Biology 102 Chapter 32 Notes
Lecture Presentations by Nicole Tunbridge and Kathleen Fitzpatrick: Chapter 32 An Overview of Animal Diversity
Key Characteristics of Animals
Heterotrophic Nature: Animals are heterotrophs, meaning they obtain energy and nutrients by consuming other organisms, which distinguishes them from autotrophic plants that produce their own organic molecules. Animals do not digest food externally (unlike fungi), but instead ingest food and digest it internally.
Development from Embryonic Layers: Animals are multicellular and their bodies are formed from layers of embryonic cells, which develop into various tissues and organs.
Efficient Consumers: Animals have digestive systems that allow them to process food inside their bodies, thereby increasing their efficiency as consumers.
Mobility: Most animals can move and have sensory mechanisms to detect and capture potential prey.
Concept 32.1: Characteristics Defining Animals
Multi-cellularity: Animals are composed of multiple cells that are specialized to perform different functions, a trait notably supported by proteins like collagen instead of cell walls.
Tissue Specialization: Two unique tissue types define animals: nervous tissue and muscle tissue. Tissues are groups of similar cells functioning as a unit.
Reproductive Modes: Predominantly, animals reproduce sexually, with a diploid lifecycle dominating. Male and female gametes (sperm and egg) develop directly via meiotic division.
Cleavage and Gastrulation: After fertilization, zygotes undergo cleavage—a series of rapid divisions without growth, leading to a blastula formation, which subsequently undergoes gastrulation to form a gastrula with different embryonic tissue layers.
Larval Stages: Most animals have a larval stage that is morphologically and behaviorally distinct from the adult form, undergoing metamorphosis to transition into juveniles resembling adults yet being sexually immature.
Hox Genes: All animals have developmental genes, particularly Hox genes, which are unique in regulating the expression of other genes influencing morphological development.
Concept 32.2: History of Animals
Over 1.3 million animal species have been documented with many more likely undiscovered, indicating vast animal diversity.
Common Ancestor: Molecular evidence suggests that the common ancestor of animals likely lived around 770 million years ago. Close living relatives include choanoflagellates, indicating a shared evolutionary history.
Ediacaran Biota: Fossils from the Ediacaran period (approx. 560 million years ago) suggest that the earliest animals resembled modern choanoflagellates and include classified fossils like Cloudina, an early predator.
Paleozoic Era: Following the Cambrian, the Paleozoic era (c. 541 to 252 million years ago) saw a series of significant evolutionary developments. It included the colonization of land by vertebrates, the rise of amphibians, and the proliferation of fish and the first reptiles. This era concluded with the Permian extinction, the largest mass extinction event in Earth's history.
Cambrian Explosion: Occurring approximately 535–525 million years ago, this event marked a significant increase in the diversity of life forms and the emergence of most major animal phyla. The Cambrian period is characterized by the rapid development of hard-bodied animals and the establishment of complex ecosystems.
Mesozoic Era: Spanning from about 252 to 66 million years ago, this era is known as the age of reptiles, particularly dinosaurs. It included the rise of mammals and birds as well. The Mesozoic era ended with the extinction of the dinosaurs, likely triggered by an asteroid impact.
Cenozoic Era: Following the Mesozoic, this era began approximately 66 million years ago and is often referred to as the age of mammals. It is characterized by the diversification of mammals and birds into new ecological niches as well as significant climatic changes, leading to the development of modern ecosystems.
Concept 32.3: Animal Body Plans and Symmetry
Animal diversity is characterized by a few primary body plans that reflect sets of shared morphological and developmental traits.
Symmetry: An important trait for comparison among animal types:
Radial Symmetry: Body parts arranged around a central axis, exemplified in sessile or planktonic forms.
Bilateral Symmetry: Body parts organized around two axes (head-tail and dorsal-ventral), allowing for active movement and central nervous systems, with specific anatomical features:
A dorsal (top) and ventral (bottom) side
A right and left side
Head and tail ends, often with concentrated sensory systems.
Tissue Organization
Tissues are differentiated groups of cells that perform distinct functions.
Different animal groups, like sponges (which lack true tissues) versus triploblastic animals (which have three germ layers: ectoderm, endoderm, and mesoderm), illustrate this organization:
Ectoderm: Forms the outer covering and nervous system.
Endoderm: Lines the digestive cavity and becomes digestive organs.
Mesoderm: The middle layer contributing to muscles and other organs in triploblastic animals.
Body Cavities and Developmental Modes
Most triploblastic animals possess a body cavity (coelom), which serves multiple functions such as cushioning organs and facilitating movement of internal organs.
Types of Body Cavities:
Coelom: A body cavity entirely surrounded by mesodermal tissue, crucial for organ suspension.
Hemocoel: A cavity filled with hemolymph, providing transport functions.
Some triploblastic animals might lack a body cavity, being compact with body fluid exchange mechanisms.
Developmental Modes: Protostome vs. Deuterostome
Animals can exhibit either protostome or deuterostome development:
Cleavage Patterns: Protostomes show spiral and determinant cleavage; deuterostomes exhibit radial and indeterminate cleavage.
Coelom Formation: Protostomes and deuterostomes form their coeloms differently during gastrulation.
Blastopore Fate: In protostomes, the blastopore forms the mouth; in deuterostomes, it forms the anus, which reflects a distinctive developmental pathway.
Concept 32.4: Phylogenetic Views of Animals
Current phylogenetic understanding indicates all animals share a common ancestry, with sponges being the basal group.
The clades Eumetazoa (animals with true tissues), Bilateria (bilaterally symmetrical animals), and distinctions between invertebrates and vertebrates are key to understanding modern animal phylogeny.
Clades: Three significant clades exist within the Bilateria:
Ecdysozoa: Includes molting animals like nematodes and arthropods.
Lophotrochozoa: Comprises animals with distinctive larval stages and feeding structures like lophophores.
Deuterostomia: Consisting of both invertebrates and vertebrates, highlighting the evolutionary diversity.
Summary of Major Events in Animal Evolution
Timeline includes critical evolutionary events:
Ediacaran animals appear (approx. 560 million years ago)
Cambrian explosion of diversity (approx. 535–525 million years ago)
Colonization of land by vertebrates (approx. 365 million years ago)
Evolution of larger mammals and modern climates during the Cenozoic era (beginning 66 million years ago).