Animal Evolution & Diversity Notes

Animal Evolution and Diversity
Learning Objectives

  • 33.1 Describe the different body plans found in animals.

  • 33.2 Distinguish between a tissue and an organ.

  • 34.1 Assess the evidence for the evolution of sponges, cnidarians, and ctenophores from single-celled ancestors.

  • 34.2 Assess evidence indicating protostomes as a distinct branch of bilaterians.

  • 34.3 Assess evidence for the diversity in arthropods.

  • 34.4 Evaluate relationships of deuterostomes on a phylogenetic tree.

  • 34.5 Evaluate relationships of vertebrates on a phylogenetic tree.

  • Explain trends in animal evolution.

Historical Context of Animal Evolution

  • 600 million years ago: Simple-bodied, asymmetrical organisms dominated the earth, characterized by basic tissues.

  • First Animals: Likely resembled choanoflagellates; marked by multicellularity, embryonic cell movement, and the formation of simple tissues.

Major Groups and Features of Early Animals

  • Sponges (Porifera):

    • Simplest animals, lacking true tissues and organs.

    • Have a cell-level organization, filtering water for food.

    • Body Plan: Asymmetrical.

  • Cnidarians:

    • Possess radial symmetry.

    • Key features include:

      • Mouth surrounded by tentacles.

      • Closed gastric cavity.

      • Two tissue layers (endodermis and epidermis) with a mesoglea layer.

    • Examples: Jellyfish, corals, anemones.

  • Ctenophores:

    • Similar to cnidarians with radial symmetry.

    • Have a gelatinous interior, simple nerve net, and rudimentary gonads.

Differentiation Between Early Animal Groups

  • Cnidarians vs. Sponges:

    • Cnidarians:

      • Have epithelial-lined compartments, true tissues, diverse cell types, and are predatory (using nematocysts).

    • Sponges: Only collections of cells without true tissues, nerves, or muscles.

Evolution of Tissues and Organs

  • Sponges lack true tissues, serving as simple collections of cells.

  • Cnidarians and Ctenophores: First true tissues, including:

    • Nerve nets for sensory input.

    • Muscular-type cells for movement.

    • Digestive cavities for improved food processing.

  • Organ: Structure made of multiple tissue types working together for specific functions.

Evolutionary Advances Around 540 MYA

  • Cambrian Explosion: Marked a rapid diversification of life, characterized by

    • Bilateral body plans, shells/exoskeletons, cephalization, and segmentation.

    • Increased predator-prey interactions contributing to natural selection.

    • Evolution influenced by HOX genes and increased oxygen levels.

Symmetry in Animals

  • Radial Symmetry:

    • Present in jellyfish: several planes of symmetry around a central axis.

  • Bilateral Symmetry:

    • Present in crabs: a single plane of symmetry that divides the body into right and left halves, having anterior and posterior ends.

    • Associated with directed movement and complexity in brain evolution.

Phylogeny and Complexity

  • The phylogenetic tree indicates increasing complexity from protists to bilaterians, where:

    • Bilaterians are closely related to cnidarians and sponges.

    • Bilaterians consist of creatures with bilateral symmetry and complex organs.

Lophotrochozoans and Ecdysozoans

  • Lophotrochozoans:

    • Include annelids and mollusks.

    • Characterized by segmentation and complex organ systems.

  • Ecdysozoans:

    • Notable for molting their external cuticle; include nematodes and arthropods.

    • Highly successful and numerous in various habitats.

Adaptations and Success in Arthropods

  • Arthropods:

    • Diverse due to adaptations such as desiccation-resistant eggs, wings, special respiratory systems, and metamorphosis.

    • Exhibit segmentation and jointed appendages facilitating movement and interaction with environments.

Deuterostome Phylogeny

  • Major Groups of Deuterostomes:

    • Hemichordata (acorn worms).

    • Echinodermata (sea stars, sea urchins).

    • Chordata (including vertebrates).

  • Chordates:

    • Characterized by a notochord, dorsal nerve cord, pharyngeal slits, and post-anal tail.

Mammalian Evolution and Adaptation

  • Amniotes:

    • Key adaptations include the amniotic egg, allowing reproduction on land.

    • Includes reptiles, birds, and mammals.

  • Mammals:

    • Defined by fur/hair, mammary glands, and varying reproductive strategies (monotremes, marsupials, placentals).

Trends in Evolution

  • Increasing complexity and specialization throughout evolutionary history, allowing for greater species diversity.

  • Mass extinctions have historically shaped the evolution of surviving species, with a current potential for a sixth extinction linked to human impact.

  • Ecological interdependence between prokaryotes and eukaryotes continues to shape ecosystem dynamics.