Education, Inc.
Horizontal gene transfer may have been common enough that the early history of life is better depicted
by a tangled web than a branching tree
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2014 Pearson Education, Inc.
Figure 20.20
Forams
Ciliates
Euglenozoans
Diatoms
COMMON ANCESTOR OF ALL LIFE
Land plants
Animals
Amoebas
Fungi
Red algae
Chlamydias
Green algae
(Mitochondria)*
Methanogens
Proteobacteria
Nanoarchaeotes
Thermophiles
Do
ma
in E
uk
ary
a
Gram-positive bacteria
(Chloroplasts)*
Spirochetes
Cyanobacteria
Do
ma
in B
ac
teria
D
om
ain
A
rch
ae
a
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2014 Pearson Education, Inc.
Figure 20.20a
Forams
Ciliates
Euglenozoans
Diatoms
Land plants
Animals
Amoebas
Fungi
Red algae
Green algae
Do
main
Eu
kary
a
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2014 Pearson Education, Inc.
Phylogeny is the evolutionary history of a species or group of related species
The discipline of systematics classifies organisms and determines their evolutionary relationships
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2014 Pearson Education, Inc.
Concept 20.1: Phylogenies show evolutionary relationships
Taxonomy is the ordered division and naming of organisms
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2014 Pearson Education, Inc.
A taxonomic unit at any level of hierarchy is called a taxon
The broader taxa are not comparable between lineages
For example, an order of snails has less genetic diversity than an order of mammals
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2014 Pearson Education, Inc.
The two-part scientific name of a species is called a binomial
The first part of the name is the genus
The second part, called the specific epithet, is unique for each species within the genus
The first letter of the genus is capitalized, and the entire species name is italicized
Both parts together name the species (not the specific epithet alone)
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2014 Pearson Education, Inc.
Figure 20.3
Species: Panthera pardus
Kingdom: Animalia
Domain: Archaea
Domain: Bacteria
Domain: Eukarya
Genus: Panthera
Order: Carnivora
Family: Felidae
Class: Mammalia
Phylum: Chordata
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Linking Classification and Phylogeny
Systematists depict evolutionary relationships in branching phylogenetic trees
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A phylogenetic tree represents a hypothesis about evolutionary relationships
Each branch point represents the divergence of two taxa from a common ancestor
Sister taxa are groups that share an immediate common ancestor
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2014 Pearson Education, Inc.
Figure 20.5
1
2
3
4
5
Branch point: where lineages diverge
This branch point represents the common ancestor of taxa AG.
This branch point forms a polytomy: an unresolved pattern of divergence.
ANCESTRAL LINEAGE
Sister taxa
Basal taxon
Taxon A
Taxon B
Taxon C
Taxon D
Taxon E
Taxon F
Taxon G
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2014 Pearson Education, Inc.
What We Can and Cannot Learn from Phylogenetic Trees
Phylogenetic trees show patterns of descent, not phenotypic similarity
Phylogenetic trees do not generally indicate when a species evolved or how much change occurred in a
lineage
It should not be assumed that a taxon evolved from the taxon next to it
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2014 Pearson Education, Inc.
Applying Phylogenies
Phylogeny provides important information about similar characteristics in closely related species
Phylogenetic trees based on DNA sequences can be used to infer species identities
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2014 Pearson Education, Inc.
Sorting Homology from Analogy
When constructing a phylogeny, systematists distinguish whether a similarity is the result of
homology or analogy
Homology is similarity due to shared ancestry
Analogy is similarity due to convergent evolution
Convergent evolution occurs when similar environmental pressures and natural selection
produce similar (analogous) adaptations in
organisms from different evolutionary lineages
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Evaluating Molecular Homologies
Molecular homologies are determined based on the degree of similarity in nucleotide sequence
between taxa
Systematists use computer programs when analyzing comparable DNA segments from
different organisms
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Cladistics
Cladistics classifies organisms by common descent
A clade is a group of species that includes an ancestral species and all its descendants
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Shared Ancestral and Shared Derived Characters
In comparison with its ancestor, an organism has both shared and different characteristics
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A shared ancestral character is a character that originated in an ancestor of the taxon
A shared derived character is an evolutionary novelty unique to a particular clade
A character can be both ancestral and derived, depending on the context
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2014 Pearson Education, Inc.
Figure 20.11
Leopard
Turtle
Frog
Bass
Lamprey
Lancelet (outgroup)
Hair
Amnion
Four walking legs
Hinged jaws
Vertebral column
Le
op
ard
Tu
rtle
Fro
g
Ba
ss
La
mp
rey
La
nc
ele
t (o
utg
rou
p)
Hair
Amnion
Four walking legs
Hinged jaws
Vertebral column
(backbone)
CH
AR
AC
TE
RS
TAXA
(b) Phylogenetic tree (a) Character table
0 1 0
1
0
1
0
1
0
1 0 1
0
1
0
1
0
1
0
1
0
1 0
1 0 1
0 1 0
1
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2014 Pearson Education, Inc.
An outgroup is a species or group of species that is closely related to the ingroup, the various species
being studied
The outgroup is a group that has diverged before the ingroup
Systematists compare each ingroup species with the outgroup to differentiate between shared derived
and shared ancestral characteristics
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2014 Pearson Education, Inc.
Characters shared by the outgroup and ingroup are ancestral characters that predate the divergence of
both groups from a common ancestor
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2014 Pearson Education, Inc.
Phylogenetic Trees with Proportional Branch Lengths
In some trees, the length of a branch can reflect the number of genetic changes that have taken place in
a particular DNA sequence in that lineage
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2014 Pearson Education, Inc.
Figure 20.12
Mouse
Human
Chicken
Frog
Zebrafish
Lancelet
Drosophila
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2014 Pearson Education, Inc.
In other trees, branch length can represent chronological time, and branching points can
be determined from the fossil record
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2014 Pearson Education, Inc.
Figure 20.13
Mouse
Human
Chicken
Frog
Zebrafish
Lancelet
Drosophila
Millions of years ago
65.5 Present 542
PALEOZOIC
251
MESOZOIC CENOZOIC
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Figure 20.14
Species I Species II Species III
Three phylogenetic hypotheses: I I
I
II II
II III
III
III
Species I
Species II
Species III
Ancestral sequence
Site
Technique
Results
6 events 7 events 7 events
1 2 3 4
A G T T
C T A T
C T T C
A G A C
I I
I
II II
II III
III
III
3/A 4/C
I I
I
II II
II III
III
III
2/T 4/C 2/T 3/A
4/C
2/T 3/A 3/A
4/C 2/T
3/A 2/T 4/C
I I
I
II II
II III
III
III 1/C 1/C
1/C
1/C
1/C
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2014 Pearson Education, Inc.
Phylogenetic Trees as Hypotheses
The best hypotheses for phylogenetic trees fit the most data: morphological, molecular, and fossil
Phylogenetic hypotheses are modified when new evidence arises
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2014 Pearson Education, Inc.
Phylogenetic bracketing allows us to predict features of ancestors and their extinct
descendants based on the features of closely
related living descendants
For example, phylogenetic bracketing allows us to infer characteristics of dinosaurs based on shared
characters in modern birds and crocodiles
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2014 Pearson Education, Inc.
Figure 20.15
Ornithischian dinosaurs
Birds
Crocodilians
Lizards and snakes
Saurischian dinosaurs
Common ancestor of crocodilians, dinosaurs, and birds
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Birds and crocodiles share several features: four-chambered hearts, song, nest building, and
brooding
These characteristics likely evolved in a common ancestor and were shared by all of its descendants,
including dinosaurs
The fossil record supports nest building and brooding in dinosaurs
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2014 Pearson Education, Inc.
Figure 20.17
Hind limb
Front limb
(a) Fossil remains of Oviraptor and eggs
(b) Artists reconstruction of the dinosaurs posture based on the fossil findings
Eggs
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2014 Pearson Education, Inc.
Concept 20.4: Molecular clocks help track evolutionary time
To extend molecular phylogenies beyond the fossil record, we must make an assumption about how
molecular change occurs over time
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2014 Pearson Education, Inc.
Molecular Clocks
A molecular clock uses constant rates of evolution in some genes to estimate the absolute time of
evolutionary change
The number of nucleotide substitutions in related genes is assumed to be proportional to the time
since they last shared a common ancestor
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2014 Pearson Education, Inc.
Molecular clocks are calibrated by plotting the number of genetic changes against the dates of
branch points known from the fossil record
Individual genes vary in how clocklike they are
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2014 Pearson Education, Inc.
Figure 20.18
Divergence time (millions of years)
Nu
mb
er
of
mu
tati
on
s
120 90 60 30 0
90
60
30
0
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Differences in Clock Speed
Some mutations are selectively neutral and have little or no effect on fitness
Neutral mutations should be regular like a clock
The neutral mutation rate is dependant on how critical a genes amino acid sequence is to survival
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2014 Pearson Education, Inc.
Potential Problems with Molecular Clocks
Molecular clocks do not run as smoothly as expected if mutations were selectively neutral
Irregularities result from natural selection in which some DNA changes are favored over others
Estimates of evolutionary divergences older than the fossil record have a high degree of uncertainty
The use of multiple genes may improve estimates
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2014 Pearson Education, Inc.
Applying a Molecular Clock: Dating the Origin of HIV
Phylogenetic analysis shows that HIV is descended from viruses that infect chimpanzees and other
primates
HIV spread to humans more than once
Comparison of HIV samples shows that the virus evolved in a very clocklike way
Application of a molecular clock to one strain of HIV suggests that that strain spread to humans during
the 1930s
Animation: Class Schemes
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Figure 20.19
Year
Ind
ex o
f b
ase c
han
ges b
etw
een
H
IV g
en
e s
eq
uen
ces
1940 1920 1960 1900 0
0.15
0.10
0.05
1980 2000
HIV
Range
Adjusted best-fit line (accounts for uncertain dates of HIV sequences
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2014 Pearson Education, Inc.
Figure 20.21
Cyanobacteria
Proteobacteria
Thermophiles
Do
main
Eu
kary
a
Do
main
Bacte
ria
Do
main
A
rch
ae
a
Plantae
Methanogens
Ancestral cell populations
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2014 Pearson Education, Inc.
Figure 20.21a
Do
ma
in E
uk
ary
a
Plantae
Ancestral cell populations
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2014 Pearson Education, Inc.
Figure 20.21b
Cyanobacteria
Proteobacteria
Thermophiles
Do
main
Bacte
ria
Do
main
A
rch
aea
Methanogens
Ancestral cell populations