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


Pseudocoelom


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)


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


Muscle layer(frommesoderm)


(b) Pseudocoelomate

• Triploblastic animals that lack a body cavity are called acoelomates

Fig. 32-8c

(c) Acoelomate


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


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


annelids)


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


annelids)


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

Documents

Chapter 26 Phylogeny and the Tree of Life. Fig. 26-1