Chapter 26 Phylogeny and the Tree of Life. Fig. 26-1.
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Transcript of Chapter 26 Phylogeny and the Tree of Life. Fig. 26-1.
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. 26-3Species:Pantherapardus
Genus: Panthera
Family: Felidae
Order: Carnivora
Class: Mammalia
Phylum: Chordata
Kingdom: Animalia
ArchaeaDomain: EukaryaBacteria
Fig. 26-3a
Class: Mammalia
Phylum: Chordata
Kingdom: Animalia
ArchaeaDomain: EukaryaBacteria
Fig. 26-3b
Species:Pantherapardus
Genus: Panthera
Family: Felidae
Order: Carnivora
Linking Classification and Phylogeny
• Systematists depict evolutionary relationships in branching phylogenetic trees
Fig. 26-4Species
Canislupus
Pantherapardus
Taxideataxus
Lutra lutra
Canislatrans
Order Family Genus
Carn
ivora
Felid
aeM
ustelid
aeC
anid
ae
Can
isL
utra
Taxid
eaP
anth
era
• 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
Sistertaxa
ANCESTRALLINEAGE
Taxon A
PolytomyCommon ancestor oftaxa A–F
Branch point(node)
Taxon B
Taxon C
Taxon D
Taxon E
Taxon F
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
Fin(Mediterranean)Fin (Iceland)
RESULTS
Unknown #10,11, 12
Unknown #13
Blue(North Pacific)
Blue(North Atlantic)
Gray
Unknown #1b
Humpback(North Atlantic)Humpback(North Pacific)
Unknown #9
Minke(North Atlantic)
Minke(Antarctica)Minke(Australia)Unknown #1a,2, 3, 4, 5, 6, 7, 8
Fig. 26-6a
Unknown #9
Minke(North Atlantic)
Minke(Antarctica)Minke(Australia)Unknown #1a,2, 3, 4, 5, 6, 7, 8
RESULTS
Fig. 26-6b
Blue(North Pacific)
Blue(North Atlantic)
Gray
Unknown #1b
Humpback(North Atlantic)
Humpback(North Pacific)
Fig. 26-6c
Fin(Mediterranean)Fin (Iceland)
Unknown #13
Unknown #10,11, 12
• Phylogenies of anthrax bacteria helped researchers identify the source of a particular strain of anthrax
Fig. 26-UN1
A
B
A A
B
B
C
CC
D
D
D
(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 are, the more likely it is that they are homologous
Evaluating Molecular Homologies
• Systematists use computer programs and mathematical tools when analyzing comparable DNA segments from different organisms
Fig. 26-8
Deletion
Insertion
1
2
3
4
Fig. 26-8a
Deletion
Insertion
1
2
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
BBB
C C C
DDD
E E E
FFF
G G G
Group IIIGroup II
Group I
(a) Monophyletic group (clade) (b) Paraphyletic group (c) Polyphyletic group
Fig. 26-10a
A
B
C
D
E
F
G
Group I
(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
F
G
Group II
(b) Paraphyletic group
• A polyphyletic grouping consists of various species that lack a common ancestor
Fig. 26-10c
A
B
C
D
E
F
G
Group III
(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
Lan
cele
t(o
utg
rou
p)
Lam
pre
y
Sal
aman
der
Leo
par
d
Tu
rtle
Tu
na
Vertebral column(backbone)
Hinged jaws
Four walking legs
Amniotic (shelled) egg
CH
AR
AC
TE
RS
Hair
(a) Character table
Hair
Hinged jaws
Vertebralcolumn
Four walking legs
Amniotic egg
(b) Phylogenetic tree
Salamander
Leopard
Turtle
Lamprey
Tuna
Lancelet(outgroup)
0
0 0
0
0
0
0 0
0
0
0 0
0 0 0 1
11
111
1
11
1
1
11
11
Fig. 26-11aTAXA
La
nc
ele
t(o
utg
rou
p)
La
mp
rey
Sa
lam
and
er
Le
op
ard
Tu
rtle
Tu
na
Vertebral column(backbone)
Hinged jaws
Four walking legs
Amniotic (shelled) eggCH
AR
AC
TE
RS
Hair
(a) Character table
0
0 0
0
0
0
0 0
0
0
0 0
0 0 0 1
11
111
1
11
1
1
11
11
Fig. 26-11b
Hair
Hinged jaws
Vertebralcolumn
Four walking legs
Amniotic egg
(b) Phylogenetic tree
Salamander
Leopard
Turtle
Lamprey
Tuna
Lancelet(outgroup)
• 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
Human
Chicken
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
Human
Chicken
Mouse
CENOZOIC
Present65.5
MESOZOIC
251
Millions of years ago
PALEOZOIC
542
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
15%
Tree 1: More likely Tree 2: Less likely
(b) Comparison of possible trees
15% 15%
5%
5%
10%
25%20%
40%
40%
30%0
0
0
(a) Percentage differences between sequences
Human Mushroom
Mushroom
Tulip
Tulip
Fig. 26-14a
Human 40%
40%
30%0
0
0
(a) Percentage differences between sequences
Human Mushroom
Mushroom
Tulip
Tulip
Fig. 26-14b
15%
Tree 1: More likely Tree 2: Less likely
(b) Comparison of possible trees
15% 15%
5%
5%
10%
25%20%
• Computer programs are used to search for trees that are parsimonious and likely
Fig. 26-15-1
Species I
Three phylogenetic hypotheses:
Species II Species III
I
II
III
I
III
IIIII
III
Fig. 26-15-2
Species I
Site
Species II
Species III
I
II
III
I
III
IIIII
III
Ancestralsequence
1/C1/C
1/C
1/C
1/C
4321
C
C C
C
T
T
T
T
T
T A
AA
A G
G
Fig. 26-15-3
Species I
Site
Species II
Species III
I
II
III
I
III
IIIII
III
Ancestralsequence
1/C1/C
1/C
1/C
1/C
4321
C
C C
C
T
T
T
T
T
T A
AA
A G
G
I I
I
II
II
II
III
III
III3/A
3/A
3/A3/A
3/A
2/T2/T
2/T 2/T
2/T4/C
4/C
4/C
4/C
4/C
Fig. 26-15-4
Species I
Site
Species II
Species III
I
II
III
I
III
IIIII
III
Ancestralsequence
1/C1/C
1/C
1/C
1/C
4321
C
C C
C
T
T
T
T
T
T A
AA
A G
G
I I
I
II
II
II
III
III
III3/A
3/A
3/A3/A
3/A
2/T2/T
2/T 2/T
2/T4/C
4/C
4/C
4/C
4/C
I I
I
II
II
II
III
III
III
7 events7 events6 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
Commonancestor ofcrocodilians,dinosaurs,and birds
Birds
Lizardsand snakes
Crocodilians
Ornithischiandinosaurs
Saurischiandinosaurs
• This has been applied to infer features of dinosaurs from their descendents: birds and crocodiles
Animation: The Geologic RecordAnimation: The Geologic Record
Fig. 26-17
Eggs
Front limb
Hind limb
(a) Fossil remains of Oviraptor and eggs
(b) Artist’s reconstruction of the dinosaur’s posture
Fig. 26-17a
Eggs
Front limb
Hind limb
(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
(b) Paralogous genes
(a) Orthologous genes
Ancestral gene
Paralogous genes
Ancestral species
Speciation withdivergence of gene
Gene duplication and divergence
Species A after many generations
Species A Species B
Species A
Orthologous genes
Fig. 26-18a
(a) Orthologous genes
Ancestral gene
Ancestral species
Speciation withdivergence of gene
Species A Species BOrthologous genes
Fig. 26-18b
(b) Paralogous genes
Paralogous genes
Gene duplication and divergence
Species A after many generations
Species A
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
Divergence time (millions of years)
Nu
mb
er o
f m
uta
tio
ns
120
90
90
60
60
30
300
0
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
Year
Ind
ex o
f b
ase
chan
ges
bet
wee
n H
IV s
equ
ence
s
1960
0.20
1940192019000
1980 2000
0.15
0.10
0.05
Range
Computer modelof HIV
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 data from many sequenced genomes
Animation: Classification SchemesAnimation: Classification Schemes
Fig. 26-21
Fungi
EUKARYA
Trypanosomes
Green algaeLand plants
Red algae
ForamsCiliates
Dinoflagellates
Diatoms
Animals
AmoebasCellular slime molds
Leishmania
Euglena
Green nonsulfur bacteria
Thermophiles
Halophiles
Methanobacterium
Sulfolobus
ARCHAEA
COMMONANCESTOR
OF ALLLIFE
BACTERIA
(Plastids, includingchloroplasts)
Greensulfur bacteria
(Mitochondrion)
Cyanobacteria
ChlamydiaSpirochetes
Fig. 26-21a
Green nonsulfur bacteria
COMMONANCESTOR
OF ALLLIFE
BACTERIA
(Plastids, includingchloroplasts)
Greensulfur bacteria
(Mitochondrion)
Cyanobacteria
Chlamydia
Spirochetes
Fig. 26-21b
Thermophiles
Halophiles
Methanobacterium
Sulfolobus
ARCHAEA
Fig. 26-21c
Fungi
EUKARYA
Trypanosomes
Green algaeLand plants
Red algae
ForamsCiliates
Dinoflagellates
Diatoms
Animals
AmoebasCellular slime molds
Leishmania
Euglena
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
3
Archaea
Bacteria
Eukarya
Billions of years ago
4 2 1 0
• 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
Is the Tree of Life Really a Ring?
Fig. 26-23
ArchaeaBacteria
Eukarya
Fig. 26-UN2
Taxon F
Sister taxa
Node
Polytomy
Most recentcommonancestor
Taxon E
Taxon D
Taxon C
Taxon B
Taxon A
Fig. 26-UN3
F
Polyphyletic group
Monophyletic group
Paraphyletic group
E
D
C
B
A
G
AA
BB
CC
D D
E E
FF
GG
Fig. 26-UN4
Lizard
Salamander
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 ReefVideo: Coral Reef
Fig. 32-1
• There are exceptions to nearly every criterion for distinguishing animals from other life-forms
• Several characteristics, taken together, sufficiently define the group
Concept 32.1: Animal are multicellular, heterotrophic
eukaryotes with tissues that develop from embryonic layers
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 embryonic tissues
Video: Sea Urchin Embryonic DevelopmentVideo: Sea Urchin Embryonic Development
Fig. 32-2-1
Zygote
Cleavage
Eight-cell stage
Fig. 32-2-2
Zygote
Cleavage
Eight-cell stage
Cleavage Blastula
Cross sectionof blastula
Blastocoel
Fig. 32-2-3
Zygote
Cleavage
Eight-cell stage
Cleavage Blastula
Cross sectionof blastula
Blastocoel
Gastrulation
BlastoporeGastrula
Archenteron
Ectoderm
Endoderm
Blastocoel
• 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
OTHEREUKARYOTES
Choanoflagellates
Sponges
Other animals
An
imals
Individualchoanoflagellate
Collar cell(choanocyte)
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
(a) Mawsonites spriggi (b) Spriggina floundersi
1.5 cm 0.4 cm
Fig. 32-4a
(a) Mawsonites spriggi
1.5 cm
Fig. 32-4b
(b) Spriggina floundersi
0.4 cm
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
RESULTS
Site ofgastrulation
100
µm
Site ofgastrulation
Fig. 32-6a
RESULTS
100
µm
Fig. 32-6b
RESULTS
Site ofgastrulation
Fig. 32-6c
RESULTS
Site ofgastrulation
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. 32-8Coelom
Body covering(from ectoderm)
Digestive tract(from endoderm)
Tissue layerlining coelomand suspendinginternal organs(from mesoderm)
(a) Coelomate
Body covering(from ectoderm)
Pseudocoelom
Digestive tract(from endoderm)
Muscle layer(frommesoderm)
(b) Pseudocoelomate
Body covering(from ectoderm) Tissue-
filled region(frommesoderm)
Wall of digestive cavity(from endoderm)
(c) Acoelomate
Fig. 32-8a
CoelomBody covering(from ectoderm)
Digestive tract(from endoderm)
Tissue layerlining coelomand suspendinginternal organs (from mesoderm)
(a) Coelomate
• A pseudocoelom is a body cavity derived from the mesoderm and endoderm
• Triploblastic animals that possess a pseudocoelom are called pseudocoelomates
Fig. 32-8b
Pseudocoelom
Body covering(from ectoderm)
Muscle layer(frommesoderm)
Digestive tract(from endoderm)
(b) Pseudocoelomate
• Triploblastic animals that lack a body cavity are called acoelomates
Fig. 32-8c
(c) Acoelomate
Body covering(from ectoderm)
Wall of digestive cavity(from endoderm)
Tissue-filled region(from mesoderm)
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 Radial and indeterminate
Coelom
Archenteron
(a) Cleavage
(b) Coelom formation
Coelom
KeyEctodermMesodermEndoderm
Mesoderm MesodermBlastopore Blastopore
Solid masses of mesodermsplit and form coelom.
Folds of archenteronform coelom.
Anus Mouth
Digestive tube
Mouth AnusMouth develops from blastopore. Anus develops from blastopore.
(c) Fate of the blastopore
Fig. 32-9a
Eight-cell stage Eight-cell stage(a) Cleavage
Spiral and determinate Radial and indeterminate
Protostome development(examples: molluscs,
annelids)
Deuterostome development(examples: echinoderms,
chordates)
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
Coelom
Protostome development(examples: molluscs,
annelids)
Deuterostome development(examples: echinoderms,
chordates)
(b) Coelom formation
Key
EctodermMesoderm
Endoderm
MesodermMesoderm
Coelom
Archenteron
Blastopore Blastopore
Solid masses of mesodermsplit and form coelom.
Folds of archenteronform 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
Anus
Protostome development(examples: molluscs,
annelids)
Deuterostome development(examples: echinoderms,
chordates)
Anus
Mouth
Mouth
Digestive tube
(c) Fate of the blastopore
Key
EctodermMesoderm
Endoderm
Mouth develops from blastopore. Anus develops from blastopore.
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
ANCESTRALCOLONIALFLAGELLATE
Me
tazo
a
Eu
me
tazo
a
“Porifera”
Bila
teria
De
ute
ros
tom
iaP
roto
sto
mia
Cnidaria
Ctenophora
Ectoprocta
Brachiopoda
Echinodermata
Chordata
Platyhelminthes
Rotifera
Mollusca
Annelida
Arthropoda
Nematoda
• One hypothesis of animal phylogeny is based mainly on molecular data
Fig. 32-11
Silicea
ANCESTRALCOLONIALFLAGELLATE
Metazo
a
Eu
metazo
a
“Po
rifera”
Bilateria
Deu
tero
stom
ia
Lo
ph
otro
cho
zoa
Ecd
ysozo
a
Calcarea
Ctenophora
Cnidaria
Acoela
Echinodermata
Chordata
Platyhelminthes
Rotifera
Ectoprocta
Brachiopoda
Mollusca
Annelida
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 tuftof cilia
Mouth
(a) An ectoproct (b) Structure of a trochophore larva
100
µm
Anus
Future Directions in Animal Systematics
• Phylogenetic studies based on larger databases will likely provide further insights into animal evolutionary history
Fig. 32-UN1
Common ancestorof all animals
Truetissues
Sponges(basal animals)
Ctenophora
Cnidaria
Acoela (basalbilaterians)
Deuterostomia
Lophotrochozoa
Ecdysozoa
Metazo
a
Eu
metazo
a
Bilateria (m
ost an
imals)
Bilateralsummetry
Three germlayers
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 grades
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