chapter 32 Flashcards
Evolution of the Animal Body Plan
Five key transitions can be noted in animal evolution Tissues Symmetry Body cavity Development Segmentation
Evolution of the Animal Body Plan
. 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
Evolution of the Animal Body Plan
- 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
Evolution of the Animal Body Plan
Radial symmetry
- Body parts arranged around central axis-Can be bisected into two equal halves in any 2-D plane
- Bilateral symmetry (Bilateria)
- Body has right and left halves that are mirror images
- Only the sagittal plane bisects the animal into two equal halves
Evolution of the Animal Body Plan
Bilaterally symmetrical animals have two main advantages over radially symmetrical ones
- Cephalization
- Evolution of a definite brain area - Greater mobility
Evolution of the Animal Body Plan
- Evolution of a body cavity
- Three basic kinds of body plans
- Acoelomates = No body cavity
- Pseudocoelomates = Body cavity between mesoderm and endoderm
- Called the pseudocoelom
- Coelomates = Body cavity entirely within the mesoderm-Called the coelom
Evolution of the Animal Body Plan
The body cavity made possible the development of advanced organ 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—more efficient
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
. 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-layer-thick ball with:
- Blastopore = Opening to outside
- Archenteron = Primitive gut
Deuterostomes differ from protostomes in three other fundamental embryological features: -1. Cleavage pattern of embryonic cells -Protostomes = Spiral cleavage -Deuterostomes = Radial cleavage -2. Developmental fate of cells -Protostomes = Determinate development fate determined early -Deuterostomes = Indeterminate development—fate determined later
-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
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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Traditional Classification of Animals
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
A New Look At Metazoans
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 until recently have been allied with annelids
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 do not necessarily reflect evolutionary relationships.
Molecular systematics uses unique sequences within certain genes to identify clusters of related groups
Most new phylogenies agree on two revolutionary features:
- Separation of annelids and arthropods into different clades
- Division of the protostome group into Ecdysozoa and Spiralia
- The latter is then broken down into Lophotrochozoa and Platyzoa
--lophophore--feeding structure horseshoe-shaped crown of ciliated tentacles around the mouth—used in filter-feeding Bryozoa and Brachiopoda --Free-living larva called a trochophore Mollusca and Annelida
Evolutionary Developmental Biology
Most taxonomists agree that the animal kingdom is monophyletic
-Three prominent hypotheses have been proposed for the origin of metazoans from single-celled protists
- The multinucleate hypothesis—evolved from a multinucleated protistan
- The colonial flagellate hypothesis—
evolved from a choanoflagellate protistan - The polyphyletic origin hypothesis—
evolved more than once
Molecular systematics using rRNA sequences settles this argument in favor of the colonial flagellate hypothesis
Molecular analysis may also explain the Cambrian explosion
- The enormous expansion of animal diversity in the Cambrian period (543–525 MYA)
- The homeobox (Hox) developmental gene complex evolved
- Provided a tool that can produce rapid changes in body plan
Metazoa—multicellular
Parazoa—”near animals”
Platyzoa—acoelomate protostomes
Lophotrochozoa—coelomate protostomes
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