Overview of Animal Diversity

  • 1.3 million living species identified; total species may be 100-200 million.

Characteristics of Animals

  • Multicellular, heterotrophic eukaryotes

  • Obtain nutrition through ingestion

  • No cell walls, held by collagen

  • Specialized tissues: nervous and muscle tissues.

Ploidy

  • Diploid (2n): two sets of chromosomes

  • Haploid (n): one set of chromosomes

Reproductive Cycle

  • Most reproduce sexually; diploid stage dominant.

  • Zygote formed post-fertilization; undergoes cleavage.

Early Embryonic Development

  • Cleavage leads to blastula (hollow ball of cells) and gastrula (layer formation).

  • Gastrulation: rearrangement leading to germ layers:

    • Ectoderm: outer covering & nervous tissue

    • Endoderm: digestive tract lining

    • Mesoderm: muscles & most organs.

Life Stages

  • Some species have larval stages that undergo metamorphosis.

Hox Genes

  • Regulatory genes (Homeoboxes) that influence morphology.

Historical Context

  • Early fossils from Neoproterozoic Era (Ediacaran biota).

  • Cambrian Explosion: rapid diversification of life (542 - 251 mya).

  • Mesozoic and Cenozoic Eras marked by declines and adaptations in biodiversity.

Animal Classification

  • Defined by:

    1. Symmetry (asymmetry, radial, bilateral)

    2. Tissue organization (true tissues vs. germ layers)

    3. Body cavity (coelom types: coelomates, pseudocoelomates, acoelomates)

    4. Blastopore fate (protostome vs. deuterostome).

Major Animal Clades

  1. Deuterostomia: Includes vertebrates.

  2. Lophotrochozoa: Trochophore larval stage.

  3. Ecdysozoa: Animals with exoskeletons that undergo ecdysis (molting).

Extinction Cycles

  • 99% of all animal species extinct; major events: Cambrian explosion and K-T extinction.