jimtrue.com : school : BSC2011 : CH 32: Introduction to Animal Evolution

Posted by Jim True on October 21, 2004 6:30 PM. Last Updated October 22, 2006 9:23 PM

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CH 32: Introduction to Animal Evolution

Animal Characteristics

• Highly diverse group of multicellular eukaryotes.
• All are heterotrophic, taking in food mainly via ingestion.
• All are capable of movement, at least during some part of life cycle. Just because a form is attached (like a barnacle or a sea anemone, doesn't mean they do not move, they are just stationary.
• Usually exhibit rapid response to a stimulus due to the presence of two unique tissues, muscle and nervous (which enervates the muscle to contract).
• Cell wall absent in all.
• Most animals reproduce sexually, with development following a specific series of stages:
• Flagellated sperm (n) fertilized n ovum.
• 2n zygote undergoes cleavage to blastula stage, then gastrula stage of cellular differentiation.
• Larva - sexually immature form which does not resemble parent and may live in different habitat. Undergoes metamorphosis - transformation to adult form.
• Hox (= homeobox) genes - Regulatory genes that control the expression of other genes. (these control other genes)
• Homeotic genes - master regulatory genes that control the identity, location and formation of specific body parts.
• Only in the animal kingdom are homeobox genes found WITHIN homeotic genes.
• The number of Hox genes is directly related to the animal's complexity.
• It is currently thought that the likeliest ancestral form for the modern day animal kingdom was a colonial, flagellated protistan type that lived ~700MYA (Pre-Cambrian era).
• Systematists agree that the kingdom is monophyletic. (LUCA holds true for the animal kingdom).

Animal Systematics

• There are currently two slightly different schematic schemes:
• the "traditional" outline based on various body characteristics
• The "modern" plan that incorporates molecular biology into the evaluation, especially by examining single strand RNa sequences.
• There are currently 35 different animal phyla recognized. We will examine 9 that represent the entire spectrum of systematic organization.
• We will look at the body plan characteristics first because many hold in the molecular biology plan as well.
• In order of complexity, the 9 phyla we will study are the:
  • Porifera - all are called "sponges".
  • Cnidaria - jellyfishes, sea anemones, man-o-war, corals
  • Platyhelminthes - flatworms, flukes and tapeworms.
  • Nematoda - roundworms.
  • Mollusca - clams, oysters, mussels, snails, conchs, whelks, slugs, nudibranchs, octopi, nautiluses, cuttlefishes and squids.
  • Annelida - the segmented worms, including "earthworms".
  • Arthropoda - spiders, scorpions, ticks, mites, shrimp, crabs, lobsters, horseshoe crabs, insects, centipedes, millipedes.
  • Echinodermata - sea urchins, sea stars (starfishes), sand dollars, sea lilies, sea cucumbers.
  • Chordata - sea squirts, lancelets, all fishes, amphibians, reptiles, birds and mammals.
  • (1)Traditional Systematics - This plan is based on several observable physical characteristics:
  • (A)Grades of Organization - Differing levels of structural complexity. Lower (more primitive) animals are less complex than more advanced forms.
  • (i)Cellular - There may be different cells in individuals performing different functions, but cells don't work together.
  • Some of the phylum Porifera possess cellular grade body construction. (Parazoa - beside animals).
  • (ii)Tissue - Groups of similar cells which together perform a specific function.
  • Some of the phylum Porifera may have simple tissues (?), also the phylum Cnidaria
  • (iii)Organ - Group of different tissues which perform a specific function.
  • Phylum Platyhelminthes exhibit an organ grade of organization.
  • (iv)Organ System - Group of organs which together perform specific functions.
  • Of phyla we'll examine, the phylum Nematoda and all more advanced forms.
  • (B)Symmetry - Refers to body proportions on either side of an imaginary plane drawn through the central axis of body.
  • (i)Asymmetry - No matching proportions on either side of the central axis.
  • Most members of the phylum Porifera
  • (ii)Radial - Closely matched proportions ("mirror images") on either side of ANY single plane through axis.
  • The phyla Cnidaria (Radiata) and Echinodermata are radially symmetrical animals .
  • (iii) Bilateral - Only a single central plane divides the body into two mirror image halves (left and right) (Mid-sagital division).
  • All other animal phyla we'll examine are bilateral (referred to as the Bilateria). Echinoderms are considered Bilateria because they begin life as bilateral symmetry and develop radial symmetry later in life.
  • In addition to left and right, body directions include:
    • (the back), ventral (the belly), Anterior (towards the front), and posterior (towards the rear).
    • In humans, dorsal and posterior are the same region, as is ventral and anterior.
  • Cephalization - concentration of sensory structures at anterior end ("head"). Better to put primary sensors in the portion of the body that hits the environment first (sensory towards the head).
  • (C)Tissue Layers - Tissues are produced by germ layers (ecto-, meso-, and endoderm).
  • (i)Zero Tissue Layers - No germ layers, animal is cellular grade of organization. Phylum Porifera.
  • (ii)Diploblastic - 2 tissue layers, one from endoderm, one from ectoderm. Phylum Cnidaria
  • (iii)Triploblastic - 3 tissue layers, all 3 germ layers involved. All other phyla are triploblastic.
  • (D>Coelom - this applies ONLY to bilaterally symmetrical organisms, therefore these terms have no application to the phyla Porifera or Cnidaria.
  • Coelom - ("cavity") - Refers to a fluid-filled space within the body walls OTHER than the cavity within the digestive system.
  • (i)Acoelomate - No coelom present. This was originally thought to be a primitive condition, but now is thought to be a later reduction in the Phylum Platyhelminthes.
  • (ii)Pseudocoelomate - Coelom present, arises from the blastocoel, does not possess a thin lining of connective tissue known as mesentery tissue..
  • Of phyla e'll examine, the Nematoda are pseudocoelomates.
  • (iii)Coelomate - Coelom present, arises from mesodermal tissue, is lined with mesentery. All other phyla. (Could be referred to as "eucoelomate' or 'True cavity'.
  • (E)Embryonic Development - Refers to common developmental structures or processes that indicate common ancestry among animal phyla.
  • There are two developmental lines, the Prostomia and the Deuterostomia.
  • Terms apply ONLY to bilateral coelomates. According to the traditional plan, the Porifera, Cnidaria, Platyhelminthes and Nematoda excluded.
  • According to the molecular plan, both the Platyhelminthes and Nematoda would be included.
  • (i)Protostomia ("first" + "mouth") - In early development, all of the following are observed:
  • Spiral cleavage - In early cleavage (8-16 cells), new cells develop around an imaginary axis at an offset angle to the cells from which they arise.Cleavage is the term for cell division in a developing organism
  • Thus, it appears that new cells "spiral" around the imaginary axis.
  • Determinate cleavage - Developmental fate of each cell is "set" as early as the 4 cell stage. Removal of any cell after this point results in embryonic death. The removal of any cell will kill the embryo.
  • Fate of the blastopore - the blastopore becomes the mouth (protostome).
  • Coelom originates by schizocoely ("splitting" + "cavity") - The mesoderm splits, creating coelom.
  • Originally included Annelida, Mollusca and Arthropoda; now includes Nematoda and Platyhelminthes
  • (ii)Deuterostomia ("second" + "mouth") - these possess:
  • Radial cleavage - New cells radiate out from imaginary axis directly in line with original cells.
  • Indeterminate cleavage - Developmental fate of cells not set until 16-32 cell stage.
  • Fate of the blastopore - Blastopore, or location close to it, becomes the anus.
  • Enterocoely ("gut" + "cavity") - The coelom initially forms as pouches that develop along the gut wall and gradually enlarge to become the coelom.
  • Deuterostomes include the phyla Echinoderma and Chordata.
  • This has not changed from the traditional to the molecular systematic concept.
  • (2)Molecular Systematics:
    • Still retains the non-tissue vs. tissue branch point, as well as the Radiata vs. Bilateria.
    • However, the acoelomate Platyhelminthes and the pseudocoelomate Nematoda are now included in two different subgroups of Protostomia:
    • (i)Lophootrochozoa - refers to the possession of either a special ciliated larva, the trocophore (Platyhelminthes, Mollusca and Annelida) or a horseshoe shaped ring of ciliated feeding tentacles, the lophophore (no phyla we will be studying).
    • (ii)Ecdysozoa - ("ecdysis" - to shed). refers to the shedding of the acellular exoskeleton (Arthropoda) or cuticle (Nematoda).
    • In order to continue growing, the animal must molt the old outer covering and secrete a new one.

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