BIO 111 Comprehensive Study Notes: Speciation, Phylogenetics, History of Life, Prokaryotes, and Viruses

Species Concepts and Reproductive Barriers

• Species Concepts:

  • Morphological Species Concept: Defines species by distinct, observable physical traits. Useful for fossils and field identification; fails to resolve cryptic species (morphologically identical but genetically distinct) or polymorphic species (different phenotypes within a single interbreeding species), and can be misled by convergent evolution.

  • Biological Species Concept (BSC): Defines species as groups of actually or potentially interbreeding natural populations that are reproductively isolated from other groups. Directly links speciation to gene flow, but cannot be applied to fossils, asexual organisms, or geographically isolated allopatric populations.

  • Phylogenetic Species Concept (PSC): Defines a species as the smallest diagnosable monophyletic group sharing a common ancestor and unique synapomorphies. Applies well to molecular/DNA datasets and cryptic taxa, but can over-split lineages.

  • Ecological Species Concept (ESC): Defines species based on their ecological niche and adaptive role. Useful for organisms with distinct resource utilization (e.g., host-specific parasites or cichlids), but limited by overlapping or shifting niches.

• Prezygotic Isolating Mechanisms (prevent fertilization):

  • Temporal: Breeding occurs at different times or seasons (e.g., distinct breeding peaks across frog species from March to July).

  • Habitat: Breeding occurs in distinct geographic locations or microhabitats (e.g., Arctic fox adapted to northern cold vs. Gray fox adapted to southern heat).

  • Behavioral: Divergence in courtship behaviors, displays, or mating songs (e.g., Black-capped Chickadee vs. Carolina Chickadee; eastern vs. western meadowlarks).

  • Gametic: Incompatibility between sperm and egg surface proteins prevents fertilization (e.g., distinct sea urchin surface recognition proteins).

  • Mechanical: Structural incompatibilities of reproductive organs prevent copulation (e.g., left-handed vs. right-handed shell coiling in Bradybaena snails preventing genital pore alignment).

• Postzygotic Isolating Mechanisms (reduce hybrid viability or fertility):

  • Hybrid Inviability: Hybrid zygotes undergo developmental arrest or exhibit high embryonic mortality (e.g., sheep-goat hybrids or "geeps").

  • Hybrid Sterility: Hybrids survive to adulthood but cannot produce functional gametes (e.g., mules, ligers, and tigons).

Speciation Mechanisms and Macroevolutionary Patterns

• The Speciation Cascade: Speciation progresses through reduced gene flow $\rightarrow$ genetic divergence (via mutation, natural selection, and genetic drift) $\rightarrow$ the evolution of reproductive isolating barriers $\rightarrow$ complete reproductive isolation.

• Allopatric Speciation (geographic isolation):

  • Vicariance: A physical barrier bisects a previously continuous population (e.g., the formation of the Isthmus of Panama around $3.5\text{--}3.0\,\text{mya}$ splitting marine populations such as Alpheus snapping shrimp, forming sister-species pairs across the land bridge).

  • Dispersal / Founder Effect: A small subpopulation colonizes a distant, isolated area (e.g., Channel Islands fox, Urocyon littoralis, diverging across six islands following rafting or introduction).

  • Peripatric Speciation: A small, peripheral isolate at the boundary of a range undergoes accelerated divergence due to strong genetic drift and founder effects (e.g., Tanysiptera paradise kingfishers showing dramatic phenotypic divergence on New Guinea peripheral islands compared to mainland forms).

• Sympatric Speciation (without geographic barriers):

  • Driven by disruptive selection (e.g., host-plant preference shifts in apple maggot flies Rhagoletis pomonella selecting for apple vs. hawthorn fruit scent) or polyploidization (abrupt genome duplication producing instant reproductive isolation).

• Adaptive Radiation:

  • Rapid production of multiple descendant lineages exhibiting diverse ecological and morphological adaptations from a single common ancestor.

  • Mechanisms: Driven by ecological opportunity (vacant niches post-extinction or colonization of isolated archipelagos, such as Hawaiian honeycreepers) and key morphological innovations (e.g., angiosperm flowers; Hox gene duplications and active predation during the Cambrian Explosion ~$540\,\text{mya}$).

• Macroevolutionary Dynamics:

  • Tempo Models: Gradualism (slow, steady phenotypic change); Stasis (long intervals with little net change); Punctuated Equilibrium (prolonged stasis interrupted by rapid morphological change during speciation events).

  • Extinction: Background extinction refers to regular baseline losses; mass extinction represents geologically rapid, global loss of biodiversity. The Cretaceous-Paleogene (K-Pg) extinction ($66\,\text{Ma}$) eliminated ~$75\%$ of species, including non-avian dinosaurs, precipitating the adaptive radiation of surviving mammalian lineages.

Systematics and Reading Phylogenetic Trees

• Core Components:

  • Systematics unites Taxonomy (naming and hierarchical classification) and Phylogenetics (hypothesizing evolutionary relationships).

  • Tree Topography: Root (ancestor of the entire ingroup), Nodes (most recent common ancestors and speciation branching points), Branches (lineages evolving through time), and Tips / Sister Taxa (terminal extant or extinct entities sharing an immediate common ancestor).

  • Interpretation Rules: Relatedness is defined strictly by the Most Recent Common Ancestor (MRCA), not the arrangement of taxa along tree tips. Nodes rotate freely without altering phylogenetic relationships; unscaled branch lengths contain no temporal or character-change information.

• Character Evolution:

  • Homology: Trait similarity inherited from a common ancestor (e.g., mammalian forelimb bone arrangement).

  • Homoplasy: Trait similarity resulting from convergent evolution, parallel evolution, or evolutionary reversals rather than common descent (e.g., wings in birds vs. bats).

  • Synapomorphies: Shared derived characters unique to a specific evolutionary branch; the exclusive basis for building clades and recovering monophyletic lineages.

  • Plesiomorphies: Ancestral character states inherited from distant ancestors; uninformative for resolving sub-relationships within a derived group.

• Clade Classification:

  • Monophyletic Group (Clade): Includes a common ancestor and all of its descendants; can be severed from the tree with a single cut.

  • Paraphyletic Group: Includes a common ancestor and some, but not all, of its descendants (e.g., traditional "prokaryotes" or non-avian reptiles).

  • Polyphyletic Group: Composed of taxa that do not share an immediate common ancestor inside that defined group (e.g., grouping flying tetrapods like birds and bats together).

• Evaluating Lineages with Fossils:

  • Fossils provide direct anatomical timelines and minimum lineage ages via sedimentary superposition (relative dating) and radioactive isotope decay (numerical dating).

  • Cetacean Transition: DNA sequence data places Cetacea sister to Hippopotamidae; transitional fossil series (Indohyus, Pakicetus, Ambulocetus, Kutchicetus, Rodhocetus, Dorudon) corroborates gradual aquatic transitions (thickened tympanic bone, reduction of weight-bearing hindlimbs, tail-powered swimming, and subsequent divergence into Odontocetes and Mysticetes).

Prokaryotic Cell Organization, Diversity, and Evolution

• Deep Tree of Life and LUCA:

  • Life splits into three domains: Bacteria, Archaea, and Eukarya.

  • Archaea and Eukarya share a more recent common ancestor with each other than either does with Bacteria.

  • LUCA (Last Universal Common Ancestor) occurred $3.5\text{--}4.0\,\text{bya}$; it represents the MRCA of all extant cellular life, not the first living organism.

  • Prokaryotes lack a nucleus and do not form a monophyletic clade.

• Domain Distinctions:

• Bacterial Structures and Functions:

  • Cell Wall: Peptidoglycan mesh resists osmotic lysis in hypotonic media and maintains structural integrity. Gram-positive cells retain crystal violet (purple) due to a thick, exposed peptidoglycan wall; Gram-negative cells stain pink/red due to a thin peptidoglycan layer enclosed by an outer lipopolysaccharide membrane. Gram status reflects envelope morphology rather than monophyly.

  • Surface Projections: Rotary flagella power motility; fimbriae mediate host surface adherence; pili mediate cell-to-cell DNA conjugation.

• Reproduction and Horizontal Gene Transfer (HGT):

  • Binary Fission: Asexual division yielding two genetically identical daughter cells; initiated by bidirectional chromosome replication from a single origin, followed by separation and membrane constriction.

  • HGT Mechanisms:

  • Transformation: Direct uptake of exogenous, free environmental DNA by competent cells.

  • Transduction: Accidental packaging and transfer of donor bacterial DNA by a bacteriophage virus.

  • Conjugation: Transfer of plasmid or chromosomal DNA via direct contact through a pilus.

  • Phylogenetic Discordance: HGT transfers genes across divergent taxa, causing single-gene phylogenetic trees to contradict true organismal/species phylogenies.

• Antibiotic Resistance Mechanics:

  • Rapid evolution is fueled by short generation times (e.g., E. coli dividing in $20\text{--}25\,\text{minutes}$) and large population sizes. Mutation rates per replication are not unusually elevated.

  • Antibiotics apply strong non-random selective pressure; they do not induce resistance mutations.

  • Neisseria gonorrhoeae: Penicillinase-encoding plasmids spread via conjugation and vertical inheritance. Reported US cases rose, with penicillinase-producing isolates jumping from $3.2\%$ in 1988 to $7.4\%$ in 1989, ending penicillin recommendations by 1989. Ceftriaxone is now the primary US therapy, with $543{,}409$ total gonorrhea cases reported in 2024.

Prokaryotic Metabolism and Global Biogeochemistry

• Metabolic Classification Framework:

  • Energy Source: Phototroph (light) vs. Chemotroph (chemical reactions).

  • Electron Donor: Organotroph (organic molecules: glucose, acetate, amino acids) vs. Lithotroph (inorganic molecules: $H_2$, $NH_3$, $H_2S$, $Fe^{2+}$).

  • Carbon Source: Autotroph (inorganic carbon, usually $CO_2$) vs. Heterotroph (preformed organic carbon compounds).

  • Examples:

  • Cyanobacteria: Photolithoautotroph (energy: light; donor: $H_2O$; carbon: $CO_2$).

  • E. coli: Chemoorganoheterotroph (energy: chemicals; donor: glucose; carbon: glucose).

  • Ammonia-oxidizing bacteria: Chemolithoautotroph (energy: chemicals; donor: $NH_3$; carbon: $CO_2$).

  • Purple non-sulfur bacteria: Photoorganoheterotroph.

• Energy Generation Pathways:

  • Aerobic Respiration: Electron transport chain (ETC) coupled to ATP synthase; $O_2$ acts as the terminal electron acceptor; produces high ATP yields.

  • Anaerobic Respiration: ETC coupled to ATP synthase; terminal electron acceptor is an inorganic molecule other than $O_2$ (e.g., $NO_3^-$, $SO_4^{2-}$); yields variable ATP, typically less than aerobic respiration.

  • Fermentation: No respiratory electron transport chain; organic molecules serve as both electron donor and terminal acceptor; ATP generated solely via substrate-level phosphorylation; low ATP yield.

• Biogeochemical Cycles:

  • Oxygen: Cyanobacterial oxygenic photosynthesis sparked the Great Oxygenation Event, fundamentally altering the redox state of Earth's atmosphere and oceans.

  • Nitrogen: Bacterial/archaeal nitrogen fixers reduce atmospheric $N_2$ to biological ammonia ($NH_3$); nitrifiers and denitrifiers control global nitrogen availability.

  • Carbon, Sulfur, Iron, and Phosphorus: Microbial decomposition, chemoautotrophic carbon fixation, sulfur reduction/oxidation, and iron oxidation ($Fe^{2+} \rightarrow Fe^{3+}$) drive Earth's elemental recycling.

Viral Structure, Replication, and Evolutionary Roles

• Viral Architecture:

  • Noncellular infectious agents consisting of a genome (single- or double-stranded DNA or RNA, linear or circular, single or segmented) encased within a protective protein shell (capsid).

  • Some viruses feature an outer envelope consisting of a lipid bilayer acquired from host membranes studded with viral glycoproteins.

• Host Specificity and Replication Cycles:

  • Tropism: Attachment depends entirely on physical interactions between viral surface proteins and specific host cell membrane entry receptors.

  • Obligate Intracellular Replication: Viruses lack independent metabolic machinery, ribosomes, and ATP synthesis; replication strictly depends on hijacking host cellular machinery to replicate viral genomes, transcribe mRNAs, translate structural proteins, assemble virions, and release progeny via lysis or budding.

• Evolutionary Significance:

  • Facilitate horizontal gene transfer through transduction (transferring host DNA segments between distinct bacterial strains).

  • Act as selective agents, driving frequency changes in host immunity and receptor alleles.

  • Viral genomes can integrate into host lineages, permanently altering host genetic architecture.