Transcript The Living World

The Living World
Fourth Edition
GEORGE B. JOHNSON
21
Evolution of the
Animal Phyla
PowerPoint® Lectures prepared by Johnny El-Rady
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21.1 General Features of Animals
All animals are multicellular heterotrophs
They all require oxygen for respiration
Animals are diverse in form
There are ~ 10 million living species
~ 99% are invertebrates (lacking a backbone)
~ 1% are vertebrates (possessing a backbone)
There are about 36 phyla
Most occur in the sea
Three phyla dominate life on land
Arthropoda; Mollusca; Chordata
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Animals lack cell walls
They are usually quite flexible
Animals are mobile
They move more rapidly and in more complex ways than
members of other kingdoms
Most animals reproduce sexually
An animal develops from a zygote by a characteristic
process of embryonic development
Morula  Blastula  Gastrula
Details vary widely between phyla
Provide clues to evolutionary relatedness
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21.2 The Animal Family Tree
Taxonomists have traditionally created phylogenies
(family trees) based on two main criteria
Anatomical features
Embryonic development
Fig. 21.3
The first branch: Tissues
Parazoa (“beside animals”)
Lack tissues and symmetry
Eumetazoa (“true animals”)
Possess tissues and symmetry
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The second branch: Symmetry
Eumetazoa have two branches
Radiata
Fig. 21.3
Have radial symmetry
Bilateria
Have bilateral symmetry
Further branches
Based on key features of the body plan shared
by all animals belonging to that branch
This either-or nature has produced a family tree
with a lot of paired branches
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Fig. 21.2
Evolutionary
trends
among the
animals
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A New Look at the Animal Family Tree
Puzzling minor groups do not fit well into the
standard either-or organization
Myzostomids
Fig. 21.4
Marine animals
Parasites or symbionts of
echinoderms
Have no body cavity and
incomplete segmentation
Have been aligned in some fashion with annelids
Recent molecular comparison has revealed that they
are more closely aligned with flatworms
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Molecular systematics uses unique sequences within
certain genes to identify clusters of related groups
The animal phylogenetic tree is viewed as a hierarchy of
clades nested within larger clades
Using DNA/RNA/protein data, a variety of molecular
phylogenies have been produced in the last decade
They all have the same deep branch structure as the
traditional animal family tree
However, most agree on a revolutionary difference
The protostomes are broken into two distinct clades
Lophotrochozoa
Ecdysozoa
Grow by adding
mass to an
existing body
Grow by molting
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Fig. 21.5
Proposed
revision of
the animal
tree of life
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“Evo-Devo” and Roots of the Animal Family Tree
There are three prominent hypotheses for the origin
of metazoans from single-celled protists
1. The multinucleate hypothesis
The ancestor was a multinuclear protist like today’s
ciliates
2. The colonial flagellate hypothesis
The ancestor was a colonial protist
Hollow spherical colony of flagellated cells
3. The polyphyletic origin hypothesis
Sponges evolved independently of eumetazoans
Molecular evidence clearly favors hypothesis #2
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“Evo-Devo” and Roots of the Animal Family Tree
Nearly all the major animal body plans can be seen
in Cambrian rocks dating from 543-525 mya
However, the branches of the animal family tree
arose in the earlier Ediacaran era
Biologists have long debated what caused this
Cambrian explosion of animal diversity
Proposed reasons include
Emergence of predatory lifestyles
Geological factors, such as the buildup of
minerals in the oceans
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“Evo-Devo” and Roots of the Animal Family Tree
A third possibility for the Cambrian explosion comes
from the new field of “evo-devo”
A synthesis of evolutionary and devolepmental
biology
Much of the variation in animal body plan is
associated with changes in the Hox gene complex
Family of genes controlling animal development
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21.3 Sponges: Animals
Without Tissues
The Kingdom Animalia consists of two subkingdoms
Parazoa
Animals that lack symmetry and possess
neither tissues nor organs
1 phylum: Porifera
Eumetazoa
Animals that have symmetry and in most cases
tissues and organs
About 35 phyla
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Sponges are the simplest animals
Bodies consist of little more than masses of
specialized cells embedded in a gel-like matrix
The adult sponge is shaped like a vase
It is anchored in place on the seafloor
Fig. 21.7
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Sponges are perforated by tiny holes
Basis of the Phylum name Porifera
Unique flagellated cells called choanocytes or
collar cells, line the body cavity of the sponge
Beating of the flagella draws water in through the
pores and drives it through the cavity
The sponge is a ”filter-feeder”
The choanocytes of sponges very closely resemble
a kind of protist called choanoflagellates
These may be the ancestors of all animals
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Fig. 21.8
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Fig. 21.8
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21.4 Cnidarians: Tissues Lead to
Greater Specialization
The structure of eumetazoans is much more
complex than that of sponges
Radially symmetric eumetazoans form two distinct
embryonic layers
An outer ectoderm  epidermis
An inner endoderm  gastrodermis
A jelly-like layer called the mesoglea forms
between the epidermis and gastrodermis
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There are two radially symmetric phyla
Together, they are called Radiata
1. Cnidaria
Hydra
Corals
Fig. 21.9
Jellyfish
Sea anemone
2. Ctenophora
A minor phylum that includes the comb jellies
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Cnidarians
Carnivores that capture their prey with tentacles
Bear unique stinging cells called cnidocytes
Contain a small but powerful harpoon called a
nematocyst
A major evolutionary innovation among the radiates
is extracellular digestion of food
In radiates, digestion begins in the gastrovascular
cavity
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Fig. 21.10
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Cnidarians have two basic body forms
Medusae
Free-floating,
gelatinous and often
umbrella-shaped
Polyps
Cylindrical, pipeshaped and usually
attached to a rock
Fig. 21.11
Cnidarians may exist exclusively as either/or
Others alternate between the two phases
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Fig. 21.12 The life cycle of Obelia, a marine colonial hydroid
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21.5 Solid Worms: Bilateral Symmetry
Radiates are radially
symmetrical
Have a regular arrangement
of parts around a central axis
All other eumetazoans are
bilaterally symmetrical
Have right and left halves
that are mirror images
Fig. 21.13
Dorsal (top) vs.
Ventral (bottom)
Anterior (front) vs.
Posterior (back)
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Bilaterally symmetrical animals have evolved a
definite head end, a process called cephalization
Solid worms are the simplest of all bilaterally
symmetrical animals
Nervous system
Ectoderm
Mesoderm
Endoderm
Lack internal
cavities, except
for digestive tract
Acoelomate
+
Fig. 21.14
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Flatworms
Members of Platyhelminthes
Planaria
The largest phylum of solid worms
Simplest animals in which
organs occur
Some species are free-living
Most species are parasitic
Tapeworms
Fig. 21.15a
Flukes
Many require two or more hosts
to complete their life cycle
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Fig. 21.16
Life cycle of
the human
liver fluke,
Clonorchis
sinensis
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Fig. 21.17
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Flatworms
Have incomplete gut with only one opening
Cannot eat, digest and excrete food simultaneously
Have an excretory system consisting of a network
of tubules running throughout the body
Have a simple nervous system
Lack a circulatory system
Most are hermaphroditic
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21.6 Roundworms: The Evolution
of a Body Cavity
The evolution of an internal body cavity was
important for three reasons
Circulation
Rapid passage of material
Movement
Muscle-driven body movement
Organ function
Little deformation by surrounding muscles
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Three kinds of body plans
Fig. 21.19
Acoelomates
Have no body cavity
Pseudocoelomates
Have body cavity between
mesoderm and endoderm
Pseudocoel
Coelomates
Have body cavity entirely
within mesoderm
Coelom
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Roundworms: Pseudocoelomates
There are seven phyla
The pseudocoel serves as a hydrostatic skeleton
Gains rigidity from being filled with fluid under pressure
Therefore muscles can work against this “skeleton”
Lack a defined circulatory system
Two important phyla
Cilia aid in feeding
and locomotion
Nematoda
Fig. 21.20
Rotifera
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Phylum Nematoda
Nematodes are bilaterally symmetrical, cylindrical,
unsegmented worms
Covered by a thick, flexible cuticle
Mouth is equipped with piercing organs called stylets
Food passes through the mouth by the sucking
action of the pharynx
Lack flagella or cilia
Reproduction is sexual
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Fig. 21.21
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Phylum Nematoda
Caenorhabritis elegans
Only animal whose complete cellular anatomy is
known
Trichinella sp.
Cause trichinosis
Acquired from pigs
Ascaris lumbricoides
Intestinal roundworm
Infects one out of every six people worldwide
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21.7 Mollusks: Coelomates
The bulk of the animal kingdom consists of
coelomates
Development of specialized tissues in animals
involves primary induction
In this process one of the three primary tissues
interacts with another
A major advantage of the coelomate body plan is
that it allows mesoderm–endoderm contact
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Mollusks
Only major phylum of coelomates without a
segmented body
The second largest animal phylum, after Arthropods
The body consists of three distinct parts
Head-foot; Visceral mass; Mantle
Gills capture O2 from water and release CO2
The radula is a rasping, tongue-like organ
Used to scrape algae off rocks
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The three major groups
of mollusks
Fig. 21.22
Gastropods
Snails and slugs
Bivalves
Clams, oysters and
scallops
Cephalopods
Octopuses and squids
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Fig. 21.23
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21.8 Annelids: The Rise
of Segmentation
Segmentation is the building of a body from a series
of similar segments
It offers evolutionary flexibility
Small change in existing segment can produce
a new segment with a different function
The first segmented animals to evolve were the
annelid worms, phylum Annelida
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Most annelid species are marine
About one-third are terrestrial
Representative annelids
Fig. 21.24
Earthworm
Shiny bristle
worm
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The basic body plan is a tube within a tube
Three characteristics
Repeated segments
Separate segments able to expand or contract
independently
Specialized segments
Front segments contain the worm’s sensory organs
Connections
Materials and information pass through partitions in
the segments
Segmentation underlies the body organization of all
complex coelomate animals
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Fig. 21.25
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21.9 Arthropods: Advent of
Jointed Appendages
Arthropods belong to
the phylum Arthropoda
The most successful of
all animal groups
2/3rd of all named species
80% of all arthropods are
insects
Scientists estimate that a
quintillion insects are
Fig. 21.26
alive at any one time
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All arthropods have jointed appendages
They have a rigid external skeleton made up of chitin
This exoskeleton protects the animals and provides sites
for muscle attachment
It is brittle, so its thickness limits arthropod body size
Arthropod bodies are
segmented
Most larval stages
have many segments
These fuse into
functional groups
in the adult
Fig. 21.27
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Fig. 21.28
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Chelicerates
Arthropods that lack jaws, or mandibles
Their mouthparts, called chelicerae, are the foremost
appendages of the body
Mandibulates are arthropods that have mandibles
Jumping spider
Bullfrog ant
Fig. 21.29
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Chelicerates
Arachnida is the largest class of chelicerates
57,000 named species
Largely terrestrial organisms
Spiders, ticks, mites, scorpions
Brown recluse
Black widow
Fig. 21.31
Poisonous spiders
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Mandibulates
Crustaceans
Large, diverse group of primarily aquatic organisms
Differ from insects, but resemble millipedes and
centipedes, in having legs on their abdomen and thorax
May be
compound
Used for
swimming and
reproduction
Fig. 21.33
Body of a
lobster
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Mandibulates
Crustaceans
Fig. 21.32
Edible crab
Three general types
1. Decapods
Crabs, shrimp, lobsters
2. Pillbugs and sowbugs
Terrestrial crustaceans
3. Barnacles
Sessile as adults
Sowbug
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Mandibulates
Millipedes and Centipedes
Consist of a head region followed by numerous similar
segments
Centipedes have one pair of legs on each segment
Millipedes have two pairs of legs on each segment
Active
carnivores
Fig. 21.34
Sedentary
herbivores
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Mandibulates
Insects
The largest group of arthropods by far
~ 1 million species identified
Fig. 21.36
Beetle
Flea
Honeybee
Grasshoppers
Moth
Dragonfly
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Mandibulates
Insects
Butterfly
Mosquito
Have three body sections
1. Head
With a pair of
antennae and
elaborate mouthparts
2. Thorax
Fig. 21.35
Three segments, each with a pair of legs
Wings are attached
3. Abdomen
Up to 12 segments
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Housefly
21.10 Protostomes and
Deuterostomes
In the coelomates there are two different
developmental patterns
In protostomes, the mouth develops from or near
the blastopore
The anus (if present) develops later from
another region of the embryo
In deuterostomes, the anus develops from or near
the blastopore
The mouth develops later from another region
of the embryo
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Fig. 21.37
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Fig. 21.37
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Deuterostomes differ from protostomes in three
other fundamental embryological features
1. Cleavage pattern
In protostomes, the egg cleaves spirally
In deuterostomes, the egg cleaves radially
2. Developmental fate of cells
In protostomes, the cells are committed even early on
In deuterostomes, the commitment occurs later
3. Origination of coelom
In protostomes, it forms directly from the mesoderm
In deuterostomes, it forms indirectly via the archenteron
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21.11 Echinoderms:
The First Deuterostomes
Echinoderms have an endoskeleton composed of
hard calcium-rich ossicles that are often fused
They consist of about 6,000 living marine species
Sand dollar
Feather star
Sea urchin
Sea star
Fig. 21.38
Sea cucumber
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Echinoderms are bilaterally symmetrical as larvae
But they become radially symmetrical as adults
This could be an environmental adaptation
Adults have a five-part body plan
The key evolutionary innovation is the development
of a water vascular system
A fluid-filled system with a central ring canal and
five radial canals
Thousands of tiny, hollow tube feet extend from
each radial canal
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Fig. 21.39
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21.12 Chordates:
Improving the Skeleton
Chordates are members of the phylum Chordata
Their nearest animal relations are the echinoderms
However, chordates employ a truly internal endoskeleton
Chordates are quite diverse
Lancelets
Tunicate
Fig. 21.40
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Distinguishing features of chordates
1. Notochord
A stiff, but flexible rod, that forms beneath the nerve cord
2. Nerve cord
A single dorsal nerve to which other nerves are attached
3. Pharyngeal slits
A series of slits behind the mouth into the pharynx
4. Postanal tail
A tail that extends beyond the anus
All chordates have all four of these at some time in
their life
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Fig. 21.41
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Vertebrates
In their body plan,
chordates are
segmented
Segments are
called somites
Muscle somites
Fig. 21.42 A mouse embryo
(at 11.5 days of development)
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With the exception of tunicates and lancelets, all
chordates are vertebrates
Distinguishing features of vertebrates
1. Backbone
A bony vertebral column replaces the notochord
2. Head
Well-differentiated, with skull and brain
All vertebrates have an internal skeleton made of
bone and cartilage against which the muscles work
This makes possible great size and movement
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