Introduction to

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Chapter 32 Introduction to Animal Evolution
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26.16 Our changing view of biological diversity
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26.1 Some major episodes in the history of
life.Note that molecular evidence puts origin
of animals at 1 bya.
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26.2 Clock analogy for some key events in
evolutionary history
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32.4 A traditional view of animal diversity
based on body-plan grades
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Kingdom Animalia general characteristics (and
yes, there are exceptions)

• Multicellular, heterotrophic eukaryotes.
• Feed by ingestion.
• Lack cell walls. Bodies held together by
  structural proteins embedded in tissues
  (collagen) and between cells.
• Presence of nervous and muscle tissue
• Store carbohydrate reserves as glycogen
• Reproduce sexually with characteristic
  development of the early 2N embryo.
• Transformation and development of the zygote
  controlled by special regulatory genes (Hox
  genes).

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I.  What is an animal? A.  Structure, nutrition,
and life history define animals 1. Animals are
multicellular, heterotrophic eukaryotes.          
                           - Animals must take
in organic molecules by ingestion they eat other
organisms or organic material that is
decomposing.   2.  Animal cells have no cell
walls.  - Bodies are held together by proteins,
especially collagen.   3.  Animals have two
unique types of tissues                          
           a.  Nervous tissue and                 
                    b.  Muscle tissue
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4.  Most animals reproduce sexually.              
                       a.  Dominant stage is
typically diploid. b.  Motile sperm fertilizes a
larger, non-motile egg (both 1n). c.  The
resulting zygote (2n) goes through embryonic
development as follows       i.  The zygote
undergoes cleavage, a series of mitotic cell
divisions.   ii.  Cleavage results in the
formation of a blastula, a hollow ball of
cells.   iii.  Further development results
in the formation of a gastrula, a two-layered,
cup-shaped cluster of cells.                      
                                       - This is
the stage where tissue differentiation
occurs     - Endoderm digestive tract     -
Ectoderm skin, nerves
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      iv.  Many embryos develop directly into
adults (sexually mature organisms)                
                                       v.  Some
develop into larvae (sexually immature organisms)
and later undergo metamorphosis to the adult
stage  
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32.1 Early embryonic development
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47.8 Cleavage in a frog embryo
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47.6 Cleavage in an echinoderm (sea urchin)
embryo
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Sea urchin development, from single cell to larva
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•  5.  Growth from embryo to adult is modulated
  (controlled and organized) by Hox genes. 
• Many genes are the same or similar in all
  animals.  The sequence in which they are turned
  on and off during development causes embryos to
  develop into different animals. 
• Thus, for example, the same genes that give rise
  to dolphins and humans are in both organisms, but
  the sequence and time in which they are turned on
  creates either the dolphin or human. This is
  controlled by the Hox genes.
•  

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Animals probably evolved from colonial,
flagellated protists, like this choanoflagellate
colony. https//youtu.be/JVqxyYBuI_U
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One hypothesis for the origin of animals from a
flagellated protist.
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• II.  Two views of animal diversity
•            
• The phylogenetic tree of animals has about 35
  phyla.
• Continuous remodeling of the tree
•                        
• 1.  The traditional view of animal diversity is
  based on anatomy and embryology.
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• 2.  Newer trees are based on molecular evidence.
•                        

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https//youtu.be/8UBBOEBfBEc
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This section will focus on the four main branch
points in this phylogenetic tree.   1.  Porifera
(Sponges) early branch à Parazoa structural
simplicity no true tissue differentiation        
                 ? Separated from Eumetazoa all
have true tissue differentiation. The next phyla
are all Eumetazoa   2.  Radiata radial
symmetry in body structure (no left/right side)
Example  jellyfish Separated from Bilateria
two-sided symmetry à left/right, dorsal/ventral,
anterior/posterior cephalization sensory
organs concentrated on anterior end
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Body symmetry
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3.  Acoelomates no body cavity                  
             Example  flatworms                  
       Separated from Pseudocoelomates (e.g.,
nematodes) and Coelomates (e.g., humans) have
body cavities                                    
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 4.  Protostomia body cavity forms from cell
masses, blastopore becomes mouth                  
                   Examples  snails, worms,
insects                         Separated from
Deuterostomia body cavity develops from
digestive tube, blastopore becomes
anus                                    
Examples  starfish, chordates (humans)  
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A comparison of early development in protostomes
and deuterostomes
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Two views of animal phylogeny Which is right?
Both are useful?
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III.  Origins of animal diversity A.  Most
animal phyla originated in a relatively brief
span of geologic time.                        
1.  Modern phyla developed in about 40 million
years total.   2.  During the Cambrian Explosion
(543 to 524 million years ago), nearly all major
body plans appeared. Really 3 explosions, 1 each
within lophotrochozoa, ecdysozoa,
deuterostomia?
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26.8 The Cambrian radiation of animals
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B.  What caused the Cambrian explosion?           
              1.  Development of predators and
evolution toward prey escaping/predator hunting.
Increased need for speed and better sensory
equipment.   2.  Oxygen levels reached present
levels that allow for rapid metabolism exhibited
by animals.   3.  Hox genes evolved at that time
and allowed for differential development.