Download ECOLOGY SPRING 2009 - Florida International University

Animals are the consumers of the Earth
They are a very diverse group
-However, they share major characteristics
-Are heterotrophs
-Are multicellular
-Have cells without cell walls
-Most are able to move
-Are very diverse in form and habitat
-Most reproduce sexually
-Have a characteristic pattern of
embryonic development
-Cells of all animals (except sponges) are
organized into tissues
Five key transitions can be noted in animal evolution
1. Tissues
2. Symmetry
3. Body cavity
4. Development
5. Segmentation
1. Evolution of tissues
-Parazoa (Sponges - the simplest animals) lack
defined tissues and organs
-Have the ability to disaggregate and
aggregate their cells
-Eumetazoa (all other animals) have distinct and
well-defined tissues
-Have irreversible differentiation for most cell
types
2. Evolution of symmetry
-Sponges also lack any definite symmetry
-Eumetazoa have a symmetry defined along an
imaginary axis drawn through the animal’s body
-There are two main types of symmetry
-Radial symmetry
-Body parts arranged around central axis
-Can be bisected into two equal halves in any 2D plane
-Bilateral symmetry
-Body has right and left halves that are
mirror
images
-Only the sagittal plane bisects the animal into
two equal halves
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Bilaterally symmetrical animals have two main
advantages over radially symmetrical ones
1. Cephalization
-Evolution of a definite brain area
2. Greater mobility
3. Evolution of a body cavity
-Eumetazoa produce three germ layers
-Outer ectoderm (body coverings and nervous
system)
-Middle mesoderm (skeleton and muscles)
-Inner endoderm (digestive organs and intestines)
Body cavity = Space surrounded by mesoderm tissue
that is formed during development
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3. Evolution of a body cavity
-Three basic kinds of body plans
-Acoelomates = No body cavity
-Pseudocoelomates = Body cavity between
mesoderm and endoderm
-Called the pseudocoel
-Coelomates = Body cavity entirely within the
mesoderm
-Called the coelom
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-The body cavity made possible the development of
advanced organs systems
-Coelomates developed a circulatory system to
flow nutrients and remove wastes
-Open circulatory system: blood passes from
vessels into sinuses, mixes
with body fluids
and reenters the vessels
-Closed circulatory system: blood
moves
continuously through vessels that
are
separated from body fluids
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4. Evolution of different patterns of development
-The basic Bilaterian pattern of development:
-Mitotic cell divisions of the egg form a
hollow
ball of cells, called the blastula
-Blastula indents to form a two-layerthick
ball with:
-Blastopore = Opening to outside
-Archenteron = Primitive body cavity
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Bilaterians can be divided into two groups:
-Protostomes develop the mouth first from or near
the blastopore
-Anus (if present) develops either from
blastopore or another region of embryo
-Deuterostomes develop the anus first from the
blastopore
-Mouth develops later from another region
of the embryo
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Deuterostomes differ from protostomes in three other
fundamental embryological features:
-1. Cleaveage pattern of embryonic cells
-Protostomes = Spiral cleavage
-Deuterostomes = Radial cleavage
-2. Developmental fate of cells
-Protostomes = Determinate development
-Deuterostomes = Indeterminate
development
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-3. Origination of coelom
-Protostomes = Forms simply and directly
from the mesoderm
-Deuterostomes = Forms indirectly from the
archenteron
Deuterostomes evolved from protostomes more than
500 MYA
5. Evolution of segmentation
-Segmentation provides two advantages
-1. Allows redundant organ systems in adults
such as occurs in the annelids
-2. Allows for more efficient and flexible
movement because each segment can move
independently
Segmentation appeared several times in the
evolution of animals
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Multicellular animals, or metazoans, are traditionally
divided into 36 or so distinct phyla based on shared
anatomy and embryology
Metazoans are divided into two main branches:
-Parazoa = Lack symmetry and tissues
-Eumetazoa = Have symmetry and tissues
-Diploblastic = Have two germ layers
-Triploblastic = Have three germ layers
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The traditional animal phylogeny is being reevaluated
using molecular data
Myzostomids are marine animals that are parasites of
echinoderms
-Have no body cavity and only incomplete
segmentation
-And so have been allied with annelids
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Recent analysis of the translation machinery revealed
that myzostomids have no close link to the annelids
at all
-Instead, they are
more closely allied
with the flatworms
(planaria and
tapeworms)
Therefore, key morphological characters used in
traditional classification are not necessarily
conservative
Molecular systematics uses unique sequences
within certain genes to identify clusters of related
groups
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Most new phylogenies agree on two revolutionary
features:
1. Separation of annelids and arthropods into
different clades
2. Division of the protostome group into
Ecdysozoa and Spiralia
-The latter is then broken down into
Lophotrochozoa and Platyzoa
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Most taxonomists agree that the animal kingdom is
monophyletic
Three prominent hypotheses have been proposed
for the origin of metazoans from single-celled
protists
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1. The multinucleate hypothesis
2. The colonial flagellate hypothesis
3. The polyphyletic origin hypothesis
Molecular systematics using rRNA sequences settles
this argument in favor of the colonial flagellate
hypothesis
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Molecular analysis may also explain the Cambrian
explosion
-The enormous expansion of animal diversity in the
Cambrian period (543 to 525 MYA)
-The homeobox (Hox) developmental gene
complex evolved
-Provided a tool that can produce rapid
changes in body plan
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