Zoology 121A

1. Introduction to Zoology & Evolutionary Thought

• Zoology Definition: The scientific study of animals, derived from Greek and Latin word roots.

• Core Themes:

  1. The mechanisms responsible for animal diversity.

  2. Form and function.

  3. Survey of animal diversity.

• Principles of Biodiversity:

  • Exhibits uniformity: all animals share a common ancestor and cellular construction.

  • Phylogenetic trees represent connections, where nodes signify the last common ancestor.

• Levels of Biological Organization (Smallest to Largest):   $\text{Atom} \rightarrow \text{Molecule} \rightarrow \text{Cell} \rightarrow \text{Organ} \rightarrow \text{Organism} \rightarrow \text{Population} \rightarrow \text{Community} \rightarrow \text{Ecosystem} \rightarrow \text{Biosphere}$

• Keystone Species:

  • A species whose removal causes ecosystem collapse.

  • Concept developed through studies on the purple ochre sea star.

• Core Quote: "Nothing in biology makes sense except in the light of evolution" — Theodosius Dobzhansky (1954/1973).

What Evolution Is and Is Not

• Definition: All changes in the characteristics and diversity of life on Earth throughout history; descent with modification (Darwin); heritable change in gene frequencies over time.

• Historical Antecedents to Darwin:

  • Al-Jahiz and Jean-Baptiste Lamarck: Proposed early ideas, including the theory of inheritance of acquired characteristics.

  • Sir Charles Lyell: Proposed Uniformitarianism:

  • Laws of physics and chemistry have remained constant throughout Earth's history.

  • Past geological events occurred via natural processes.

  • Demonstrated that Earth is extremely old through gradual geological change.

• Charles Darwin & Alfred Russel Wallace:

  • Co-founders of the theory of evolution by natural selection.

  • Darwin traveled from Australia to the Galápagos Islands aboard the HMS Beagle, studying barnacles, finches (beak adaptations), and artificial selection in pigeons.

• Darwin's Theories:

  • Observational: Perpetual change, common descent, multiplication of species.

  • Proposed Mechanisms: Gradualism, natural selection.

2. Evidence of Evolution & Speciation

Perpetual Change

• Living world is continuously changing; traits are modified over time, best observed via the fossil record.

• Key Evolutionary Milestones:

  • First eukaryotic cells: $\sim 2.5\,\text{BYA}$

  • First multicellular organisms: $1.7\,\text{BYA}$

  • Cambrian Explosion: $540\,\text{MYA}$ (yielded both extinct and extant lineages)

  • Tetrapods on land: $400\,\text{MYA}$

• Fossil Record Biases:

  • Over-represents hard-bodied organisms.

  • Under-represents soft-bodied organisms.

Common Descent

• Homology: Traits inherited from a common ancestor, which may be modified for different functions (e.g., vertebrate limb bones modified for walking, swimming, or flying).

• Analogous Traits: Superficially similar features driven by convergent evolution (e.g., fusiform body shape in aquatic animals).

• Ontogeny: The developmental history of an organism from embryo to death.

  • Homologous structures may only exist during early embryonic development before being modified or lost (e.g., human pharyngeal slits and embryonic tails).

Multiplication of Species (Speciation)

• Species Definition: A group of interbreeding natural populations with common ancestry that are reproductively isolated from other groups.

• Allopatric (Geographic) Speciation: Geographic barriers separate a population, preventing gene flow and allowing reproductive isolation to evolve.

• Reproductive Barriers:

  • Prezygotic: Geographic, temporal, mechanical, and behavioral barriers.

  • Postzygotic: Fertilization occurs, but hybrids are non-viable or sterile.

• Adaptive Radiation: Rapid emergence of many ecologically diverse species from a single ancestral lineage over short geological timescales.

• Gradualism: Continuous, incremental morphological changes in natural populations over long periods ($10{,}000 - 15{,}000\,\text{years}$) leading to speciation.

Natural Selection Mechanism

• Darwin lacked a mechanism for inheritance because genetics (Mendel's work) was not yet integrated.

• Natural selection acts as the primary agent of evolutionary change, altering heritable gene frequencies in populations rather than modifying individual organisms.

3. Natural Selection, Micro/Macroevolution & Systematics

The VISTA Model of Natural Selection

• V - Variation: Random mutations and DNA recombination accumulate neutral, beneficial, or deleterious traits.

• I - Inheritance: Offspring inherit variations from parents.

• S - Selection: Environmental pressures favor organisms with traits that confer higher evolutionary fitness (survival and reproduction).

• T - Time: Gene frequencies shift over successive generations.

• A - Adaptation: Novel traits and distinct species gradually emerge.

Modes of Selection

• Stabilizing Selection: Favors intermediate phenotypes, selecting against extremes.

• Directional Selection: Favors one extreme phenotype over others (e.g., selection against lighter snails).

• Disruptive Selection: Favors both phenotypic extremes over intermediate traits.

Neo-Darwinism & Microevolution

• Neo-Darwinism (Modern Synthesis): Integrates Darwinian selection with Mendelian genetics, chromosomal theory, population genetics, systematics, and paleontology.

• Microevolution: Genetic changes within a population driven by:

  1. Random mutation

  2. Natural selection (including non-random/sexual selection)

  3. Genetic drift

• Sexual Selection: Selection for traits that enhance mating success, even if detrimental to survival (e.g., peacock tail display).

• Genetic Drift: Random fluctuations in allele frequencies across generations.

  • Bottleneck Effect: Dramatic population reduction due to a random event causes loss of genetic diversity.

  • Founder Effect: A small migrating group establishes a new population with limited genetic variation.

Macroevolution & Systematics

• Macroevolution: Large-scale evolutionary patterns over geological time.

  • Occurs when extinction rate exceeds speciation rate, or vice versa.

  • Mass Extinctions: Permian extinction (loss of $90\,\%$ marine invertebrates); Cretaceous extinction (dinosaur extinction).

  • Ecological Vacuum: Speciation accelerates when new ecological niches open up.

• Systematics: Study of animal variation to reconstruct evolutionary relationships.

• Taxonomy: Formal naming and grouping of species into hierarchical categories (taxa).

• Phylogenetics & Cladistics:

  • Phylogenetic Trees: Visual representations of evolutionary relationships based on shared characters.

  • Clades: Monophyletic groups comprising an ancestral lineage and all of its descendants, defined by shared derived traits (synapomorphies).

  • Sister Groups: The closest lineage outside a given clade.

  • Parsimony: Selecting the tree hypothesis that requires the fewest evolutionary transitions.

  • Groupings:

  • Monophyletic: Ancestor + ALL descendants (ideal clade).

  • Paraphyletic: Ancestor + SOME descendants.

  • Polyphyltic: Grouping lacking the recent common ancestor.

• Evolutionary Arms Race: Continuous co-evolutionary cycle of adaptations and counter-adaptations between competing species.

4. Animal Body Plans & Development

Classifying Animals by Form and Function

• Body Symmetry:

  • Asymmetrical

  • Spherical symmetry

  • Radial symmetry (e.g., pentaradial in adult sea stars)

  • Bilateral symmetry

• Gut Openings:

  • Incomplete / Blind Gut: Single opening serves as mouth and anus (e.g., Platyhelminthes).

  • Complete Gut: Distinct mouth and anus (e.g., mammals/rabbits).

  • No Gut: Secondary loss of gut and mouth associated with parasitic lifestyles (e.g., tapeworms absorbing nutrients across the tegument).

• Germ/Tissue Layers:

  • Ectoderm: Skin and nervous system.

  • Mesoderm: Muscles, skeleton, and circulatory system.

  • Endoderm: Internal organs and gut lining.

  • Diploblastic: Possesses endoderm and ectoderm only.

  • Triploblastic: Possesses all three germ layers.

• Body Cavities (Coelom in Triploblasts):

  • Acoelomate: No body cavity.

  • Pseudocoelomate: Cavity not completely lined by mesoderm.

  • Coelomate (Eucoelomate): True body cavity completely enclosed within mesoderm.

• Appendages: Number and arrangement serve as key synapomorphies (e.g., insect body segmentation).

• Body Orientation Axes:

  • Anterior / Cephalic: Head end.

  • Posterior / Caudal: Tail end.

  • Dorsal: Back side.

  • Ventral: Front / Belly side.

Developmental Pathways

• Protostomes vs. Deuterostomes:

  • Protostomes: Blastopore develops into the mouth first; spiral cleavage.

  • Deuterostomes: Blastopore develops into the anus first; radial cleavage.

Basic Developmental Stages:   $\text{Zygote} \rightarrow \text{Cleavage Stages} \rightarrow \text{Blastula} \rightarrow \text{Gastrula} \rightarrow \text{Larval / Embryonic} \rightarrow \text{Juvenile} \rightarrow \text{Adult}$

Eukaryotes

All eukaryotic life shares a common ancestor (~2BYA)

• Membrane-bound Nucleus

• Specialized organelles and subcellular processes

• Mitochondria from symbiogenesis > = Sequential endosymoiotic theory

Kingdom Protista

• Originally called Phylum Protozoa

• Mostly unicellular

• All lack collagen and chitin in their cell walls

• Now has become a catch all taxon for groups that aren’t quite plant, animal, or fungi

Unicellular Eukaryotes Overview of Chapter 5

• Focus on adaptations of select UEs in 5 phyla

  • Euglenozoa, e.g. Hemoflagellates parasite, Trypanosoma

  • Dinoflagellata

  • Metamonada

  • Cilliophora

  • Apicomplexa

Traditional VS. Modern Classification Schemes

• Traditional = Locomotory mode groups

  • Flagellates

  • Amoebas

  • Cilliates

• Modern = RNA-based supergroups

  • Based on molecular evidence, modes of locomotion evolved independently multiple times.

Locomotory Mode:

• Motile structures made of cytoskeletal protein sub-units (microtybules) and motor proteins (dyeins)

• Used to propel water along the cell for locomotion

• “Fagellates” have one or more falgella

• “Ciliates” have 100s of cilia

Pseudopodia

• Extensions of the cell cytoplasm in the direction of movement

Feeding and Digestion

• Autotrophs (Primary Producers) — Make their own food

• Photosynthesis or Chemosynthesis

  • Sometimes appear green

• Chloroplasts (Photosynthetic)

Heterotrophs (Consumers) — Obtain food extrenally

Mixotrophs — autotrophy + heterophy

Reproduction; Sexual & Asexual

Wide range of reproduction processes in unicellular Eukaryotes

• Asexual reproduction

  • Binary fission — Individual divides into two identical daughter cells

  • Multiple fission — Numerous individuals are produced almost simultaneously = Schizogony

• Sexual reproduction

  • Conjugation — an exchange of genetic material in micronuclei occurs between two individuals

Encystment

Unicellular Eukaryotes are successful in harsh habitats and conditions

• Cysts — dormant stages with resistant cyst wall

  • Adaptation of many parasitic UEs that must survive external environments between hosts (dispersal)

Osmoregulation and Excretion

Marine and parasitic species — isosmotic environment

Freshwater species (FW) — Hypoosmotic environment

• Contractile vacuoles

  • Act as a pump to remove excess water & some waste products

• Diffusion through plasma membrane

  • Passive transport

Ecology of Unicellular Eukaryotes

• found wherever moisture exists

  • free-living species in all marine & freshwater habitats, damp soils

  • Form a large part of phytoplankton & zooplankton

  • primary producers & primary consumers — food webs

  • Thousands of species symbiotic in or on other animals

Symbiotic relationships

• 3 types of symbiotic relationships

  • Mutualism—both benefit

  • commensalism—one benefits without affecting the other

  • parasitism—one benefits at the expense of the other

“Phytoflagellates”

• Protists with one or more flagella for locomotion

• have chloroplasts, photosynthesis (sometimes)

• Ecologically important primary producers

• representative groups

  • Euglenozoa

  • “Green algae”

  • Dinoflagellata

Phylum Euglenozoa

• Euglenids/euglenoids/euglenophytes

• Mostly freshwater

• Mixotrophs

  • switching feeding modes based on the environment

Euglena Gracilis

• Beneath cell membrane is a matrix of rigid protein called the pelicle

• it allows the cell to contract and change shape

Colonial Euglenid Colacium

The flagella are modified into mucus covered stalks but can be re-grown in 1 hour

“Green algae”

• Close relatives of plants

• Mostly freshwater

• Autotrophic

Solitary Green Algae

• Have 2 flagella

• Extremely common

Colonial Green Algae

• Forms spherical, cooperative colonies

  • cell differentiation

  • colony polarity

• Primitive multicellularity

Phylum Dinoflagellata

• Two dissimilar flagella in different directions, called dinoknot flagellation

• Marine and freshwater

• mixotrophic

Blooms of Ceratium and other Dinoflagellates

• Can cause red tide

  • Shellfish accumulate toxin while filter-feeding on dinoflagellates

  • shell fish cinsumption

    • Paralytic shellfish poisoning (PSP)

• Can Bioluminese

  • E.g. notciluca

Symiodinum

Endosymbiotic dinoflagellates that live in the inner layer of cnidarians

• Provide photosynthetic products

• Receive nutrients and protection

“Zooflagellates”

• Protists with one or more flagella for locomotion

• Are heterotrophic or parasitic

• Representative groups;

  • Parabasalids

  • Choanoflagellates

Parabasalids

• Commensal and parasitic forms in animals and plants

• Responsible for human disease

  • e.g sleeping sickness.

• Reproduces by binary fission in human blood stream

• Transmission by blod-feeding insect vector

Choanoflagellates

• Heterotrophic

• Sessile and motile forms

• Solitary or colonial

Amebas

• Protists with pseudopods used for crawling and feedings

• Are heterotrophic or parasitic

• Representative groups;

  • Amebozoa

  • Foraminifera

  • Radiolaria

Phylum Amebozoa

• Blunt and tubular