Animal Evolution and Diversity

18.1 WHAT IS AN ANIMAL?

  • General Characteristics:

    • Eukaryotic cells with no cell walls

    • Multicellular organisms

    • Heterotrophic: acquire nutrients through ingestion

  • Classification of Animals:

    • Invertebrates: Make up 96% of all animal species

    • Vertebrates: Comprise the remaining 4%

  • Reproductive Cycle:

    • Most animals are diploid and reproduce sexually

    • Gametes are produced via meiosis and fuse to form a diploid zygote, which then develops into an adult through mitosis

  • Animal Development Stages:

    1. Blastula: Hollow ball of cells formed after fertilization

    2. Gastrula: Invagination leads to tissue layering and development of the digestive tract

    3. Larval Stage: Many undergo changes before reaching adulthood

  • Germ Layers:

    • Endoderm: Innermost layer forming the digestive tract

    • Mesoderm: Middle layer forming muscles and internal organs

    • Ectoderm: Outermost layer forming the nervous system and outer body covering

18.2 ANIMAL DIVERSIFICATION BEGAN MORE THAN HALF A BILLION YEARS AGO

  • Origins:

    • Likely evolved from a flagellated protist, similar to modern choanoflagellates

  • Cambrian Explosion (542 million years ago):

    • An adaptive radiation event resulting in diverse new animal forms in a relatively short time frame

    • Factors involved: ecological, geological, and genetic conditions

18.3 ANIMALS CAN BE CHARACTERIZED BY BASIC FEATURES OF THEIR "BODY PLAN"

  • Body Symmetry:

    • Radial Symmetry: No back/front distinction (e.g. sea anemones)

    • Bilateral Symmetry: Distinct anterior/posterior and dorsal/ventral surfaces (e.g. humans)

  • Cephalization:

    • Concentration of sensory organs and nerve cells at the anterior end, seen in many bilateral animals

  • Tissue Organization:

    • Sponges: Lack true tissues

    • Others: Have organized tissues derived from germ layers

  • Development Types:

    • Protostomes: Mouth develops from the gastrula’s opening (blastopore)

    • Deuterostomes: Anus develops from blastopore

  • Body Cavities:

    • Solid Body: Found in flatworms

    • Pseudocoelom: Partial mesoderm lining (example: nematodes)

    • True Coelom: Fully lined by mesoderm (example: annelids)

18.5 SPONGES HAVE A RELATIVELY SIMPLE, POROUS BODY

  • Phylum Porifera:

    • Lack true tissues, simple organisms

    • Feed by filtering food from water drawn through porous body

18.6 CNIDARIANS ARE RADIAL ANIMALS WITH TENTACLES AND STINGING CELLS

  • Phylum Cnidaria:

    • Two tissue layers: outer epidermis and inner digestive cell layer

    • Possess cnidocytes (stinging cells) on tentacles for prey capture

    • Body forms: Polyp (attached) and Medusa (free-floating)

18.7 FLATWORMS ARE THE SIMPLEST BILATERAL ANIMALS

  • Phylum Platyhelminthes:

    • Simplest bilateral organisms with distinct body symmetry

    • Can be free-living or parasitic

18.8 NEMATODES HAVE A PSEUDOCOELOM & A COMPLETE DIGESTIVE TRACT

  • Phylum Nematoda:

    • Roundworms characterized by bilateral symmetry, pseudocoelom, three tissue layers

    • Diverse group including many free-living and parasitic species

18.9 DIVERSE MOLLUSCS ARE VARIATIONS ON A COMMON BODY PLAN

  • Phylum Mollusca:

    • Soft-bodied animals with true coelom and sophisticated body plan

    • Modes of movement include jet propulsion in more advanced species

18.10 ANNELIDS ARE SEGMENTED WORMS

  • Phylum Annelida:

    • Possess segmented bodies and closed circulatory systems

    • Earthworms are significant for their role in soil aeration and nutrient cycling

18.11 ARTHROPODS ARE SEGMENTED ANIMALS WITH JOINTED APPENDAGES AND AN EXOSKELETON

  • Phylum Arthropoda:

    • Insects, arachnids, and crustaceans characterized by segmentation, jointed appendages, and exoskeleton made of chitin

    • Key features include body segments (head, thorax, and abdomen)

18.12 EVOLUTION CONNECTION: INSECTS ARE THE MOST SUCCESSFUL GROUP OF ANIMALS

  • 70% of all animal species are insects

  • Insect Anatomy:

    • Include head, thorax, abdomen, three pairs of legs, and often wings

18.13 ECHINODERMS HAVE SPINY SKIN, AN ENDOSKELETON, AND A WATER VASCULAR SYSTEM FOR MOVEMENT

  • Phylum Echinodermata:

    • Radially symmetrical; possess a water vascular system that facilitates movement and feeding

18.15 OUR OWN PHYLUM, CHORDATA, IS DISTINGUISHED BY FOUR FEATURES

  • Phylum Chordata:

    • Key features include:

    • Dorsal hollow nerve cord

    • Notochord

    • Pharyngeal slits

    • Post-anal tail

  • Simplest chordates are tunicates and lancelets; lack a backbone

19.2 FISH

  • Jawless fish: E.g., hagfishes and lampreys; lack jaws and paired fins

19.3 FISH

  • Chondrichthyans: Sharks and rays have cartilaginous skeletons; Ray-finned fishes have bony skeletons and swim bladders

19.4 NEW FOSSIL DISCOVERIES ARE FILLING IN THE GAPS OF TETRAPOD EVOLUTION

  • Evolution from lobe-finned fish to tetrapods: development of limbs for land movement

19.5 AMPHIBIANS ARE TETRAPODS VERTEBRATES WITH TWO PAIRS OF LIMBS

  • First tetrapods on land; include frogs, salamanders, and some caecilians

19.6 REPTILES ARE AMNIOTES—TETRAPODS WITH A TERRESTRIALLY ADAPTED EGG

  • Amniotic eggs: Key adaptation allowing reproduction on land

19.7 BIRDS ARE FEATHERED REPTILES WITH ADAPTATIONS FOR FLIGHT

  • Evolved from theropod dinosaurs; adapted for flight through various physical features like feathers and hollow bones

19.8 MAMMALS ARE AMNIOTES THAT HAVE HAIR AND PRODUCE MILK

  • Key features include:

    • Hair for insulation, mammary glands for milk production

    • Differentiated teeth and efficient organ systems

  • Monotremes: Egg-laying mammals (e.g., platypus)

  • Marsupials: Partially developed young born and complete development in pouches

  • Eutherians: Fully developed young born, also known as placental mammals.