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The Evolution of Animal Diversity

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The Evolution of Animal Diversity TheEvolution of Animal Diversity Objectives Introduction Describethe difficultiesof classifyingthe duck-billedplatypus and otherAustralian mammals. AnimalEvolution and Diversity 18.1 Define animalsand distinguishthem from other forms of life. 18.1 Describethe generalanimal life cycle and the basic body plan. 18.2 Describethe five-stagehypothesis for the evolutionof animalsfrom protists' 18.2 Describethe Cambrianexplosion and list threehypotheses to explainits occurrence. lnvertebrates 1E.!18.15 Describethe characteristicsof anddistinguish between the following phyla: Porifera, Cnidaria,Platyhelminthes, Nematoda, Mollusca, Annelida, Arthropoda, Echinodermata, Chordata. 1831E.15 Distinguishbetween radially and bilaterallysymmetric animals. Note which body type is found in each of the phyla examinedin this chapter. 18.7,1E.8 Distinguishbetween a pseudocoelomand a coelom.Describe the functionsof eachand note the animal phyla where they occur. 18.10 Define segmentation,explain its functions, and note the animal phyla where it occurs. 18.13 Describethe commoncharacteristics of insects.Distinguish between the seveninsect orders describedin this chaPter. Vertebrates 18.16 Describethe definingcharacteristics of vertebrates. 18.17-1E.22 Describethe characteristicsof and distinguishbetween the following vertebrategroups: agnathans,Chon&ichthyes, Osteichthyes, Amphibia, Reptilia,Aves, Mammalia. Phylogenyof the Animal Kingdom 18.23 Describethe fwo main classificationsof the major animal groups.Explain why there are differencesin thesetwo systems. 18.24 Describethe consequencesof introducedspecies in Australia. KeyTerms kingdomAnimalia mesoderm radial symmetry ingestion larva choanocyte blastula metamorphosis amoebocyte gastrula invertebrate suspensionfeeder ectoderm sponge choanoflagellate endoderm Porifera Cnidaria t73 r74 Instructor's Guide to Textand Media polyp segmentatron lancelet medusa Annelida skull gasfrovascularcavity earthworm backbone cnidocyte polychaete vertebra bilateral syulmetry leech agnathan anterior Arthropoda Chondrichthyes posterior arthropod Osteichthyes dorsal exoskeleton cartilaginousfish ventral molting lateral line system lateral horseshoe crab bony fish flatworm arachnid operculum Platyhelminthes crustacean swim bladder free-living flatworm millipede ray-finnedfish fluke centipede lobe-finnedflsh tapeworm entomology lungfish bodycavity incomplete metamorphosis Amphibia pseudocoelom complete metamorphosii Reptilia coelom echinoderm amniotic egg roundworm Echinodermata ectothermic Nematoda endoskeleton endothermic mollusk water vascular system Aves Mollusca Chordata Mammalia foot dorsal, hollow nerve cord monotreme visceralmass notochord placenta mantle pharyngeal slit marsupial radula post-anal tail placental gastropod chordate eutherian bivalve vertebrate protostome cephalopod tunicate deuterostome Word Roots choano- : a funnel; -cyte : cell (choanocyte:flagellated collar cells of a sponge) cnido- : a nettle (cnidocytes:unique cells that function in defenseand prey capturein cnidarians) cuti- : the skin (cuticle: the exoskeletonof an arthropod) deutero- : second(deuterostome.' one of two distinct evolutionarylines of coelomatescharacterized by radial, indeterminatecleavage, enterocoelous formation of the coelom, and developmentof the anusfrom the blastopore) echino- : spiny;-derm : skin (echinoderm.'sessile or slow-movinganimals with a thin skin that cov- ers an exoskeleton;the group includessea stars, sea urchins, brittle stars,crinoids, and basketstars) ecto- : outside; -derm : skin (ectoderm:the outermostof the three primary germ layers in animal embryos) endo- : inner; themr- : heat (endotherm.'an animal that uses metabolicenergy to maintain a constant body temperature,such as a bird or mammal) gastro- : stomach;-vascula : a little vessel(gastrovascular cavity: the centraldigestive compartment, usuallywith a singleopening that functionsas both mouth and anus) in- : without (irwenebrate:an animal without a backbone) Chapter 18 TheEvolution of Animal Diversity t75 : l marsupi- a bag, pouch (marsupial:a mammal,such as a koala,kangaroo, or opossum,whose young completetheir embryonicdevelopment inside a maternalpouch called the marsupium) meso- : middle (rnesoderm:the middle primary germ layer of an early embryo) meta- : boundary,tuming point; -morph : form (metarnorphosis;the resurgenceof developmentin an animal larva that transforms it into a sexually mature adult) mono- : one (rnonotreme;an egg-layingmammal, represented by the platypusand echidna) noto- : the back; -chord : a string (notochord:a longitudinal,flexible rod formed from dorsalmeso- derm and locatedbetween the gut and the nervecord in all chordateembryos) proto- : first; -stoma : mouth Qtrotostome.'a memberof one of two distinctevolutionary lines of coelomatescharactenzed by spiral,determinate cleavage, schizocoelous formation of the coelom,and developmentof the mouth from the blastopore) LectureOutline Intrcduction WhatAm I? A. Most known organismsare animals. 1. Of the 1.5 million known species,two-thirds are animals. Animal diversity progressedthrough millions of yearsof evolutionand naturalselection. 2. Humanshave a tendencyto appreciateand studyanimals, but what is an animal? Do we know one when we seeone and how do we classifydifferent animals? B. What am I (Figure18.0)? 1. The duck-billedplatypus was a taxonomicnighnnare when initially found by the first Europeanswho visited Ausftalia. It lays eggs,has a duck bill and webbedfeet . it's a bird-no, it has fur, a tail like a beaver,and mammary glands . it's a mammal- well . a specialmammal called a monofteme(an eggJayingmammal)! 2. Australia is full of specialmammals called monotremes and marsupials.The niches that were being filled with placentalmammals on other continentswere left empty when Australia broke away from Pangaea.Without competitionfrom other placental mammals,monotremes and marsupialsfilled in the empty niches and becamethe dominantmammals in Australia. Preview:Zoologists distinguish between animals with internalskeletons (vertebrates)and those without internal skeletons(invertebrates). All but one animal phylum (our own, phylum Chordata)are invertebrates(Module 18.15). I. Animal Evolution and DiversitY Module 1E.1 What is an animal? A. Animals are eukaryotic,multicellular heterotrophsthat lack cell walls and obtain nutri- tion by ingestion (Figure 18.1A).This distinguishesanimals from fungi that digesttheir food first and then absorbit. Most animals havemuscle cells for movementand nerve cells for sensoryperception. B. The life cycle of most animalsincludes a dominant,diploid adult that produceseggs or spermby meiosis.These gametes fuse to form a zygote.The zygotedevelops into the adult animal, passingthrough a seriesof embryonicstages, many of which are shared by most membersof the animalkingdom (Figure18.1). Review:This is an exampleof homology(Module 15.11). 176 Instructor'sGuide to Textand Media C. In all animalsthe embryonicstages include the blastula (hollow ball of cells) and, in most,a gastrula(a saclikeembryo with one openingand two layers of cells). Most further developan additionallayer of cells. Preview:Embryonic development (Modules 27.9-27.I5). D. In many animals,the gastruladevelops into one or more immaturestages, for example, larvae that developinto the sexually mature adults only after metamorphosis. NOTE: Larvaeare dispersivestages that help an animal'soffspring find suitablehabitats before continuingto grow. They are also extremely important sourcesof food for many other animalsin aquatichabitats. E. Animals haveunique types of cell junctions(Module 4.19). F. Animals are,with the exceptionof the gametes,composed of diploid cells. G. Specialregulatory genes called Hox genesconftol zygotedevelopment. These homeotic genesare presentonly in animals. Module L8.2 The animalkingdom probably originated from colonial protists. A. Fossilsof the oldestknown animalsdate from the late Precambrianera, =600 mya. B. A hypotheticalevolutionary scenario leading to the first animalproceeds as follows: 1. Colonialprotists of a few, identical,flagellated cells 2. coloniesthat ingested organic materials suspended in the li::';::ti:-;:rl;f;:1 3. Colonieswith cells ,p""iuti""a for somatic(movement, digestion, etc.) and repro- ductive functions 4. Differentiatedentities with an infolded, temporary digestiveregion 5. "Protoanimals,"completely infolded, with two-layeredbody walls (Figure 18.2) C. Theseprotoanimals probably "crawled," feeding on the oceanbottom. D. The modernanimal phyla evolved during the Cambrianperiod, =545 mya, a ten-mil- lion-year period of explosiveevolution. E. Three hypothesesexplain the rapid expansionof animal diversity during the early Cam- brian era: L Ecologicalcauses; increased dependency on the predator/preyrelationship. 2. Geologicchanges; atmospheric oxygen may have reacheda critical threshold that was high enoughto supportthe active lifestyle of animals. 3. Geneticcauses; the developmentof the Hox genessupported the diversity associ- ated with variationsin the spatial and temporal arrangementof the genesduring embryonicstages of development. NOTE: Somemutualistic synthesis of the three hypothesesis likely the correct answer. II. Invertebrates Module 18.3 Spongeshave relatively simple, porous bodies. A. Spongesare classifiedin the phylum Porifera. Most are marine and live singly, attachedto a substrate,and rangein height from I cm to 2 m (Figure 18.34). B. Many spongesare built on a body plan havingradial s5rmmetry,that is, similar shapes as mirror
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