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Chapter 26
Phylogeny and the Tree of Life
Fig. 26-1
Overview: Investigating the Tree
of Life
• Phylogeny is the evolutionary history of
a species or group of related species
• The discipline of systematics classifies
organisms and determines their
evolutionary relationships
• Systematists use fossil, molecular, and
genetic data to infer evolutionary
relationships
Fig. 26-2
Concept 26.1: Phylogenies show
evolutionary relationships
• Taxonomy is the ordered division and
naming of organisms
Binomial Nomenclature
• In the 18th century, 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
• 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)
Hierarchical Classification
• Linnaeus introduced a system for
grouping species in increasingly broad
categories
• The taxonomic groups from broad to
narrow are domain, kingdom, phylum,
class, order, family, genus, and
species
• A taxonomic unit at any level of hierarchy
is called a taxon
Fig. 26-3
Species:
Panthera
pardus
Genus: Panthera
Family: Felidae
Order: Carnivora
Class: Mammalia
Phylum: Chordata
Kingdom: Animalia
Bacteria
Domain: Eukarya
Archaea
Fig. 26-3a
Class: Mammalia
Phylum: Chordata
Kingdom: Animalia
Bacteria
Domain: Eukarya
Archaea
Fig. 26-3b
Species:
Panthera
pardus
Genus: Panthera
Family: Felidae
Order: Carnivora
Linking Classification and
Phylogeny
• Systematists depict evolutionary
relationships in branching phylogenetic
trees
Fig. 26-4
Order
Family Genus
Species
Taxidea
Taxidea
taxus
Lutra
Mustelidae
Panthera
Felidae
Carnivora
Panthera
pardus
Lutra lutra
Canis
Canidae
Canis
latrans
Canis
lupus
• Linnaean classification and phylogeny
can differ from each other
• Systematists have proposed the
PhyloCode, which recognizes only
groups that include a common ancestor
and all its descendents
• A phylogenetic tree represents a
hypothesis about evolutionary
relationships
• Each branch point represents the
divergence of two species
• Sister taxa are groups that share an
immediate common ancestor
• A rooted tree includes a branch to
represent the last common ancestor of all
taxa in the tree
• A polytomy is a branch from which more
than two groups emerge
Fig. 26-5
Branch point
(node)
Taxon A
Taxon B
Taxon C
ANCESTRAL
LINEAGE
Taxon D
Taxon E
Taxon F
Common ancestor of
taxa A–F
Polytomy
Sister
taxa
What We Can and Cannot Learn
from Phylogenetic Trees
• Phylogenetic trees do show patterns of
descent
• Phylogenetic trees do not indicate when
species evolved or how much genetic
change occurred in a lineage
• It shouldn’t be assumed that a taxon
evolved from the taxon next to it
Applying Phylogenies
• Phylogeny provides important information
about similar characteristics in closely
related species
• A phylogeny was used to identify the
species of whale from which “whale meat”
originated
Fig. 26-6
RESULTS
Minke
(Antarctica)
Minke
(Australia)
Unknown #1a,
2, 3, 4, 5, 6, 7, 8
Minke
(North Atlantic)
Unknown #9
Humpback
(North Atlantic)
Humpback
(North Pacific)
Unknown #1b
Gray
Blue
(North Atlantic)
Blue
(North Pacific)
Unknown #10,
11, 12
Unknown #13
Fin
(Mediterranean)
Fin (Iceland)
Fig. 26-6a
RESULTS
Minke
(Antarctica)
Minke
(Australia)
Unknown #1a,
2, 3, 4, 5, 6, 7, 8
Minke
(North Atlantic)
Unknown #9
Fig. 26-6b
Humpback
(North Atlantic)
Humpback
(North Pacific)
Unknown #1b
Gray
Blue
(North Atlantic)
Blue
(North Pacific)
Fig. 26-6c
Unknown #10,
11, 12
Unknown #13
Fin
(Mediterranean)
Fin (Iceland)
• Phylogenies of anthrax bacteria helped
researchers identify the source of a
particular strain of anthrax
Fig. 26-UN1
(a)
A
B
D
B
D
C
C
C
B
D
A
A
(b)
(c)
Concept 26.2: Phylogenies are
inferred from morphological and
molecular
data
• To infer phylogenies, systematists gather
information about morphologies, genes,
and biochemistry of living organisms
Morphological and Molecular
Homologies
• Organisms with similar morphologies or
DNA sequences are likely to be more
closely related than organisms with
different structures or sequences
Sorting Homology from Analogy
• When 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
Fig. 26-7
• Convergent evolution occurs when similar
environmental pressures and natural
selection produce similar (analogous)
adaptations in organisms from different
evolutionary lineages
• Bat and bird wings are homologous as
forelimbs, but analogous as functional
wings
• Analogous structures or molecular
sequences that evolved independently
are also called homoplasies
• Homology can be distinguished from
analogy by comparing fossil evidence
and the degree of complexity
• The more complex two similar structures
Evaluating Molecular Homologies
• Systematists use computer programs and
mathematical tools when analyzing
comparable DNA segments from different
organisms
Fig. 26-8
1
Deletion
2
Insertion
3
4
Fig. 26-8a
1
Deletion
2
Insertion
Fig. 26-8b
3
4
• It is also important to distinguish
homology from analogy in molecular
similarities
• Mathematical tools help to identify
molecular homoplasies, or coincidences
• Molecular systematics uses DNA and
other molecular data to determine
evolutionary relationships
Fig. 26-9
Concept 26.3: Shared characters
are used to construct
phylogenetic
trees
• Once homologous characters have been
identified, they can be used to infer a
phylogeny
Cladistics
• Cladistics groups organisms by common
descent
• A clade is a group of species that
includes an ancestral species and all its
descendants
• Clades can be nested in larger clades,
but not all groupings of organisms qualify
as clades
• A valid clade is monophyletic, signifying
that it consists of the ancestor species
and all its descendants
Fig. 26-10
A
A
A
B
B
C
C
C
D
D
D
E
E
F
F
F
G
G
G
B
Group I
(a) Monophyletic group (clade)
Group II
(b) Paraphyletic group
E
Group III
(c) Polyphyletic group
Fig. 26-10a
A
B
Group I
C
D
E
F
G
(a) Monophyletic group (clade)
• A paraphyletic grouping consists of an
ancestral species and some, but not all,
of the descendants
Fig. 26-10b
A
B
C
D
E
Group II
F
G
(b) Paraphyletic group
• A polyphyletic grouping consists of
various species that lack a common
ancestor
Fig. 26-10c
A
B
C
D
E
Group III
F
G
(c) Polyphyletic group
Shared Ancestral and Shared
Derived Characters
• In comparison with its ancestor, an
organism has both shared and different
characteristics
• 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
Inferring Phylogenies Using
Derived Characters
• When inferring evolutionary relationships,
it is useful to know in which clade a
shared derived character first appeared
Fig. 26-11
TAXA
Tuna
Leopard
Lancelet
(outgroup)
Vertebral column
(backbone)
0
1
1
1
1
1
Hinged jaws
0
0
1
1
1
1
Lamprey
Tuna
Vertebral
column
Salamander
Hinged jaws
Four walking legs
0
0
0
1
1
1
Turtle
Four walking legs
Amniotic (shelled) egg
0
0
0
0
1
1
Hair
0
0
0
0
0
1
Amniotic egg
(a) Character table
Leopard
Hair
(b) Phylogenetic tree
Fig. 26-11a
Tuna
Leopard
TAXA
Vertebral column
(backbone)
0
1
1
1
1
1
Hinged jaws
0
0
1
1
1
1
Four walking legs
0
0
0
1
1
1
Amniotic (shelled) egg
0
0
0
0
1
1
Hair
0
0
0
0
0
1
(a) Character table
Fig. 26-11b
Lancelet
(outgroup)
Lamprey
Tuna
Vertebral
column
Salamander
Hinged jaws
Turtle
Four walking legs
Amniotic egg
Leopard
Hair
(b) Phylogenetic tree
• 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
ancestral characteristics
• Homologies shared by the outgroup and
ingroup are ancestral characters that
predate the divergence of both groups
from a common ancestor
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
Fig. 26-12
Drosophila
Lancelet
Zebrafish
Frog
Chicken
Human
Mouse
• In other trees, branch length can
represent chronological time, and
branching points can be determined from
the fossil record
Fig. 26-13
Drosophila
Lancelet
Zebrafish
Frog
Chicken
Human
Mouse
PALEOZOIC
542
MESOZOIC
251
Millions of years ago
CENOZOIC
65.5
Present
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
• Maximum parsimony assumes that the
tree that requires the fewest evolutionary
events (appearances of shared derived
characters) is the most likely
• 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
Fig. 26-14
Human
Mushroom
Tulip
0
30%
40%
0
40%
Human
Mushroom
0
Tulip
(a) Percentage differences between sequences
15%
5%
5%
15%
15%
10%
20%
25%
Tree 1: More likely
Tree 2: Less likely
(b) Comparison of possible trees
Fig. 26-14a
Human
Mushroom
Human
Mushroom
Tulip
0
30%
40%
0
40%
Tulip
(a) Percentage differences between sequences
0
Fig. 26-14b
15%
5%
5%
15%
15%
10%
25%
20%
Tree 1: More likely
Tree 2: Less likely
(b) Comparison of possible trees
• Computer programs are used to search
for trees that are parsimonious and likely
Fig. 26-15-1
Species I
Species III
Species II
Three phylogenetic hypotheses:
I
I
III
II
III
II
III
II
I
Fig. 26-15-2
Site
1
2
3
4
Species I
C
T
A
T
Species II
C
T
T
C
Species III
A
G
A
C
Ancestral
sequence
A
G
T
T
1/C
I
1/C
II
I
III
III
II
1/C
II
III
1/C
I
1/C
Fig. 26-15-3
Site
1
2
3
4
Species I
C
T
A
T
Species II
C
T
T
C
Species III
A
G
A
C
Ancestral
sequence
A
G
T
T
1/C
I
1/C
II
I
III
III
II
1/C
II
III
I
1/C
3/A
2/T
I
2/T
3/A
3/A
4/C
II
II
2/T 4/C
III
2/T
4/C
III
3/A 4/C
I
III
II
4/C
1/C
I
2/T 3/A
Fig. 26-15-4
Site
1
2
3
4
Species I
C
T
A
T
Species II
C
T
T
C
Species III
A
G
A
C
Ancestral
sequence
A
G
T
T
1/C
I
1/C
II
I
III
III
II
1/C
II
III
I
1/C
3/A
2/T
I
2/T
3/A
3/A 4/C
3/A
4/C
III
II
2/T
4/C
II
III
6 events
I
III
II
4/C
1/C
I
2/T 3/A
2/T 4/C
I
I
III
II
III
II
III
II
I
7 events
7 events
Phylogenetic Trees as
Hypotheses
• The best hypotheses for phylogenetic
trees fit the most data: morphological,
molecular, and fossil
• Phylogenetic bracketing allows us to
predict features of an ancestor from
features of its descendents
Fig. 26-16
Lizards
and snakes
Crocodilians
Common
ancestor of
crocodilians,
dinosaurs,
and birds
Ornithischian
dinosaurs
Saurischian
dinosaurs
Birds
• This has been applied to infer features of
dinosaurs from their descendents: birds
and crocodiles
Animation: The Geologic Record
Fig. 26-17
Front limb
Hind limb
Eggs
(a) Fossil remains of Oviraptor
and eggs
(b) Artist’s reconstruction of the dinosaur’s posture
Fig. 26-17a
Front limb
Hind limb
Eggs
(a) Fossil remains of Oviraptor
and eggs
Fig. 26-17b
(b) Artist’s reconstruction of the dinosaur’s posture
•
Concept 26.4: 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
• DNA that codes for rRNA changes
relatively slowly and is useful for
investigating branching points hundreds
of millions of years ago
• mtDNA evolves rapidly and can be used
to explore recent evolutionary events
Gene Duplications and Gene
Families
• Gene duplication increases the number of
genes in the genome, providing more
opportunities for evolutionary changes
• Like homologous genes, duplicated
genes can be traced to a common
ancestor
• Orthologous genes are found in a single
copy in the genome and are homologous
between species
• They can diverge only after speciation
occurs
• 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 and often evolve new
functions
Fig. 26-18
Ancestral gene
Ancestral species
Speciation with
divergence of gene
Species A
Orthologous genes
Species B
(a) Orthologous genes
Species A
Gene duplication and divergence
Paralogous genes
Species A after many generations
(b) Paralogous genes
Fig. 26-18a
Ancestral gene
Ancestral species
Speciation with
divergence of gene
Species A
Orthologous genes
(a) Orthologous genes
Species B
Fig. 26-18b
Species A
Gene duplication and divergence
Paralogous genes
Species A after many generations
(b) Paralogous genes
Genome Evolution
• Orthologous genes are widespread and
extend across many widely varied
species
• Gene number and the complexity of an
organism are not strongly linked
• Genes in complex organisms appear to
be very versatile and each gene can
perform many functions
Concept 26.5: 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
Molecular Clocks
• A molecular clock uses constant rates of
evolution in some genes to estimate the
absolute time of evolutionary change
• In orthologous genes, nucleotide
substitutions are proportional to the time
since they last shared a common
ancestor
• In paralogous genes, nucleotide
substitutions are proportional to the time
since the genes became duplicated
• Molecular clocks are calibrated against
branches whose dates are known from
the fossil record
Fig. 26-19
90
60
30
0
0
30
60
90
Divergence time (millions of years)
120
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
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
• The use of multiple genes may improve
estimates
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
• Application of a molecular clock to one
strain of HIV suggests that that strain
spread to humans during the 1930s
Fig. 26-20
0.20
0.15
0.10
Computer model
of HIV
Range
0.05
0
1900
1920
1940
1960
Year
1980 2000
Concept 26.6: New information
continues to revise our
understanding
of
the
tree
of
life
• Recently, we have gained insight into the
very deepest branches of the tree of life
through molecular systematics
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
Classification Schemes
data from many Animation:
sequenced
genomes
Fig. 26-21
EUKARYA
Dinoflagellates
Forams
Ciliates Diatoms
Red algae
Land plants
Green algae
Cellular slime molds
Amoebas
Euglena
Trypanosomes
Leishmania
Animals
Fungi
Sulfolobus
Green nonsulfur bacteria
Thermophiles
Halophiles
(Mitochondrion)
COMMON
ANCESTOR
OF ALL
LIFE
Methanobacterium
ARCHAEA
Spirochetes
Chlamydia
Green
sulfur bacteria
BACTERIA
Cyanobacteria
(Plastids, including
chloroplasts)
Fig. 26-21a
Green nonsulfur bacteria
(Mitochondrion)
Spirochetes
COMMON
ANCESTOR
OF ALL
LIFE
Chlamydia
Green
sulfur bacteria
BACTERIA
Cyanobacteria
(Plastids, including
chloroplasts)
Fig. 26-21b
Sulfolobus
Thermophiles
Halophiles
Methanobacterium
ARCHAEA
Fig. 26-21c
EUKARYA
Land plants
Green algae
Dinoflagellates
Forams
Ciliates
Red algae
Diatoms
Amoebas
Cellular slime molds
Animals
Fungi
Euglena
Trypanosomes
Leishmania
A Simple Tree of All Life
• 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, as these have evolved slowly
• 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 complicates
efforts to build a tree of life
Fig. 26-22
Bacteria
Eukarya
Archaea
4
3
2
Billions of years ago
1
0
Is the Tree of Life Really a Ring?
• Some researchers suggest that
eukaryotes arose as an endosymbiosis
between a bacterium and archaean
• If so, early evolutionary relationships
might be better depicted by a ring of life
instead of a tree of life
Fig. 26-23
Eukarya
Bacteria
Archaea
Fig. 26-UN2
Node
Taxon A
Taxon B
Sister taxa
Taxon C
Taxon D
Taxon E
Most recent
common
ancestor
Polytomy
Taxon F
Fig. 26-UN3
Monophyletic group
A
A
A
B
B
B
C
C
C
D
D
D
E
E
E
F
F
F
G
G
G
Paraphyletic group
Polyphyletic group
Fig. 26-UN4
Salamander
Lizard
Goat
Human
Fig. 26-UN5
Fig. 26-UN6
Fig. 26-UN7
Fig. 26-UN8
Fig. 26-UN9
Fig. 26-UN10
Fig. 26-UN10a
Fig. 26-UN10b
You should now be able to:
1. Explain the justification for taxonomy based
on a PhyloCode
2. Explain the importance of distinguishing
between homology and analogy
3. Distinguish between the following terms:
monophyletic, paraphyletic, and
polyphyletic groups; shared ancestral and
shared derived characters; orthologous
and paralogous genes
4. Define horizontal gene transfer and
explain how it complicates phylogenetic
trees
5. Explain molecular clocks and discuss
their limitations
Chapter 32
An Introduction to Animal
Diversity
Overview: Welcome to Your
Kingdom
• The animal kingdom extends far beyond
humans and other animals we may
encounter
• 1.3 million living species of animals have
been identified
Video: Coral Reef
Fig. 32-1
multicellular, heterotrophic
eukaryotes with tissues that
from embryonic
layers
• develop
There are exceptions
to nearly every
criterion for distinguishing animals from
other life-forms
• Several characteristics, taken together,
sufficiently define the group
Nutritional Mode
• Animals are heterotrophs that ingest their
food
Cell Structure and
Specialization
Animals are multicellular eukaryotes
•
• Their cells lack cell walls
• Their bodies are held together by
structural proteins such as collagen
• Nervous tissue and muscle tissue are
unique to animals
Reproduction and Development
• Most animals reproduce sexually, with the
diploid stage usually dominating the life
cycle
• After a sperm fertilizes an egg, the zygote
undergoes rapid cell division called
cleavage
• Cleavage leads to formation of a blastula
• The blastula undergoes gastrulation,
forming a gastrula with different layers of
Video: Sea Urchin Embryonic Development
embryonic tissues
Fig. 32-2-1
Cleavage
Zygote
Eight-cell stage
Fig. 32-2-2
Cleavage
Zygote
Cleavage Blastula
Eight-cell stage
Blastocoel
Cross section
of blastula
Fig. 32-2-3
Blastocoel
Cleavage
Endoderm
Cleavage Blastula
Ectoderm
Zygote
Eight-cell stage
Gastrulation
Blastocoel
Cross section
of blastula
Gastrula
Blastopore
Archenteron
• Many animals have at least one larval
stage
• A larva is sexually immature and
morphologically distinct from the adult; it
eventually undergoes metamorphosis
• All animals, and only animals, have Hox
genes that regulate the development of
body form
• Although the Hox family of genes has
been highly conserved, it can produce a
wide diversity of animal morphology
Concept 32.2: The history of
animals spans more than half a
billion
years
• The animal kingdom includes a great
diversity of living species and an even
greater diversity of extinct ones
• The common ancestor of living animals
may have lived between 675 and 875
million years ago
• This ancestor may have resembled
modern choanoflagellates, protists that
are the closest living relatives of animals
Fig. 32-3
Individual
choanoflagellate
Choanoflagellates
OTHER
EUKARYOTES
Sponges
Animals
Collar cell
(choanocyte)
Other animals
Neoproterozoic Era (1 Billion–524
Million Years Ago)
• Early members of the animal fossil record
include the Ediacaran biota, which dates
from 565 to 550 million years ago
Fig. 32-4
1.5 cm
(a) Mawsonites spriggi
0.4 cm
(b) Spriggina floundersi
Fig. 32-4a
1.5 cm
(a) Mawsonites spriggi
Fig. 32-4b
0.4 cm
(b) Spriggina floundersi
•
Paleozoic Era (542–251 Million
Years
Ago)
The Cambrian explosion (535 to 525
million years ago) marks the earliest
fossil appearance of many major groups
of living animals
• There are several hypotheses regarding
the cause of the Cambrian explosion
– New predator-prey relationships
– A rise in atmospheric oxygen
– The evolution of the Hox gene complex
Fig. 32-5
• Animal diversity continued to increase
through the Paleozoic, but was
punctuated by mass extinctions
• Animals began to make an impact on
land by 460 million years ago
• Vertebrates made the transition to land
around 360 million years ago
Mesozoic Era (251–65.5 Million
Years Ago)
• Coral reefs emerged, becoming important
marine ecological niches for other
organisms
• During the Mesozoic era, dinosaurs were
the dominant terrestrial vertebrates
• The first mammals emerged
Cenozoic Era (65.5 Million Years
Ago to the Present)
• The beginning of the Cenozoic era
followed mass extinctions of both
terrestrial and marine animals
• These extinctions included the large,
nonflying dinosaurs and the marine
reptiles
• Modern mammal orders and insects
diversified during the Cenozoic
Concept 32.3: Animals can be
characterized by “body plans”
• Zoologists sometimes categorize animals
according to a body plan, a set of
morphological and developmental traits
• A grade is a group whose members share
key biological features
• A grade is not necessarily a clade, or
monophyletic group
Fig. 32-6
100 µm
RESULTS
Site of
gastrulation
Site of
gastrulation
Fig. 32-6a
100 µm
RESULTS
Fig. 32-6b
RESULTS
Site of
gastrulation
Fig. 32-6c
RESULTS
Site of
gastrulation
Fig. 32-6d
RESULTS
Symmetry
• Animals can be categorized according to
the symmetry of their bodies, or lack of it
• Some animals have radial symmetry
Fig. 32-7
(a) Radial symmetry
(b) Bilateral symmetry
• Two-sided symmetry is called bilateral
symmetry
• Bilaterally symmetrical animals have:
–
–
–
–
A dorsal (top) side and a ventral (bottom) side
A right and left side
Anterior (head) and posterior (tail) ends
Cephalization, the development of a head
Tissues
• Animal body plans also vary according to
the organization of the animal’s tissues
• Tissues are collections of specialized
cells isolated from other tissues by
membranous layers
• During development, three germ layers
give rise to the tissues and organs of the
animal embryo
• Ectoderm is the germ layer covering the
embryo’s surface
• Endoderm is the innermost germ layer
and lines the developing digestive tube,
called the archenteron
• Diploblastic animals have ectoderm and
endoderm
• Triploblastic animals also have an
intervening mesoderm layer; these
include all bilaterians
Body Cavities
• Most triploblastic animals possess a
body cavity
• A true body cavity is called a coelom and
is derived from mesoderm
• Coelomates are animals that possess a
true coelom
Fig. 32-8
Coelom
Digestive tract
(from endoderm)
Body covering
(from ectoderm)
Tissue layer
lining coelom
and suspending
internal organs
(from mesoderm)
(a) Coelomate
Body covering
(from ectoderm)
Pseudocoelom
Muscle layer
(from
mesoderm)
Digestive tract
(from endoderm)
(b) Pseudocoelomate
Body covering
(from ectoderm)
Tissuefilled region
(from
mesoderm)
Wall of digestive cavity
(from endoderm)
(c) Acoelomate
Fig. 32-8a
Coelom
Body covering
(from ectoderm)
Digestive tract
(from endoderm)
(a) Coelomate
Tissue layer
lining coelom
and suspending
internal organs
(from mesoderm)
• A pseudocoelom is a body cavity derived
from the mesoderm and endoderm
• Triploblastic animals that possess a
pseudocoelom are called
pseudocoelomates
Fig. 32-8b
Body covering
(from ectoderm)
Pseudocoelom
Digestive tract
(from endoderm)
(b) Pseudocoelomate
Muscle layer
(from
mesoderm)
• Triploblastic animals that lack a body
cavity are called acoelomates
Fig. 32-8c
Body covering
(from ectoderm)
Tissuefilled region
(from
mesoderm)
Wall of digestive cavity
(from endoderm)
(c) Acoelomate
Protostome and Deuterostome
Development
• Based on early development, many
animals can be categorized as having
protostome development or
deuterostome development
Cleavage
• In protostome development, cleavage is
spiral and determinate
• In deuterostome development, cleavage
is radial and indeterminate
• With indeterminate cleavage, each cell in
the early stages of cleavage retains the
capacity to develop into a complete
embryo
• Indeterminate cleavage makes possible
identical twins, and embryonic stem cells
Fig. 32-9
Protostome development
(examples: molluscs,
annelids)
Deuterostome development
(examples: echinoderm,
chordates)
Eight-cell stage
Eight-cell stage
Spiral and determinate
(a) Cleavage
Radial and indeterminate
(b) Coelom formation
Key
Coelom
Ectoderm
Mesoderm
Endoderm
Archenteron
Coelom
Mesoderm
Blastopore
Blastopore
Solid masses of mesoderm
split and form coelom.
Mesoderm
Folds of archenteron
form coelom.
Anus
Mouth
(c) Fate of the blastopore
Digestive tube
Mouth
Mouth develops from blastopore.
Anus
Anus develops from blastopore.
Fig. 32-9a
Protostome development
(examples: molluscs,
annelids)
Eight-cell stage
Spiral and determinate
Deuterostome development
(examples: echinoderms,
chordates)
Eight-cell stage
Radial and indeterminate
(a) Cleavage
Coelom Formation
• In protostome development, the splitting
of solid masses of mesoderm forms the
coelom
• In deuterostome development, the
mesoderm buds from the wall of the
archenteron to form the coelom
Fig. 32-9b
Protostome development
(examples: molluscs,
annelids)
Deuterostome development
(examples: echinoderms,
chordates)
(b) Coelom formation
Coelom
Key
Ectoderm
Mesoderm
Endoderm
Archenteron
Coelom
Mesoderm
Blastopore
Solid masses of mesoderm
split and form coelom.
Blastopore
Mesoderm
Folds of archenteron
form coelom.
Fate of the Blastopore
• The blastopore forms during gastrulation
and connects the archenteron to the
exterior of the gastrula
• In protostome development, the
blastopore becomes the mouth
• In deuterostome development, the
blastopore becomes the anus
Fig. 32-9c
Protostome development
(examples: molluscs,
annelids)
Deuterostome development
(examples: echinoderms,
chordates)
Anus
Mouth
(c) Fate of the blastopore
Key
Digestive tube
Anus
Mouth
Mouth develops from blastopore. Anus develops from blastopore.
Ectoderm
Mesoderm
Endoderm
Concept 32.4: New views of
animal phylogeny are emerging
from
molecular
data
• Zoologists recognize about three dozen
animal phyla
• Current debate in animal systematics has
led to the development of two
phylogenetic hypotheses, but others exist
as well
• One hypothesis of animal phylogeny is
based mainly on morphological and
developmental comparisons
Fig. 32-10
“Porifera”
Eumetazoa
Metazoa
ANCESTRAL
COLONIAL
FLAGELLATE
Cnidaria
Ctenophora
Deuterostomia
Ectoprocta
Brachiopoda
Echinodermata
Bilateria
Chordata
Platyhelminthes
Protostomia
Rotifera
Mollusca
Annelida
Arthropoda
Nematoda
• One hypothesis of animal phylogeny is
based mainly on molecular data
Metazoa
Silicea
Calcarea
Ctenophora
Eumetazoa
ANCESTRAL
COLONIAL
FLAGELLATE
“Porifera”
Fig. 32-11
Cnidaria
Acoela
Bilateria
Deuterostomia
Echinodermata
Chordata
Platyhelminthes
Lophotrochozoa
Rotifera
Ectoprocta
Brachiopoda
Mollusca
Annelida
Ecdysozoa
Nematoda
Arthropoda
Points of Agreement
• All animals share a common ancestor
• Sponges are basal animals
• Eumetazoa is a clade of animals
(eumetazoans) with true tissues
• Most animal phyla belong to the clade
Bilateria, and are called bilaterians
• Chordates and some other phyla belong
to the clade Deuterostomia
Progress in Resolving Bilaterian
Relationships
• The morphology-based tree divides
bilaterians into two clades:
deuterostomes and protostomes
• In contrast, recent molecular studies
indicate three bilaterian clades:
Deuterostomia, Ecdysozoa, and
Lophotrochozoa
• Ecdysozoans shed their exoskeletons
through a process called ecdysis
Fig. 32-12
• Some lophotrochozoans have a feeding
structure called a lophophore
• Other phyla go through a distinct
developmental stage called the
trochophore larva
Fig. 32-13
Lophophore
Apical tuft
of cilia
100 µm
Mouth
(a) An ectoproct
Anus
(b) Structure of a trochophore
larva
Future Directions in Animal
Systematics
• Phylogenetic studies based on larger
databases will likely provide further
insights into animal evolutionary history
Fig. 32-UN1
Common ancestor
of all animals
Metazoa
Sponges
(basal animals)
Eumetazoa
Ctenophora
Cnidaria
Acoela (basal
bilaterians)
Deuterostomia
Bilateral
summetry
Three germ
layers
Lophotrochozoa
Ecdysozoa
Bilateria (most animals)
True
tissues
Fig. 32-T1
Fig. 32-UN2
You should now be able to:
1. List the characteristics that combine to
define animals
2. Summarize key events of the Paleozoic,
Mesozoic, and Cenozoic eras
3. Distinguish between the following pairs
or sets of terms: radial and bilateral
symmetry; grade and clade of animal
taxa; diploblastic and triploblastic; spiral
and radial cleavage; determinate and
indeterminate cleavage; acoelomate,
pseudocoelomate, and coelomate
4. Compare the developmental differences
between protostomes and
deuterostomes
5. Compare the alternate relationships of
annelids and arthropods presented by
two different proposed phylogenetic
trees
6. Distinguish between ecdysozoans and
lophotrochozoans