Chapter 20

Animal Evolution and Diversity - Study Notes

Learning Objectives

  • Describe the uniting characteristics that define animals (Metazoa).

  • Understand the characteristics used to classify animals.

  • Understand where different characteristics arose on the animal phylogenetic tree.

Differentiating the Multicellular Kingdoms

  • Plants: Autotrophs

  • Fungi: Absorptive heterotrophs

  • Animals: Ingestive heterotrophs

Cell Structure and Specialization

  • Animals are multicellular eukaryotes.

  • Structural proteins, such as collagen, support animal cells instead of cell walls.

  • Unique Defining Characteristics of Animals:

    • Nervous tissue.

    • Muscle tissue.

    • Defined by tissues: groups of similar cells acting as a functional unit.

Tissues

  • Tissues are composed of integrated groups of cells with common structure, function, or both.

  • Except in sponges (Phylum Porifera), animal cells are organized into various well-defined tissues.

  • Four Tissue Types in Complex Animals:

    • Epithelial

    • Connective

    • Muscle

    • Nervous

Hierarchical Body Plans

  • More complex organisms are composed of compact masses of cells with complex internal organization.

  • A complex body plan helps animals maintain a relatively stable internal environment despite variable external conditions.

  • Correlation of Animal Form and Function: Occurs at all levels of organization.

Reproduction

  • Most animals reproduce sexually, with the diploid (2n) stage dominating the life cycle.

  • Unlike plants, sperm and egg cells are produced directly by meiotic division in animals.

  • Plant gametes undergo meiotic division followed by mitotic division to produce gametophytes.

Development

  • Animal zygotes undergo cleavage, a succession of cell division without growth between divisions.

  • Blastula Formation: Cleavage leads to the formation of a blastula, often in the form of a hollow ball of cells.

  • The blastula undergoes gastrulation, forming a gastrula with different layers of embryonic tissues.

Animal Body Plans - Symmetry

  • Animals exhibit different types of symmetry:

    • Radial Symmetry: No distinct front and back, or left and right. Found chiefly in Cnidarians.

    • Bilateral Symmetry: Only one imaginary cut divides the animal into mirror-image halves. Observed in the remaining animal groups.

Animal Body Plans - Cephalization

  • Cephalization: Concentration of the nervous system and special sensory organs at the front of the body.

    • Important for directed locomotion.

    • Adaptation for locomotion: Enables detection of sensory information ahead as the animal moves forward.

    • Adaptation for predation: Facilitates prey detection and predator avoidance for animals with cephalization.

Animal Body Plans - Segmentation

  • Segmentation: Organization of the body into repeated units along the anterior-posterior (head-tail) axis.

  • Repeated units modified based on their location in the body.

  • Notable segmented body plans are found in:

    • Arthropods

    • Annelids

    • Chordates

Embryonic Development

  • Animals generally reproduce sexually, where the diploid (2n) stage predominates.

  • After fertilization (sperm (n) + egg (n) = zygote (2n)), the zygote undergoes rapid cell division (cleavage).

  • Formation of a multicellular, hollow blastula ensues following cleavage.

  • Blastula undergoes gastrulation, forming a gastrula with differentiated embryonic tissue layers.

Embryonic Development - Time Lapse

  • Example: Cell division in a frog egg observed from two cells into millions over approximately 33 hours.

Animal Classification - Embryo Cleavage

  • Protostomes:

    • Cleavage: Spirally.

    • Type: Determinate.

  • Deuterostomes:

    • Cleavage: Radially.

    • Type: Indeterminate.

Animal Classification - Embryonic Layers

  • Tissues are specialized cells organized into distinct layers by membranous layers.

  • During development, three germ layers emerge, giving rise to tissues and organs:

    • Ectoderm

    • Endoderm

    • Mesoderm

Classification by Tissue Organization

  • Sponges (Phylum Porifera):

    • Lack true tissues

  • Diploblastic Animals: (E.g., Cnidarians)

    • Have ectoderm and endoderm.

    • Include organisms like jellyfish.

  • Triploblastic Animals: (E.g., flatworms, arthropods, vertebrates)

    • Have an additional mesoderm layer (between ectoderm and endoderm).

Phylogeny of Early Embryological Development

  • Animals divided into two major groups based on germ layers:

    • Diploblasts: Develop from embryos with two germ layers (endoderm and ectoderm).

    • Triploblasts: Develop from embryos with three germ layers, including a mesoderm that forms muscles and connective tissues.

Fate of the Blastopore

  • Embryological features show bilaterian animals organized into:

    • Protostomes: The blastopore forms the mouth.

    • Deuterostomes: The blastopore forms the anus.

Bilaterian Body Cavities

  • A coelom is a fluid-filled body cavity that cushions internal organs.

  • Early classifications of bilaterians identified three groups based on the presence or absence of coelom:

    • Acoelomates: Without a body cavity.

    • Coelomates: With a complete body cavity.

    • Pseudocoelomates: Having a body cavity that does not completely surround the internal organs.

  • Molecular studies have revised this traditional classification of bilaterians.

Learning Objectives Review

  • Describe the uniting characteristics that define animals (Metazoa).

  • Understand characteristics used in animal classification.

  • Comprehend the evolutionary development of characteristics on the animal phylogenetic tree.

Major Groups of Eukaryotes

  • Excavata: Includes groups like Diplomonads, Parabasalids, and Euglenozoans.

  • Plants: Green algae and related groups.

  • SAR Clade: Includes Stramenopiles (Diatoms, Brown algae), Alveolates (Dinoflagellates, Apicomplexans, Ciliates), and Rhizarians (Radiolarians).

  • Unikonta: Includes Amoebozoans and Opisthokonts (Fungi and Animals).

Choanoflagellates

  • Morphological and molecular data suggest ancestors of the animal kingdom resemble modern choanoflagellates:

    • Single-celled aquatic microorganisms.

    • Feature: Collar of microvilli surrounding a single flagellum for feeding.

    • Can exist solitarily or form colonies.

Kingdom Animalia (Metazoa)

  • Differ from choanoflagellates because they have persistent multicellularity

  • Animals evolved from unicellular, filter-feeding ancestors similar to choanoflagellates.

  • Key evolutionary innovations include:

    • Stable, developmentally regulated multicellularity integrating cells into specialized tissues.

    • Emerged approximately 770 million years ago.

Porifera - Basal Animals Retaining Primitive Traits

  • Sponges lack true tissues and organs.

  • They are primarily sedentary, living in marine or freshwater environments.

  • Function as suspension feeders, capturing food particles in water passing through their bodies.

The Animal Genomic Toolkit: Innovations Behind Multicellularity

  • Innovations in genomic tools enhanced:

    • Cell adhesion and communication through specialized genes.

    • Developmental control networks enabling patterned growth and differentiation.

    • Expansion of extracellular matrix (ECM) components providing structural support and new tissue organization modes.

Porifera Anatomy

  • A collection of key cell types includes:

    • Choanocytes:

    • Flagella circulate water and capture food particles.

    • Amoebocytes:

    • Transport nutrients and produce skeletal fibers (spicules).

    • Porocytes:

    • Span the body wall to create pores for water flow.

    • Osculum:

    • Large opening for water expulsion.

Eumetazoa: Transition from No True Tissues to True Tissues

  • Porifera (Sponges): Lack true tissues; cells function independently with minimal coordination.

  • Eumetazoa (All Other Animals): Possess true tissues formed by specialized cells organized into layers (ectoderm and endoderm).

  • Evolution of true tissues facilitated:

    • Nervous and muscular systems.

    • Coordinated movement and complex body architectures.

Molecular Clock Analyses

  • Suggest that sponges diverged around 680-700 million years ago.

  • Ctenophora and Cnidaria branch off as early groups, exhibiting radial symmetry and diploblastic organization.

Bilaterians

  • Most animals fall into the Bilateria clade.

  • Bilaterians show:

    • Anterior-Posterior Axis: Evolution of bilateral symmetry and a through-gut (mouth and anus).

    • Innovations enabling complex behaviors and directional movement.

  • Molecular-clock and trace-fossil evidence support their divergence around 650-700 million years ago, preceding the Cambrian explosion.

Cambrian Explosion (535 to 525 million years ago)

  • Marks the first fossil appearance of many major animal groups.

  • Most fossils from this period belong to bilaterians.

Three Major Clades of Bilaterian Animals

  • Ecdysozoa: Invertebrates that undergo ecdysis (shedding of exoskeletons).

    • Includes Arthropoda and Nematoda.

    • Feature a hard exoskeleton composed of chitin

    • segmented bodies with jointed appendages to enable movement.

  • Lophotrochozoa: Includes familiar phyla such as Mollusca and Annelida, many of which possess a lophophore or trochophore larval stage.

  • Deuterostomia: Defined by embryonic development where the blastopore forms the anus.

    • Includes Chordata, Echinodermata, Hemichordata.

    • monophyletic and distinct from protostomes

      • split between deuterostomes and protostomes occurred early in bilaterian evolution as the bilaterian body-plans diversified

Phylogeny Summary of Animal Relationships

  1. All animals share a common ancestor (monophyletic).

  2. Sponges (Porifera) are basal animals.

  3. Eumetazoa comprise those animals with true tissues.

  4. Most animal phyla are bilaterians.

  5. Three major clades exist among bilaterian animals, primarily invertebrates, except for Chordata, which are vertebrates (animals with backbone).