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Biology 102 Week 2
The Origin of Species.
Phylogeny and Systematics.
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: The “Mystery of Mysteries”
• In the Galápagos Islands Darwin discovered
plants and animals found nowhere else on Earth
Video: Galápagos Tortoise
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Speciation, the origin of new species, is at the
focal point of evolutionary theory
• Evolutionary theory must explain how new species
originate and how populations evolve
• Microevolution consists of adaptations that evolve
within a population, confined to one gene pool
• Macroevolution refers to evolutionary change
above the species level
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• Two basic patterns of evolutionary change:
– Anagenesis (phyletic evolution) transforms
one species into another
– Cladogenesis (branching evolution) is the
splitting of a gene pool, giving rise to one or
more new species
Animation: Macroevolution
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LE 24-2
Anagenesis
Cladogenesis
Concept 24.1: The biological species concept
emphasizes reproductive isolation
• Species is a Latin word meaning “kind” or
“appearance”
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The Biological Species Concept
• Members of a biological species are
reproductively compatible, at least potentially; they
cannot interbreed with other populations.
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LE 24-3
Similarity between different species.
Diversity within a species.
Reproductive Isolation
• Reproductive isolation is the existence of
biological factors (barriers) that impede two
species from producing viable, fertile hybrids
• Two types of barriers: prezygotic and postzygotic
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• Prezygotic barriers impede mating or hinder
fertilization if mating does occur:
– Habitat isolation
– Temporal isolation
– Behavioral isolation
– Mechanical isolation
– Gametic isolation
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• Habitat isolation: Two species encounter each
other rarely, or not at all, because they occupy
different habitats, even though not isolated by
physical barriers
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LE 24-4a
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Habitat
isolation
Temporal
isolation
Behavioral
isolation
Individuals
of
different
species
Mechanical
isolation
Gametic
isolation
Mating
attempt
HABITAT ISOLATION
Fertilization
TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION
GAMETIC ISOLATION
Postzygotic barriers prevent a hybrid zygote from
developing into a viable, fertile adult
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
Viable,
fertile
offspring
Fertilization
REDUCED HYBRID
VIABILITY
REDUCED HYBRID
FERTILITY
HYBRID BREAKDOWN
• Temporal isolation: Species that breed at different
times of the day, different seasons, or different
years cannot mix their gametes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Behavioral isolation: Courtship rituals and other
behaviors unique to a species are effective
barriers
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Mechanical isolation: Morphological differences
can prevent successful mating
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Gametic isolation: Sperm of one species may not
be able to fertilize eggs of another species
Video: Albatross Courtship Ritual
Video: Blue-footed Boobies Courtship Ritual
Video: Giraffe Courtship Ritual
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 24-4aa
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Habitat
isolation
Individuals
of
different
species
HABITAT ISOLATION
Temporal
isolation
Behavioral
isolation
Mechanical
isolation
Mating
attempt
TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION
Gametic
isolation
Fertilization
GAMETIC ISOLATION
• Postzygotic barriers prevent the hybrid zygote
from developing into a viable, fertile adult:
– Reduced hybrid viability
– Reduced hybrid fertility
– Hybrid breakdown
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• Reduced hybrid viability: Genes of the different
parent species may interact and impair the
hybrid’s development
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Reduced hybrid fertility: Even if hybrids are
vigorous, they may be sterile
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Hybrid breakdown: Some first-generation hybrids
are fertile, but when they mate with another
species or with either parent species, offspring of
the next generation are feeble or sterile
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 24-4ab
Postzygotic barriers prevent a hybrid zygote from
developing into a viable, fertile adult
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
Viable,
fertile
offspring
Fertilization
REDUCED HYBRID
VIABILITY
REDUCED HYBRID
FERTILITY
HYBRID BREAKDOWN
Limitations of the Biological Species Concept
• The biological species concept does not apply to
– Asexual organisms
– Fossils
– Organisms about which little is known
regarding their reproduction
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Other Definitions of Species
• Morphological: defines a species by structural
features
• Paleontological: focuses on morphologically
discrete species known only from the fossil record
• Ecological: views a species in terms of its
ecological niche
• Phylogenetic: defines a species as a set of
organisms with a unique genetic history
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Concept 24.2: Speciation can take place with or
without geographic separation
• Speciation can occur in two ways:
– Allopatric speciation
– Sympatric speciation
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LE 24-5
Allopatric speciation
Sympatric speciation
Allopatric (“Other Country”) Speciation
• In allopatric speciation, gene flow is interrupted or
reduced when a population is divided into
geographically isolated subpopulations
• One or both populations may undergo
evolutionary change during the period of
separation
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LE 24-6
A. harrisi
A. leucurus
• To determine if allopatric speciation has occurred,
reproductive isolation must have been established
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LE 24-7a
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies
raised on
starch medium
Mating experiments
after several generations
Some flies
raised on
maltose medium
LE 24-7b
22
9
8
20
Mating frequencies
in experimental group
Male
Same
Different
populations population
Male
Maltose Starch
Female
Starch Maltose
Female
Different
Same
population populations
18
15
12
15
Mating frequencies
in control group
Sympatric (“Same Country”) Speciation
• In sympatric speciation, speciation takes place in
geographically overlapping populations
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Polyploidy
• Polyploidy is presence of extra sets of
chromosomes due to accidents during cell division
• It has caused the evolution of some plant species
• An autopolyploid is an individual with more than
two chromosome sets, derived from one species
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LE 24-8
Failure of cell division
in a cell of a growing
diploid plant after
chromosome duplication
gives rise to a tetraploid
branch or other tissue.
2n = 6
Gametes produced
by flowers on this
tetraploid branch
are diploid.
Offspring with
tetraploid karyotypes may be
viable and fertile—
a new biological
species.
2n
4n = 12
4n
• An allopolyploid is a species with multiple sets of
chromosomes derived from different species
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LE 24-9
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Unreduced gamete
with 7 chromosomes
Viable fertile hybrid
(allopolyploid)
Meiotic error;
Species A chromosome
number not
2n = 4
reduced from
2n to n
2n = 10
Normal gamete
n=3
Species B
2n = 6
Normal gamete
n=3
Habitat Differentiation and Sexual Selection
• Sympatric speciation can also result from the
appearance of new ecological niches
• In cichlid fish, sympatric speciation has resulted
from nonrandom mating due to sexual selection
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LE 24-10
P. pundamilia
P. nyererei
Normal light
Monochromatic orange light
Allopatric and Sympatric Speciation: A Summary
• In allopatric speciation, a new species forms while
geographically isolated from its parent population
• In sympatric speciation, a reproductive barrier
isolates a subset of a population without
geographic separation from the parent species
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Adaptive Radiation
• Adaptive radiation is the evolution of diversely
adapted species from a common ancestor upon
introduction to new environmental opportunities
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The Hawaiian archipelago is one of the world’s
great showcases of adaptive radiation
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LE 24-12
Dubautia laxa
KAUAI
5.1
million
years
1.3
million
years
MOLOKAI
MAUI
OAHU
3.7 LANAI
million
years
Argyroxiphium sandwicense
HAWAII
0.4
million
years
Dubautia waialealae
Dubautia scabra
Dubautia linearis
Studying the Genetics of Speciation
• The explosion of genomics is enabling
researchers to identify specific genes involved in
some cases of speciation
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The Tempo of Speciation
• The fossil record includes many episodes in which
new species appear suddenly in a geologic
stratum, persist essentially unchanged through
several strata, and then apparently disappear
• Niles Eldredge and Stephen Jay Gould coined the
term punctuated equilibrium to describe periods of
apparent stasis punctuated by sudden change
• The punctuated equilibrium model contrasts with a
model of gradual change in a species’ existence
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LE 24-13
Time
Gradualism model
Punctuated equilibrium model
Concept 24.3: Macroevolutionary changes can
accumulate through many speciation events
• Macroevolutionary change is cumulative change
during thousands of small speciation episodes
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Evolutionary Novelties
• Most novel biological structures evolve in many
stages from previously existing structures
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• Some complex structures, such as the eye, have
had similar functions during all stages of their
evolution
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LE 24-14
Pigmented cells
(photoreceptors)
Pigmented
cells
Epithelium
Nerve fibers
Patch of pigmented cells
Fluid-filled cavity
Epithelium
Optic
nerve
Nerve fibers
Eyecup
Cellular
fluid
(lens)
Pigmented
layer (retina)
Pinhole camera-type eye
Optic nerve
Eye with primitive lens
Cornea
Lens
Retina
Optic nerve
Complex camera-type eye
Cornea
Evolution of the Genes That Control Development
• Genes that program development control the rate,
timing, and spatial pattern of changes in an
organism’s form as it develops into an adult
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Changes in Rate and Timing
• Heterochrony is an evolutionary change in the rate
or timing of developmental events
• It can have a significant impact on body shape
• Allometric growth is the proportioning that helps
give a body its specific form
Animation: Allometric Growth
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LE 24-15a
Newborn
2
15
5
Age (years)
Differential growth rates in a human
Adult
• Different allometric patterns contribute to the
contrasting shapes of human and chimpanzee
skulls
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LE 24-15b
Chimpanzee fetus
Chimpanzee adult
Human adult
Human fetus
Comparison of chimpanzee and human skull growth
• Heterochrony has also played a part in the
evolution of salamander feet
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LE 24-16
Ground-dwelling salamander
Tree-dwelling salamander
• In paedomorphosis, the rate of reproductive
development accelerates compared with somatic
development
• The sexually mature species may retain body
features that were juvenile structures in an
ancestral species
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Changes in Spatial Pattern
• Substantial evolutionary change can also result
from alterations in genes that control the
placement and organization of body parts
• Homeotic genes determine such basic features as
where wings and legs will develop on a bird or
how a flower’s parts are arranged
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• The products of one class of homeotic genes
called Hox genes
• Hox genes provide positional information in the
development of fins in fish and limbs in tetrapods
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LE 24-18
Chicken leg bud
Region of
Hox gene
expression
Zebrafish fin bud
• Evolution of vertebrates from invertebrate animals
was associated with alterations in Hox genes
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LE 24-19
Hypothetical vertebrate
ancestor (invertebrate)
with a single Hox cluster
First Hox
duplication
Hypothetical early
vertebrates (jawless)
with two Hox clusters
Second Hox
duplication
Vertebrates (with jaws)
with four Hox clusters
Evolution Is Not Goal Oriented
• The fossil record often shows apparent trends in
evolution that may arise because of adaptation to
a changing environment
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LE 24-20
Recent
Equus
Hippidion and other genera
Pleistocene
Nannippus
Pliohippus
Hipparion Neohipparion
Pliocene
Sinohippus
Megahippus
Callippus
Archaeohippus
Merychippus
Miocene
Anchitherium
Hypohippus
Parahippus
Miohippus
Oligocene
Mesohippus
Paleotherium
Epihippus
Propalaeotherium
Eocene
Pachynolophus
Orohippus
Key
Hyracotherium
Grazers
Browsers
• According to the species selection model, trends
may result when species with certain
characteristics endure longer and speciate more
often than those with other characteristics
• The appearance of an evolutionary trend does not
imply that there is some intrinsic drive toward a
particular phenotype
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Chapter 25
Phylogeny and Systematics
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: Investigating the Tree of Life
• Phylogeny is the evolutionary history of a species
or group of related species
• Biologists draw on the fossil record, which
provides information about ancient organisms
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Systematics is an analytical approach to
understanding the diversity and relationships of
organisms, both present-day and extinct
• Systematists use morphological, biochemical, and
molecular comparisons to infer evolutionary
relationships
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 25.1: Phylogenies are based on common ancestries
inferred from fossil, morphological, and molecular evidence
• To infer phylogenies, systematists gather
information about morphologies, development,
and biochemistry of living organisms
• They also examine fossils to help establish
relationships between living organisms
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The Fossil Record
• Sedimentary rocks are the richest source of fossils
• Sedimentary rocks are deposited into layers called
strata
Video: Grand Canyon
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LE 25-3
Rivers carry sediment to the
ocean. Sedimentary rock layers
containing fossils form on the
ocean floor.
Over time, new strata are
deposited, containing fossils
from each time period.
As sea levels change and the
seafloor is pushed upward,
sedimentary rocks are exposed.
Erosion reveals strata and fossils.
Younger stratum
with more recent
fossils
Older stratum with
older fossils
• The fossil record is based on the sequence in
which fossils have accumulated in such strata
• Fossils reveal ancestral characteristics that may
have been lost over time
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• Though sedimentary fossils are the most common,
paleontologists study a wide variety of fossils
Animation: The Geologic Record
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LE 25-4
Leaf fossil, about 40 million
years ago
Petrified trees in Arizona, about 190
million years old
Insects preserved whole in amber
Dinosaur bones being
excavated from sandstone
Casts of ammonites, about
375 million years old
Boy standing in a 150-million-year-old
dinosaur track in Colorado
Tusks of a 23,000-year-old mammoth, frozen whole
in Siberian ice
Morphological and Molecular Homologies
• In addition to fossils, phylogenetic history can be
inferred from morphological and molecular
similarities in living organisms
• Organisms with very similar morphologies or
similar DNA sequences are likely to be more
closely related than organisms with vastly different
structures or sequences
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Sorting Homology from Analogy
• In constructing a phylogeny, systematists need to
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
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• Convergent evolution occurs when similar
environmental pressures and natural selection
produce similar (analogous) adaptations in
organisms from different evolutionary lineages
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Analogous structures or molecular sequences that
evolved independently are also called
homoplasies
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Evaluating Molecular Homologies
• Systematists use computer programs and
mathematical tools when analyzing comparable
DNA segments from different organisms
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LE 25-6
1
2
Deletion
1
2
Insertion
1
2
1
2
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Concept 25.2: Phylogenetic systematics connects
classification with evolutionary history
• Taxonomy is the ordered division of organisms
into categories based on characteristics used to
assess similarities and differences
• In 1748, Carolus Linnaeus published a system of
taxonomy based on resemblances.
• Two key features of his system remain useful
today: two-part names for species and hierarchical
classification
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Binomial Nomenclature
• 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 latinized
• Both parts together name the species (not the
specific epithet alone)
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Hierarchical Classification
• Linnaeus introduced a system for grouping
species in increasingly broad categories
Animation: Classification Schemes
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LE 25-8
Panthera
pardus
Species
Panthera
Genus
Felidae
Family
Carnivora
Order
Mammalia
Class
Chordata
Phylum
Animalia
Kingdom
Domain
Eukarya
Linking Classification and Phylogeny
• Systematists depict evolutionary relationships in
branching phylogenetic trees
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Species
Mephitis
mephitis
(striped skunk)
Lutra lutra
(European
otter)
Genus
Panthera
Mephitis
Lutra
Felidae
Order
Panthera
pardus
(leopard)
Family
LE 25-9
Mustelidae
Carnivora
Canis
familiaris
(domestic dog)
Canis
lupus
(wolf)
Canis
Canidae
• Each branch point represents the divergence of
two species
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LE 25-UN497
Leopard
Domestic cat
Common ancestor
Wolf
Leopard
Domestic cat
Common ancestor
• “Deeper” branch points represent progressively
greater amounts of divergence
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Concept 25.3: Phylogenetic systematics informs the construction
of phylogenetic trees based on shared characteristics
• A cladogram depicts patterns of shared
characteristics among taxa
• A clade is a group of species that includes an
ancestral species and all its descendants
• Cladistics studies resemblances among clades
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Cladistics
• Clades can be nested in larger clades, but not all
groupings or organisms qualify as clades
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• A valid clade is monophyletic, signifying that it
consists of the ancestor species and all its
descendants
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LE 25-10a
Grouping 1
Monophyletic
• A paraphyletic grouping consists of an ancestral
species and some, but not all, of the descendants
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LE 25-10b
Grouping 2
Paraphyletic
• A polyphyletic grouping consists of various species
that lack a common ancestor
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LE 25-10c
Grouping 3
Polyphyletic
Shared Primitive and Shared Derived Characteristics
• In cladistic analysis, clades are defined by their
evolutionary novelties
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• A shared primitive character is a character that is
shared beyond the taxon we are trying to define
• A shared derived character is an evolutionary
novelty unique to a particular clade
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Outgroups
• An outgroup is a species or group of species that
is closely related to the ingroup, the various
species being studied
• Systematists compare each ingroup species with
the outgroup to differentiate between shared
derived and shared primitive characteristics
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• Outgroup comparison assumes that homologies
shared by the outgroup and ingroup must be
primitive characters that predate the divergence of
both groups from a common ancestor
• It enables us to focus on characters derived at
various branch points in the evolution of a clade
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LE 25-11
Leopard
Turtle
Salamander
Tuna
Lamprey
Lancelet
(outgroup)
TAXA
CHARACTERS
Hair
Amniotic (shelled) egg
Four walking legs
Hinged jaws
Vertebral column
(backbone)
Character table
Turtle
Leopard
Hair
Salamander
Amniotic egg
Tuna
Four walking legs
Lamprey
Hinged jaws
Lancelet (outgroup)
Vertebral column
Cladogram
Phylogenetic Trees and Timing
• Any chronology represented by the branching of a
phylogenetic tree is relative rather than absolute in
representing timing of divergences
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Phylograms
• In a phylogram, the length of a branch in a
cladogram reflects the number of genetic changes
that have taken place in a particular DNA or RNA
sequence in that lineage
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LE 25-12
Ultrametric Trees
• Branching in an ultrametric tree is the same as in
a phylogram, but all branches traceable from the
common ancestor to the present are equal length
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Millions of
years ago
Neoproterozoic
542
Paleozoic
251
Mesozoic
65.5
Cenozoic
LE 25-13
Maximum Parsimony and Maximum Likelihood
• Systematists can never be sure of finding the best
tree in a large data set
• They narrow possibilities by applying the
principles of maximum parsimony and maximum
likelihood
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• The most parsimonious tree requires the fewest
evolutionary events to have occurred in the form
of shared derived characters
• The principle of maximum likelihood states that,
given certain rules about how DNA changes over
time, a tree can be found that reflects the most
likely sequence of evolutionary events
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 25-14
Human
Mushroom
Tulip
0
30%
40%
0
40%
Human
Mushroom
Tulip
0
Percentage differences between sequences
25%
15%
15%
20%
15%
10%
5%
Tree 1: More likely
Comparison of possible trees
5%
Tree 2: Less likely
• In considering possible phylogenies for a group of
species, systematists compare molecular data for
the species.
• The most efficient way to study hypotheses is to
consider the most parsimonious hypothesis, the
one requiring the fewest evolutionary events
(molecular changes)
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LE 25-15ab
Sites in DNA sequence
1 2 3 4 5 6 7
I
Species
II
III
IV
I
II
III
IV
Bases at
site 1 for
each species
Base-change
event
Phylogenetic Trees as Hypotheses
• The best hypotheses for phylogenetic trees fit the
most data: morphological, molecular, and fossil
• Sometimes the best hypothesis is not the most
parsimonious
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LE 25-16
Lizard
Bird
Mammal
Four-chambered
heart
Mammal-bird clade
Lizard
Bird
Mammal
Four-chambered
heart
Four-chambered
heart
Lizard-bird clade
Concept 25.4: Much of an organism’s evolutionary
history is documented in its genome
• Comparing nucleic acids or other molecules to
infer relatedness is a valuable tool for tracing
organisms’ evolutionary history
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Gene Duplications and Gene Families
• Gene duplication increases the number of genes
in the genome, providing more opportunities for
evolutionary changes
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• Orthologous genes are genes found in a single
copy in the genome
• They can diverge only after speciation occurs
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LE 25-17a
Ancestral gene
Speciation
Orthologous genes
• Paralogous genes result from gene duplication, so
are found in more than one copy in the genome
• They can diverge within the clade that carries
them, often adding new functions
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LE 25-17b
Ancestral gene
Gene duplication
Paralogous genes
Genome Evolution
• Orthologous genes are widespread and extend
across many widely varied species
• The widespread consistency in total gene number
in organisms indicates genes in complex
organisms are very versatile and that each gene
can perform many functions
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Concept 25.5: Molecular clocks help track
evolutionary time
• To extend molecular phylogenies beyond the fossil
record, we must make an assumption about how
change occurs over time
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Molecular Clocks
• The molecular clock is a yardstick for measuring
absolute time of evolutionary change based on the
observation that some genes and other regions of
genomes seem to evolve at constant rates
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Neutral Theory
• Neutral theory states that much evolutionary
change in genes and proteins has no effect on
fitness and therefore is not influenced by
Darwinian selection
• It states that the rate of molecular change in these
genes and proteins should be regular like a clock
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Difficulties with Molecular Clocks
• The molecular clock does not run as smoothly as
neutral theory predicts
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Applying a Molecular Clock: The Origin of HIV
• Phylogenetic analysis shows that HIV is
descended from viruses that infect chimpanzees
and other primates
• Comparison of HIV samples throughout the
epidemic shows that the virus evolved in a very
clocklike way
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Universal Tree of Life
• The tree of life is divided into three great clades
called domains: Bacteria, Archaea, and Eukarya
• The early history of these domains is not yet clear
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings