Biology 1215 Lecture Notes

Chapter 32: Introduction to Animal Evolution

• Outline
  • General Characteristics of Animals
    
  • Major events in animal phylogeny
    
    • 1. Parazoa-Eumetazoa Split
      
    • 2. Radiata-Bilateria Split
    • 3. Acoelomate-Coelomate Split
      
    • 4. Protostome-Deuterostome Split
      
  • Origins of Animal Diversity

   

• Animal life began in Precambrian seas (Over 550 MYA) with evolution of multicellular forms that lived by eating other organisms. This allowed exploitation of untapped resources and adaptive zones and led to an evolutionary radiation of diverse forms.
  
• Over 1,000,000 species described.
  
• Based on anatomical and embryological criteria, animals are grouped into 35 phyla. Most of these are aquatic.
  
• Vertebrates consist of less than 5% of all animal species (i.e. 95% of all animals are INVERTEBRATES; very diverse).
  

What is an animal

• Defined by their mode of nutrition, structure and life cycle.
• Multicellular, heterotrophic eukaryotes.
  
• Take in preformed organic molecules by injestion.
  
• Store carbohydrate reserves as glycogen.
  
• Lack cell walls.
  
• Have intercellular junctions (i.e. tight junctions, desmosomes, gap junctions).
  
• Have nervous (impulse conduction) and muscle tissue (movement).
  
• Most reproduce sexually, with diploid stage being dominant in life cycle.
  
• Small flagellated sperm fertilizes larger, non-motile egg to form diploid zygote.
  
  • Zygote then undergoes cleavage, a succession of mitotic divisions (Fig 32.1). Cleavage leads to formation of multicellular stage called a blastula (hollow ball stage) then gastrulation (stage during which embryonic tissues of adult form are produced).
  • Hox genes of animals (involved in genetic regulation of development) contain homeobox.
    
• Life cycles of many animals include larval stages, which undergo metamorphosis into adult stage.
  
• Animals inhabit nearly all environments. Only annelids, mollusks, vertebrates and arthropods have made it onto land.
  

Major events in animal phylogeny Fig 32.3

• Animal kingdom probably eveolved from a colonial flagellated protist, related to modern day Choanoflagellates (Fig 32.2 and 3)
• Traditional view of animal diversity is based on body -plan grades and development (Fig 32.4). New molecular taxonomic evidence has made some significant alterations to this view.

1. Parazoa-Eumetazoa Split

• splits animals into those that lack true tissues (sponges) and those that don’t (all other animals)
  

2. Radiata-Bilateria Split

• Splits according to body symmetry (Fig 32.5).
  
  • radiata have only tops and bottoms (sessile animals).
    
  • bilateria have tops and bottoms, front and back, and left and right (active animals)
    
• Bilateral symmetry is associated with cephalization (concentration of sensory organs at anterior end of organism). A head end is an adaptation to forward movement.
  
• These two fundamentally different types of body plans probably arose very early in animal evolution.
  
• Equally important to body symmetry, the number of germ layers produced differs (Figure 32.6).
  
  • Radiata produce only two germ layers (diploblastic), endoderm and ectoderm..
    
  • Bilateria produce a third germ layer (mesoderm) between the ectoderm and endoderm.
    

3. Acoelomate-Coelomate Split (Fig 32.6)

• Splits between those triploblastic animals with (Coelomates) and without ( Acoelomates) a body cavity (between gut and outer body wall).
  
• Coelomates have tube-within-tube body plan, with fluid filled cavity separating digestive tract from body wall.
  
  • All Coelomates have some form of blood vascular system (blood that is circulated through a network of spaces or vessels).
    
• In Pseudocoelomates the cavity is not completely lined with tissue derived from mesoderm. (e.g. rotifers and nematodes)
  
• Functions of body cavity:
  
  • cushions suspended organs
    
  • enables internal organs to move and grow independently of outer body wall.
    
  • fluid filled cavity is under pressure and can serve as hydrostatic skeleton against which muscles can act.
    
• Coeloms evolved independently twice: in protostomes and deuterostomes.
  

4. Protostome-Deuterostome Split

• Protostomes include mollusks, annelids, arthropods.
  
• Deuterostomes include the echinoderms and chordates.
  
• Protostomes and and deuterostomes are distinguished by 3 fundamental differences in their development (Refer to Fig 32.7)
  
  • 1. Cleavage
    
    • Spiral vs Radial
      
    • Determinate vs Indeterminate
      
  • 2. Blastopore fate
    
    • In Protostomes blastopore becomes mouth.
      
    • In deuterostomes blastopore becomes anus.
      
  • 3. Coelom formation
    
    • Schizocoelous
      
    • Enterocoelous
      

Origins of Animal Diversity

• Based on anatomical and embryological criterion, animals group into 35 phyla (Fig 32.12 and 13).
  
• Most animal phyla and major body plans originated in Precambrian and early Cambrian times (565-525 million yrs ago). Early animals underwent rapid (in 40 MYA) radiation event, thus difficult to order events by fossil record.
  
• Systematists depend largely on clues from comparative anatomy and embryology.
  
• Animals are monophyletic (all had same single protistan ancestor).
  
• By early Cambrian, all animal phyla had evolved. The sudden appearance of these animals in the fossil record often referred to as the Cambrian explosion (Molecular evidence suggests 3 explosions (Fig 32.14).
  
• Ediacaran faunna is precambrian. First appearance of multicellular animals in fossil record.
  
• Early rapid radiation of animals led to many experimentations with body form and led to all present phyla.

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