Animal Evolution Lecture Notes Review

Animal Evolution

Introduction

• Transitioning from general eukaryotes to the animal kingdom.
• Focusing on the diversity of animal forms and their functions.
• Two schools of thought on form and function:
  • Form dictates function: An organism's form evolves first, and its function adapts to suit that form.
  • Function dictates form: An organism's function is required, and its form evolves to best fit that function.
• Rachel, a biology artist, captures ecosystem diversity in her artwork, particularly the Amazon basin.
  • Her work showcases the high density of organisms and provides a snapshot of overall diversity.
  • A link is provided to identify the organisms in her Amazon basin artwork.

Course Objectives

• Review phylogenetic relationships of major animal clades.
• Describe the timeline of evolution and diversification.
• Identify factors that promoted or influenced diversification, such as environmental factors.
• Describe major extinction events (if time allows).

What is an Animal?

• Focusing on key characteristics to ground our understanding as we dive deeper into this specific subgroup of eukaryotes.
• General Characteristics:
  • Multicellular: All animals are multicellular.
  • Heterotrophic: Animals are internally heterotrophic, consuming external organic matter for nutrients.
  • Nervous System: Most animals have a nervous system (sponges are an exception).
  • Mobility: Animals typically exhibit involuntary movement (though some are stationary during parts of their life cycle).
• Monophyletic: Animals share a common ancestor.
• Diagnostic Considerations:
  • No single characteristic is diagnostic on its own.
  • The combination of these characteristics is more diagnostic.
• Additional Characters:
  • Specialized Cell Junctions: Necessary for multicellularity, allowing cells to stick together.
  • Lack Cell Walls: Facilitates cell connections and junctions.
  • Hox Genes: Genes encoding for body pattern and axis orientation, crucial for diversification.
    • Mutations in Hox genes can lead to irregularities in body structures and segmentation.
    • Examples are, mutation causing legs instead of antennae of segment, and mutations causing extra set of wings instead of halters.

Essential Animal Functions

• Nutrient Acquisition: Animals must consume, extract, and absorb nutrients (internal heterotrophs).
  • Exceptions exist, such as starfish that can evert their stomachs.
• Oxygen Acquisition and Delivery: Animals need oxygen and mechanisms to deliver it to tissues and cells.
• Waste Removal: Waste products (CO2, nitrogenous waste) must be removed.
• Protection: Animals need to protect themselves with structural elements.
• Reproduction: The primary biological goal is reproduction and propagation of genetic information.
  • Animals exhibit diverse reproductive strategies (sexual, asexual, hermaphroditism).

Animal Body Plan Components

• Symmetry: Radial or bilateral symmetry.
• Digestive System: A way for digestion to occur.
• Body Cavities: Coelomates (true body cavity), pseudocoelomates (internal cavity without specialized connective tissue), and acoelomates (no true cavity).
• Segmentation: Division of body into segments.
• Appendages: Legs, arms, claws, antennae, reproductive structures.
• Nervous System: Complex sensory systems (vision, hearing, taste).

Digestive Systems

• Sac-like Structure:
  • One opening for both intake and excretion.
  • No specialized structures; relies on diffusion for nutrient and waste exchange.
  • Hydrostatic skeleton: Rigidity created by internal water pressure.
• Tube-like Structure:
  • Two openings: mouth and anus.
  • Processing and modification along the route.
  • Often associated with specialized structures for nutrient extraction and delivery.

Body Cavities

• Acoelomates:
  • No true coelom (fluid-filled body cavity).
  • Solid tissue mass instead of an open cavity.
  • Movement via cilia on the ventral side.
  • The term acoelomate is used for triploblastic organisms.
• Pseudocoelomates:
  • Have a coelom (fluid-filled space), but no connective tissue membranes separating organs.
  • Organs are suspended in the fluid.
• Coelomates:
  • Have a true coelom.
  • Organs are fluid-suspended and compartmentalized by a mesoderm layer.

Segmentation

• Division of the body into segments.
• Leads to specialization of segments and the evolution of different appendages over time.
• Arthropods are a successful lineage that have evolved segmentation.
• Segments can be specialized for:
  • Movement (arms, legs, swimmerets).
  • Prey capture (claws).
  • Housing organ systems.
  • Reproductive structures.
• Increasing the number of segments can increase the capability of moving.
• Segmentation gives rise to a radiation of arthropod diversity.
• Segmentation also happens in mammals/humans, but they're typically more internal.

Appendages

• Feet, hands, arms, legs, antennae, sensory structures, claws, mouth appendages, reproductive structures.
• Appendages are segmented.
• Organisms like frogs have different morphologies of their appendages.

Nervous Systems

• Complex Nervous System:
  • Brain, spinal cord, peripheral nerves.
  • Coordinates movement, processes information, and allows for decision-making.
• Proto Nervous System:
  • Planarian (flatworm): Two parallel bundles of nerves with ladder-like connections, simple brains.
• Primitive Nervous System:
  • Cnidarians (hydra): Neurons form an equal mesh across the body. No compartmentalization; no clusters of neurons are isolated to one part of the body. Widespread and primitive nerve nets.

Multicellularity

• Cell Adhesion: Cells must stick together to form multicellular structures.
• Cell Specialization: As more cells stick together, they can take on different jobs.
• Differentiation: Cells are assigned to their roles during embryonic development.
• Coordination: Different groups of cells must work together.
• Adhesion Molecules:
  • CAMs (cell adhesion molecules) and integrins enable cell adhesion.
  • Integrins are integral membrane proteins that attach to the cytoskeleton and the extracellular matrix.
• Cell Junctions: Different types of junctions exist.
  • Gap junctions allow for cytoplasmic connections between cells.

Centralization

• Sensory structures tend to be oriented more towards the anterior or the head region of the organism.
• Animals typically precede head first as they're moving throughout their environment.
• Bilateral symmetry or organisms that have a left-right symmetry are typically highly specialized as well, meaning that their structures are present on the anterior end.

Evolution of Animals

• The closest living relative to all animals is choanoflagellates.
• Choanocytes are internal cellular composition of sponges.
• Animals diversification was relatively slow until the Cambrian period.

Cambrian Explosion

• Beginning around 540 million years ago, there was an explosion of animal diversity.
• Many basic body forms or body types that are on Earth today are starting to form.
• Most modern animal phyla had already appeared at this point in the fossil record.
• Punctuated Equilibrium: relatively long intervals of stasis, meaning that there's relatively little change or diversification.
• Every animal in existence during the Cambrian is marine.

What the Cambrian would have looked like

• Animal phyla that exists are: Arthropods, peripherals (sponges), echinoderms, brachiopods, mollusks and clams, cnidarians.
• The reasons why diversification was able to happen:
  • A relatively massive and rapid increase in the globally available oxygen.
  • A massive increase in the available ocean calcium as well.
  • Expansion in continental shelf.
  • Evolution of the Hox genes around the time.

Post-Cambrian Evolution

• Ordovician period
  • Increase in lineages of the phyla.
  • Evolutions of spinal column occurred, which lead to the first radiation of fishes.
  • Plants colonized the land. Shortly after animals also colonize the land.
  • First colonize plants and arthropods occurred, where they were very small and nearby aquatic sources.

Ordovician Fish Evolution

• Early Ordovician: Smaller fishes had spinal columns.
• Late Ordovician: Large fishes had armored plates and bony jaws.
• Two lineages split from the common ancestor:
  • Chondrichthyans (ancestors of sharks).
  • Pachyderms (ancestors of bony fishes).
• Devonian Period:
  • Sharks and bony fish continue to diversify.
  • Lobed fin fishes evolve.
  • Lobed fin fishes became the first vertebrates to colonize land.
  • Tiktaalik fossil.

Transition From Aquatic to Land

• Pectoral and pelvic girdle have transitionary forms which are weight-bearing.
• Triassic period :Diversification of bony fishes occurring.
• Extinction events occurred at the end.
• Jurassic period: Increase in the body size of the species.
• The Cretaceous period: Closest ancestors of modern fishes and large predators.
  The fossil image where it was found to contain another fish inside as evidence that is a predator
  K-Pg boundary: At 65 million years, which extinction events occurred. Where the dinosaurs and ammonia went extinct. Which has allowed more spaces and consumption.
  Lobed Fin Fishes: originated in the Devonian, led to the evolution of tetrapods , includes amphibians, reptiles, mammals, and birds.
  Vertebrate Dissection: Will contain fishes because its a vertebrate. Which is also a fish-based.
  The evolution of tetrapods after the colonization and allowed modification of the skeleton. Where the pectoral and pelvic girdle the vertebral column and the skull and skeletal structure
  Birds crocodile reptiles and mammals all have a single ancestor between the two
  Tetrapod lineages: early amphibian as transition on land however still have challenges. Desiccation/drying water
  Amniotic egg not around at the moment, therefore amphibian has to to survive still rely on being around/ eggs in water to keep fluid balanced.