Animal Evolution Notes

ANIMAL EVOLUTION

EVOLUTION OF ANIMALS
  • Animals are traditionally placed in the kingdom Animalia (five kingdom system).
  • In the three domain system, animals are classified under the domain Eukarya.
  • Animals belong to the Opisthokonta supergroup, which also includes Fungi and Choanoflagellates.
CHARACTERISTICS OF ANIMALS
  • Eukaryotic: Organisms have complex cells with a nucleus.
  • Multicellular: Composed of multiple cells, forming complex structures.
  • Heterotrophic: Animals ingest food and digest it internally.
    • Typically feed on living or recently-dead organic matter.
  • Lack cell walls: Unlike plants and fungi.
  • Mobile: Exhibit mobility at some point in their life cycle.
ANIMAL REPRODUCTION
  • Sexual reproduction: The most common method.
    • Most animals have a diploid life cycle.
    • Some insect males (like ants, bees, wasps) are haploid.
  • Haploid gametes are used in sexual reproduction.
  • Some simpler animals can reproduce asexually through mechanisms such as budding or regeneration.
TYPES OF BODY SYMMETRY
  • Symmetry refers to the pattern of organization in an organism:
    • Asymmetry: Lack of symmetry, no division into equal halves (e.g., sponges).
    • Radial symmetry: Circular organization, with any longitudinal cut producing identical halves. Found in organisms lacking a complete digestive system.
    • Bilateral symmetry: A single cut down the center line produces two equal halves, with distinct anterior (front) and posterior (back) ends.
    • Cephalization: Localization of a brain and specialized sensory organs at the anterior end.
ANATOMICAL DESCRIPTIVE TERMS
  • Anterior vs. Posterior: Reference to head and tail.
  • Dorsal vs. Ventral: Reference to back and stomach.
  • Superior vs. Inferior: Reference to upper and lower.
  • Medial vs. Lateral: Reference to central position.
  • Distal vs. Proximal: Reference to extremity position.
  • Superficial vs. Deep: Reference to depth within the body.
ANIMAL ANCESTRY
  • Evidence suggests all animals evolved from a single common ancestor.
  • The Colonial flagellate hypothesis proposes that animals descended from a hollow spherical colony of flagellated cells:
    1. Colonial cells form a hollow sphere.
    2. Specialization for reproduction occurs.
    3. Infoldings create additional tissues.
  • Choanoflagellates are considered the closest living relatives to animals.
PHYLOGENY OF ANIMALS
  • Major Classification Traits include:
    • Tissue types present
    • Body symmetry
    • Embryonic development (Blastula formation, Cleavage)
    • Coelom development
    • Growth Patterns
  • Classification generally separates animals into Protostomes and Deuterostomes, based on developmental differences.
TISSUES AND SYMMETRY
  • Animal complexity is related to tissue organization:
    • Asymmetric animals (simplest) lack tissues.
    • Diploblasts (two tissue layers: ectoderm and endoderm) lack organs and exhibit radial symmetry.
    • Triploblasts (three tissue layers: ectoderm, mesoderm, and endoderm) have organs and demonstrate bilateral symmetry.
EMBRYONIC DEVELOPMENT
  • Triploblastic animals can be either protostomes or deuterostomes:
    • Protostomes: First opening becomes the mouth, with a spiral cleavage pattern.
    • Deuterostomes: First opening becomes the anus, with a radial cleavage pattern.
  • Cleavage: Developmental division of the fertilized egg without cell size increase.
BLASTOPORE FORMATION
  • Gastrulation is involved in forming the gut and involves:
    • Formation of the blastopore (the first opening in the developing embryo).
    • Blastula: Hollow sphere of cells before gastrulation.
  • The blastopore can become either the mouth or the anus depending on whether the organism is a protostome or deuterostome.
COELOM FORMATION
  • Coelom: Internal body cavity lined with mesoderm tissue different for protostomes and deuterostomes:
    • Protostomes: Blastopore becomes the mouth. Mesoderm comes from cells near the blastopore.
    • Deuterostomes: Blastopore becomes the anus. Mesoderm is generated from endoderm cells.
TRIPLOBLAST GERM LAYERS
  • Endoderm: Develops into the digestive and respiratory linings (e.g., intestines, lungs, liver).
  • Mesoderm: Forms muscles, bone, and the circulatory system.
  • Ectoderm: Forms the outer tube (skin, nervous system).