Animal Body Plan and Evolutionary Biology Notes

Introduction to Animals

• Focus on animal body plans and classification.

Eukaryotic Supergroup: Unikonts

• Animals fall under Opisthokonts within Unikonta.
• Other groups: slime molds, fungi, choanoflagellates, tubulinids.

Shared Features of All Animals

• Multicellular Eukaryotes:
  • All animals are multicellular but not all have true tissues (e.g., sponges).
• Heterotrophy:
  • All animals are heterotrophic (do not produce their own food).
• Internal Digestion:
  • Animals have internal digestion processes.
• Motility:
  • Animals are motile at some life stage; many become sessile when mature.
• Single Posterior Flagellum:
  • Flagellated cells (like sperm) possess a single posterior flagellum.
• Developmental Genes:
  • Similar organization and function, especially involving HOX genes, which dictate body plan development.

Differences Among Animals: Morphology and Ploidy

• Not all animals have tissues:
  • Porifera (sponges)
  • Placozoa (small, parasitic animals)
• Not all have organs:
  • Cnidaria, Ctenophora.
• Ploidy: Most are diploid (somatic/body cells are diploid; gametes are haploid). Exceptions include:
  • Haploid males (e.g., wasps, bees, ants) develop from unfertilized eggs.

Differences Among Animals: Modes of Reproduction

• Sexual Reproduction: Predominates among animals.
• Asexual Reproduction: Found in some animals, includes:
  • Fragmentation: Seen in some invertebrates.
  • Budding: Also seen in invertebrates.
  • Parthenogenesis: A form of asexual reproduction where females produce offspring from unfertilized eggs (e.g., whiptail lizards).

Evolutionary Origins of Animals

• Animals derived from the eukaryotic clade Opisthokonts.
• Choanoflagellates: Sister taxa to animals showcasing:
  • Similar morphology (cell body, microvilli, flagellum).
  • Capability for colony formation.

Colonial Flagellate Hypothesis

1. Single flagellate creates aggregates.
2. Colonies form hollow spheres.
3. Cell specialization leads to reproduction and tissue formation, marking the origin of animals.

Choanoflagellates and Sponges

• Choanoflagellates closely resemble collar cells (choanocytes) in sponges, supporting their evolutionary link.

Appearance of Animals in the Fossil Record

• Molecular Phylogeny: Common ancestor of all animals around 770 MYA.
• Biochemical Evidence: Sponges produced steroids in sediment from around 710 MYA (Cryogenian period).
• Fossils: Ediacaran biota, e.g., Dickinsonia (560 MYA).

Major Animal Groups and Their Appearance

• Animals like Porifera, Cnidarians, and Molluscs appeared in the Ediacaran.
• Cambrian Period (541 - 480 MYA): Known for the Cambrian explosion where many modern phyla appear, though controversial.

Comparison of Ediacaran and Cambrian Life

• Ediacaran: Soft-bodied, filter-feeders, mostly small.
• Cambrian: Appearance of hard structures, predator adaptations, and increase in body size.

Body Orientation and Symmetry

• Radial Symmetry: Equal in all directions around an oral-aboral axis (e.g., sea anemones).
• Bilateral Symmetry:
  • Anterior (leading direction)
  • Posterior (opposite to anterior)
  • Ventral (mouth side)
  • Dorsal (opposite side of ventral)

Levels of Organization in Animals

• Protoplasmic: Functions within a single cell (e.g., protists).
• Cellular: Aggregation of functionally differentiated cells (e.g., sponges).
• Tissue: Groups of similar cells working together (e.g., cnidarians).
• Organ: Different tissues working together for specific functions.
• Organ Systems: Organs working together for complex functions.

Embryonic Development

1. Fertilization: Fusion of egg and sperm to form a zygote.
2. Cleavage: Division of the zygote into smaller cells.
3. Blastulation: Formation of a blastula (hollow ball of cells).
4. Gastrulation: Formation of germ layers (ectoderm, mesoderm, endoderm).
5. Differentiation: Formation of body tissues and organs.
6. Growth: Increase in body size.

Germ Layers

• Diploblast: Two germ layers (no mesoderm).
• Triploblast: Three germ layers (ectoderm, mesoderm, endoderm) that develop into organs and tissues.

Coelom Formation

• Acoelomate: No body cavity.
• Pseudocoelomate: Body cavity partially lined by mesoderm.
• Coelomate: Body cavity fully lined by mesoderm on all sides.

Fossil Record Patterns

• Punctuated Equilibrium: Rapid bursts of evolutionary change, interspersed with periods of stability.
• Phyletic Gradualism: Gradual and continuous evolution within lineages (e.g., vertebrate limbs).