Evolution, Microevolution, Macroevolution, and Systematics Flashcards

Introduction to Evolution & Core Concepts

• Darwinian Definition of Evolution: Evolution is defined as "descent with modification." Through time, species accumulate structural, physiological, and genetic differences. As a result, descendants differ from their ancestors, giving rise to new species from existing ones.

• Scala Naturae vs. Evolutionary Reality:

  • Scala Naturae (the "Scale of Nature") is an incorrect, pre-evolutionary concept that viewed life as a linear ladder progressing toward perfection, with humans at the top.

  • Biological evolution is non-linear and branching. It is driven by adaptation rather than an inherent drive toward superiority or complexity.

  • Humans are not the target or pinnacle of evolution; they are simply one extant species among millions.

  • Populations evolve over generations; individual organisms do not evolve within their lifetimes.

  • Extinction is a natural consequence of evolutionary history, affecting over $99\%$ of all species that have ever existed.

• Adaptation: An inherited characteristic (structural, physiological, or behavioral) that increases an organism's fitness and ability to survive and reproduce within a specific environment.

• Natural Selection and Fitness:

  • Natural Selection: The mechanism through which individual organisms possessing favorable inherited traits are more likely to survive and reproduce than those lacking those traits.

  • Environmental Context: Traits considered "favorable" are relative to a specific environment at a specific time; there are no absolute optimal traits.

  • Fitness: A quantifiable measure of relative reproductive success (the contribution an individual makes to the gene pool of the next generation).

  • Lack of Teleology: Natural selection has no internal drive, intentionality, foresight, or awareness of what a species "needs." It acts strictly as an environmental filter on pre-existing genetic variation.

• Historical Hypotheses: Lamarck vs. Darwin:

  • Jean-Baptiste Lamarck's Theory (Acquired Characteristics): Proposed that physical alterations acquired during an organism's lifetime through use or disuse (e.g., a giraffe stretching its neck to reach high leaves) are passed directly to offspring.

  • Charles Darwin's Theory (Natural Selection): Demonstrates that variation arises randomly prior to environmental pressures due to genetic differences. Individuals with longer necks survive better and reproduce more when tall trees are present, increasing the frequency of long-neck alleles in subsequent generations.

• Requirements for Natural Selection: For natural selection to cause evolutionary change within a population, variation must meet three criteria:

  1. It must be heritable (passed genetically from parents to offspring).

  2. It must be phenotypic (expressed as observable physical, physiological, or behavioral traits).

  3. It must influence fitness (impact survival or reproductive output).

Evidence for Evolutionary Change

• Artificial Selection:

  • The process by which humans consciously selectively breed organisms with desirable phenotypic traits over generations.

  • Plant Domestication: Selective breeding of wild mustard (Brassica oleracea) produced distinct crops by targeting different plant structures: kale (selected for leaves), cabbage (selected for apical terminal buds), broccoli (selected for flowers and stems), brussels sprouts (selected for axillary side buds), and kohlrabi (selected for stems).

*   *Seed Retention in Wheat:* Wild wheat species undergo seed shattering at maturity to disperse seeds, whereas domestic wheat has been selected to retain seeds on the stalk to facilitate harvesting.
*   *Pleiotropy in Domestication:* Breeding animals for behavioral tameness (such as silver foxes) often produces unintended phenotypic side effects (e.g., floppy ears, piebald coat patterns, altered skull shapes) due to pleiotropy—where single genes influence multiple distinct phenotypic traits.
*   *Canine Diversification:* Modern domestic dog breeds were artificially selected from a common wild wolf ancestor (*Canis lupus*).

• Direct Observation of Microevolution:

  • Soapberry Bug Beak Evolution: Studies on the soapberry bug (Jadera haematoloma) demonstrate directional selection in beak length matching fruit size.

  • In Southern Florida, native soapberry bugs feed on the native balloon vine (Cardiospermum corindum), which has thick seed pods requiring long beaks (average beak length $8.5\,mm$ to $9.0\,mm$).

  • In Central Florida, the introduction of the golden rain tree (Koelreuteria elegans) from Asia provided a new food source with thinner pods. Within 20 to 50 years, soapberry bug populations adapted to feed on golden rain trees evolved significantly shorter beaks (average beak length $6.0\,mm$ to $7.5\,mm$).

• Homologous Structures:

  • Anatomical Homology: Structures present in different species that share a common ancestral origin, even if they serve entirely different functions in modern organisms.

  • Mammalian Forelimbs: Humans, cats, whales, and bats share identical skeletal arrangements in their forelimbs—consisting of a single proximal bone (humerus), two distal bones (radius and ulna), a cluster of wrist bones (carpals), palm bones (metacarpals), and finger bones (phalanges)—adapted for lifting, walking, swimming, and flying, respectively.

*   **Comparative Embryology:** Vertebrate embryos exhibit shared structural features during early development that may disappear in adult forms. Both chick and human embryos develop pharyngeal arches and a muscular post-anal tail.

• Vestigial Structures:

  • Anatomical features that represent reduced, non-functional, or altered remnants of structures that served important physiological functions in ancestral species.

  • Human Examples: Extrinsic ear muscles (Darwin's tubercle) and the appendix.

  • Cetacean Examples: Pelvic girdles and reduced hind limb bones (femur, tibia) floating within the muscular body wall of modern baleen and toothed whales.

• The Fossil Record:

  • Chronologically documents macroevolutionary transitions, demonstrating that past organisms differed structurally from modern species.

  • Transitional Fossils: Transitional species like Archaeopteryx reveal the evolutionary link between non-avian feathered dinosaurs and modern birds.

  • Equine Evolution: Fossil sequences document changes over $55\text{ million years}$ from small, forest-dwelling, multi-toed browsing ancestors (Eohippus, $55\text{--}45\,mya$) to Mesohippus ($40\text{--}30\,mya$), Hipparion ($23\text{--}2\,mya$), and modern single-toed grazing horses (Przewalski horse / Equus).

  • Cetacean Origin: Fossil series including Pakicetus, Rodhocetus, and Dorudon document the transition of terrestrial artiodactyls to fully aquatic whales. Bones such as the double-pulley astragalus ankle structure unite early fossil whales with living even-toed ungulates (pigs, hippopotamuses, deer).

• Molecular Evidence:

  • All living organisms share the same basic genetic code (DNA/RNA) and metabolic pathways.

  • Evolutionary relationships are reflected in the number of amino acid differences in shared proteins.

  • Haemoglobin $\beta$-chain Analysis: Comparing the $146\text{-amino-acid}$ sequence of the human haemoglobin $\beta$-chain to other vertebrates reveals increasing differences with time since divergence:

    • Human: $0$ differences (identical homology)

    • Rhesus monkey: $8$ amino acid differences

    • Dog: $32$ amino acid differences

    • Bird: $45$ amino acid differences

    • Lamprey / Fish: $125$ amino acid differences

• Biogeography:

  • The study of the geographic distribution of species, driven by tectonic continental drift (e.g., historical breaking of Pangea).

  • Islands often possess endemic species closely related to species on the nearest mainland or neighboring islands.

• Convergent Evolution:

  • The independent evolution of functionally similar features in distantly related lineages as a result of exposure to similar environmental pressures and natural selection pressures.

  • Structures produced by convergent evolution are analogous (serving similar functions) rather than homologous (sharing common ancestry).

  • Example: The Australian sugar glider (a marsupial) and the North American flying squirrel (a placental mammal) independently evolved gliding membranes and similar body plans to navigate arboreal environments.

Microevolution: Population Genetics & Mechanisms of Evolution

• Genetics Fundamentals:

  • Gene: A discrete unit of hereditary information located at a specific chromosomal locus, composed of a specific nucleotide sequence in DNA.

  • Allele: Alternative nucleotide variations of a specific gene.

  • Genotype: The genetic composition of an organism at a specific locus.

    • Homozygous Dominant: Possessing two copies of the dominant allele ($AA$).

    • Heterozygous: Possessing one dominant allele and one recessive allele ($Aa$).

    • Homozygous Recessive: Possessing two copies of the recessive allele ($aa$).

  • Phenotype: The observable physical, physiological, anatomical, or behavioral traits produced by the interaction of genotype and environment.

• Hardy-Weinberg Principle:

  • Describes a theoretical, non-evolving population in which allele and genotype frequencies remain constant from generation to generation (Hardy-Weinberg Equilibrium).

  • Five Mandatory Conditions for Hardy-Weinberg Equilibrium:

    1. No Selection: All individuals have equal survival and reproductive success.

    2. No Mutation: No new alleles are introduced into the gene pool.

    3. No Gene Flow (Migration): No movement of alleles into or out of the population.

    4. Extremely Large Population Size: Eliminates sampling error and chance fluctuations in allele frequencies (genetic drift).

    5. Random Mating: Individuals choose mates without preference for specific genotypes or phenotypes.

  • Mathematical Equations:

    • Allele Frequency Equation:

$p + q = 1$

    *   Genotype Frequency Equation:

$p^2 + 2pq + q^2 = 1$

    *   *Variable Definitions:* 
        *   $p$ = frequency of the dominant allele in the population
        *   $q$ = frequency of the recessive allele in the population
        *   $p^2$ = frequency of homozygous dominant individuals ($AA$)
        *   $2pq$ = frequency of heterozygous individuals ($Aa$)
        *   $q^2$ = frequency of homozygous recessive individuals ($aa$)
    *   *Rule of Thumb:* Variables lacking an exponent or coefficient of 2 represent single allele frequencies ($p, q$); variables containing 2 represent diploid genotype frequencies ($p^2, 2pq, q^2$).

• Phenotypic Plasticity:

  • The capacity of a single genotype to produce different phenotypes when exposed to different environmental conditions.

  • Example: Caterpillars of the same moth species develop distinct morphology based on diet: caterpillars raised on oak flowers mimic oak catkins, whereas those raised on oak leaves mimic rigid wooden twigs.

• The Five Mechanisms of Evolutionary Change:

  1. Mutation:

     • The ultimate origin of all genetic variation through random changes in DNA nucleotide sequences.

     • Mutations in regulatory sequences can cause major morphological changes. For example, changes in the expression of the Pitx1 gene cause marine stickleback fish to possess robust hind pelvic spines while freshwater stickleback populations lose pelvic hind fins.

  2. Gene Flow:

     • The transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes (immigration and emigration).

     • Gene flow tends to reduce genetic differences between populations over time.

     • Disruption of Adaptation: In the Great Tit (Parus major) on Vlieland island, high gene flow carrying non-adapted alleles from mainland populations reduces the survival rate of island females, disrupting local adaptation.

  3. Nonrandom Mating:

     • Occurs when the probability of two individuals mating is not equal across all possible pairs in a population.

     • Assortative Mating: Individuals choose mates that share similar phenotypes (e.g., Scarlet leaf beetles mating preferentially with individuals of similar body size). Increases population homozygosity.

     • Disassortative Mating: Individuals choose mates with phenotypes different from their own (e.g., wild pigeons choosing mates with distinct plumage patterns). Increases population heterozygosity.

  4. Genetic Drift:

     • Unpredictable fluctuations in allele frequencies from one generation to the next caused by chance events, operating most strongly in small populations.

     • Genetic drift reduces overall genetic variation and can lead to the random fixation or loss of alleles.

     • Bottleneck Effect: A sudden, drastic reduction in population size caused by environmental catastrophes, disease, or human activity. The surviving population's gene pool may no longer reflect the original population's genetic makeup.

     • Founder Effect: Occurs when a few colonizing individuals become isolated from a larger source population to establish a new colony. The new population's allele frequencies differ substantially from the parent population.

  5. Natural Selection:

     • The only evolutionary mechanism consistently causing adaptive evolution by favoring traits that enhance survival and reproduction.

Modes of Natural Selection & Maintenance of Diversity

• Three Patterns of Natural Selection on Phenotypic Distributions:

  • Directional Selection: Favors individuals at one extreme of the phenotypic distribution, shifting the population's mean phenotype toward that extreme.

  • Disruptive Selection: Favors individuals at both phenotypic extremes over intermediate phenotypes, creating a bimodal phenotypic distribution.

  • Stabilizing Selection: Favors intermediate phenotypes and selects against extreme phenotypes, reducing phenotypic variation.

• Sexual Selection:

  • A form of natural selection in which individuals with specific inherited characteristics are more likely than others to secure mates.

  • Intrasexual Selection: Direct competition among individuals of one sex (usually males) for access to mates of the opposite sex (e.g., physical combat or territory defense).

  • Intersexual Selection (Mate Choice): Individuals of one sex (usually females) actively select mates of the opposite sex based on specific phenotypic displays, vocalizations, or ornamentation.

• Preservation of Genetic Variation:

  • Diploidy: Diploidy preserves recessive alleles from elimination by natural selection because recessive alleles are sheltered from selection in heterozygous individuals ($Aa$).

  • Balancing Selection: Maintains two or more phenotypic forms in a population through two main processes:

    1. Heterozygote Advantage: Occurs when heterozygous individuals at a specific locus exhibit higher fitness than both types of homozygous individuals.

       • Sickle-Cell Anemia and Malaria: In regions of sub-Saharan Africa plagued by the parasitic eukaryote Plasmodium falciparum (malaria), the sickle-cell allele ($Hb^S$) is maintained at high frequencies ($>12.5\%$). Homozygous dominant individuals ($Hb^A Hb^A$) are vulnerable to fatal malaria infection; homozygous recessive individuals ($Hb^S Hb^S$) suffer from severe sickle-cell disease; heterozygous individuals ($Hb^A Hb^S$) are protected against severe malaria without suffering full sickle-cell pathology.

    2. Frequency-Dependent Selection: The fitness of a phenotype depends on how common it is in the population. Rare phenotypes are favored by natural selection, preventing any single phenotype from achieving complete fixation.

• Constraints on Natural Selection (Why Perfect Organisms Do Not Exist):

  1. Selection can act only on existing genetic variations; it cannot spontaneously create beneficial mutations.

  2. Evolution is constrained by historical ancestral structures (e.g., limbs are modified from pre-existing structures rather than built from scratch).

  3. Adaptations are typically structural or functional compromises (e.g., human upright posture enables bipedalism but causes back stress).

  4. Evolutionary trajectories interact unpredictably with chance events, natural selection, and shifting environments.

Macroevolution: Speciation & Reproductive Isolation

• Species Concepts:

  • Biological Species Concept (BSC): Defines a species as a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups, producing viable, fertile offspring.

  • Ecological Species Concept: Defines a species in terms of its ecological niche—the sum of its interactions with the nonliving and living parts of its environment. Applies to both sexual and asexual species.

  • Phylogenetic Species Concept: Defines a species as the smallest, irreducible group of individuals that share a common ancestor on a phylogenetic tree.

  • Morphological Species Concept: Characterizes species by structural and anatomical features.

  • Subspecies: Geographically defined subdivisions of a species that exhibit distinct morphological variations but remain capable of interbreeding where ranges overlap (e.g., geographical variants of Dark-eyed Juncos).

• Reproductive Isolating Mechanisms: Biological barriers that prevent members of two distinct species from interbreeding and producing viable, fertile offspring.

  • Prezygotic Barriers (Prevent Mating or Fertilization):

    1. Habitat Isolation: Species occupy distinct habitats within the same geographic area.

       • Liger Example: Lions (Panthera leo) and tigers (Panthera tigris) do not interbreed in nature because lions inhabit open savannas and tigers inhabit dense forests; ligers exist only in human captivity.

       • Snake Example: In the Southeastern United States, the Florida Cottonmouth (Agkistrodon conanti) is primarily aquatic, whereas the Eastern Copperhead (Agkistrodon contortrix) is terrestrial.

    2. Temporal Isolation: Species breed during different times of the day, different seasons, or different years.

       • Iris Example: The Leopard lily (Iris domestica) opens flowers in the early morning, whereas the Vesper iris (Iris dichotoma) opens flowers exclusively in the late afternoon.

       • Amphibian Example: The Wood frog (Rana sylvatica) reaches peak mating activity in March, whereas the Leopard frog (Rana pipiens) reaches peak mating activity in April.

    3. Behavioral Isolation: Unique courtship rituals, displays, or vocal signals act as species-recognition barriers.

       • Bird Examples: Western and Eastern Meadowlarks possess nearly identical morphology but do not interbreed because they sing distinct mating songs. Blue-footed boobies in the Galapagos perform specific high-stepping foot displays that are not recognized by red-footed or brown boobies.

    4. Mechanical Isolation: Morphological differences prevent successful copulation or pollen transfer.

       • Snail Example: In terrestrial snails (Bradybaena), shell coiling is genetically controlled. Snails with counterclockwise (left-handed/sinistral) spiral shells cannot align their genital openings with snails possessing clockwise (right-handed/dextral) spiral shells.

    5. Gametic Isolation: Sperm of one species is unable to fertilize the eggs of another species due to biochemical incompatibility.

       • Urchin Example: Red and purple sea urchins release gametes into water simultaneously, but species-specific cell-surface receptor proteins prevent cross-fertilization.

  • Postzygotic Barriers (Prevent Hybrid Zygotes from Developing into Viable, Fertile Adults):

    1. Reduced Hybrid Viability: Genes of different parent species interact adversely, impairing hybrid development or survival in the environment.

    2. Reduced Hybrid Fertility: Hybrid individuals develop into healthy adults but are completely sterile due to chromosomal mismatches during meiosis.

       • Equine Example: Mating a male donkey ($2n = 62$) with a female horse ($2n = 64$) produces a robust but sterile mule ($2n = 63$).

    3. Hybrid Breakdown: First-generation ($F_1$) hybrids are viable and fertile, but when they mate with one another or with parent species, offspring in the $F_2$ generation are feeble or sterile (e.g., specific cultivated rice strains).

• Limitations of the Biological Species Concept:

  • Cannot be applied to evaluate asexual organisms (e.g., all prokaryotes and obligate asexual eukaryotes).

  • Cannot be applied to extinct fossil organisms.

  • Struggles with species that remain morphologically and ecologically distinct despite ongoing gene flow (e.g., Grizzly bears, Ursus arctos, and Polar bears, Ursus maritimus, can cross-breed in nature to produce fertile "grolar bear" hybrids).

Geographical & Biological Modes of Speciation

• Allopatric Speciation:

  • Speciation that occurs when populations become geographically isolated from one another by a physical barrier (e.g., mountain ranges, rivers, land bridges, or glaciers), interrupting gene flow.

  • Porkfish Example: The formation of the Isthmus of Panama approximately $2.7\text{--}3.5\text{ million years ago}$ separated continuous marine populations, leading to allopatric speciation into the Panamic porkfish (Anisotremus taeniatus) in the Pacific Ocean and the Porkfish (Anisotremus virginicus) in the Caribbean Sea.

  • Tamarin Monkey Example: Along the Amazon River, populations of Tamarin monkeys (Saguinus) separated by wide river tributaries cannot cross, leading to reproductive isolation and genetic divergence. Along narrow headwater tributaries, monkeys swim across or cross branches, maintaining gene flow.

• Sympatric Speciation:

  • Speciation that takes place in geographic populations that overlap spatially, occurring without geographic physical separation.

  • Mechanisms Driving Sympatric Speciation:

    1. Habitat Differentiation: A subpopulation begins utilizing a habitat, host plant, or resource not used by the parent population (e.g., Apple maggot fly, Rhagoletis pomonella, shifting from native hawthorn trees to introduced domestic apple trees).

    2. Sexual Selection: Mate choice isolates subpopulations (e.g., Cichlids in Lake Victoria: Pundamilia pundamilia females prefer blue males, whereas Pundamilia nyererei females prefer red males under clear light conditions).

    3. Polyploidy: The presence of extra sets of chromosomes due to errors during cell division. Common in plants.

       • Autopolyploidy: An individual possessing more than two chromosome sets derived entirely from a single species ($2n = 6 \rightarrow 4n = 12$ tetraploid). Tetraploids are reproductively isolated from diploids because triploid ($3n$) offspring have reduced fertility.

        *   **Allopolyploidy:** Occurs when two different species interbreed to produce a hybrid. Although the hybrid is usually sterile, mitotic or meiotic non-disjunction can double chromosome numbers ($2n_1 + 2n_2$), transforming a sterile hybrid into a fertile, self-compatible allopolyploid species.
        *   *Tragopogon Example:* European goatsbeard weeds (*Tragopogon dubius*, $2n = 12$; *T. pratensis*, $2n = 12$; *T. porrifolius*, $2n = 12$) introduced to North America formed sympatric allopolyploid species: *Tragopogon miscellus* ($2n = 24$) and *Tragopogon mirus* ($2n = 24$).

• Hybrid Zones & Outcomes:

  • Hybrid Zone: A geographic region in which members of different species meet, mate, and produce hybrid offspring.

  • Three Outcomes of Secondary Contact in Hybrid Zones:

    1. Reinforcement: Natural selection strengthens prezygotic reproductive barriers over time because hybrid offspring have lower fitness than parental species.

    2. Fusion: Reproductive barriers weaken, gene flow increases, and the two hybridizing species fuse back into a single species.

    3. Stability: The hybrid zone remains stable, with continued production of hybrid individuals over time (e.g., hybrid zone between Black-capped chickadees, Poecile atricapillus, and Carolina chickadees, Poecile carolinensis).

Adaptive Radiation & Evolutionary Dynamics

• Adaptive Radiation:

  • Periods of rapid evolutionary change during which groups of organisms form many new species whose adaptations allow them to fill distinct ecological roles (niches) within their communities.

  • Requires both speciation events and ecological adaptation.

  • Most common in newly formed, uncolonized, or ecologically depauperate environments (e.g., oceanic island archipelagos or post-mass-extinction ecosystems).

• Key Innovations:

  • The evolution of a novel structural, physiological, or functional trait that allows a lineage to exploit previously inaccessible resources, conferring a major selective advantage.

  • Lake Victoria Cichlid Jaws: Cichlid fish evolved a modified second set of pharyngeal jaws in their throat. This key innovation allowed species to specialize in diverse diets (e.g., fish eaters, snail crushers, algae scrapers, zooplankton strainers, leaf cutters, insect larvae eaters), fueling adaptive radiation into hundreds of endemic species.

• Character Displacement:

  • The tendency for phenotypic traits to be more divergent in sympatric populations of two species than in allopatric populations of the same two species due to natural selection favoring reduced competitive overlap.

• Classic Examples of Adaptive Radiation:

  • Galapagos Finches: Diverged from an ancestral seed-eating ground finch into ground finches (Geospiza), tree finches (Camarhynchus), warbler finches (Certhidea), and Cocos Island finches (Pinaroloxias), specializing in seed crushing, cactus feeding, insect catching, and bud eating.

*   **Hawaiian Silversword Alliance:** Over 30 species of plants (e.g., *Dubautia laxa*, *Dubautia waialealae*, *Argyroxiphium sandwicense*) adapted to rainforests, alpine deserts, and lava fields, all descended from a single ancestral North American tarweed (*Carlquistia muirii*).

• Mass Extinctions:

  • Global environmental crises cause extinction rates to spike far above background levels, destroying entire ecological communities.

  • The fossil record reveals five major mass extinction events (including the Permian mass extinction at $252\,mya$ and the Cretaceous mass extinction at $66\,mya$).

  • Mass extinctions remove dominant species, opening ecological niches that trigger widespread adaptive radiations of surviving lineages.

• Patterns of Macroevolution:

  • Gradualism: Species diverge slowly and continuously over long geological periods.

  • Punctuated Equilibrium: Species undergo long periods of apparent morphological stasis punctuated by brief episodes of rapid evolutionary change during speciation.

History of Life on Earth & Prebiotic Synthesis

• Early Earth Conditions:

  • The Earth formed approximately $4.6\text{ billion years ago}$, with the first life emerging between $4.2$ and $3.55\text{ billion years ago}$.

  • The early atmosphere was a reducing atmosphere composed of carbon dioxide ($\text{CO}_2$), nitrogen ($\text{N}_2$), water vapor ($\text{H}_2\text{O}$), hydrogen ($\text{H}_2$), carbon monoxide ($\text{CO}$), methane ($\text{CH}_4$), and ammonia ($\text{NH}_3$), with virtually no free molecular oxygen ($\text{O}_2$).

• The Miller-Urey Experiment (1953):

  • Stanley Miller and Harold Urey constructed an apparatus simulating prebiotic Earth conditions.

  • They heated water (simulating primordial oceans) to produce steam, passed it through a reducing gas atmosphere ($\text{H}_2\text{O}, \text{N}_2, \text{NH}_3, \text{CO}_2, \text{CO}, \text{CH}_4, \text{H}_2$), exposed the mixture to continuous electrical sparks (simulating lightning strikes), and condensed the resulting liquid.

  • Within one week, the apparatus synthesized organic compounds, including formaldehyde, hydrogen cyanide, and multiple amino acids.

  • Re-analysis of preserved 1953 sample vials in 2008 revealed that prebiotic chemistry produced an even wider array of amino acids than originally reported.

• Protocells & Vesicle Formation:

  • Abiotic precursor molecules self-assemble into fluid-filled vesicles bounded by a membrane-like lipid bilayer.

  • Adding precursor montmorillonite clay (a volcanic ash clay) greatly accelerates the rate of vesicle self-assembly.

  • Vesicles exhibit basic metabolic properties: self-assembly, swelling, division (reproduction), and selective absorption of RNA molecules.

• The RNA World Hypothesis:

  • RNA, rather than DNA, was likely the first genetic material.

  • RNA molecules called ribozymes possess catalytic properties capable of making complementary copies of short stretches of RNA and catalyzing biological reactions.

  • Natural selection acted on early self-replicating RNA strands: molecules that folded into stable configurations or replicated faster left more descendant RNA, establishing an early "RNA world" prior to the evolution of DNA.

Systematics, Phylogenetics, & Constructing Trees of Life

• Taxonomic Hierarchy (Linnaean Classification):

  • Organisms are classified into nested, hierarchical categories based on shared characteristics.

  • Hierarchy (from broadest to most specific): Domain $\rightarrow$ Kingdom $\rightarrow$ Phylum $\rightarrow$ Class $\rightarrow$ Order $\rightarrow$ Family $\rightarrow$ Genus $\rightarrow$ Species.

  • Three Domains: Bacteria, Archaea, and Eukarya.

  • Six Kingdoms: Bacteria, Archaebacteria, Protista, Plantae, Fungi, Animalia.

• Key Vocabulary in Systematics:

  • Systematics: The scientific study of biological diversity and the evolutionary relationships among organisms.

  • Phylogeny: The evolutionary history of a species or group of related species, represented as a branching tree diagram (cladogram/phylogenetic tree).

  • Derived Trait (Apomorphy): A trait that evolved in the most recent common ancestor of an entire group, distinguishing it from ancestral lineages.

  • Synapomorphy: A shared, derived trait present in an ancestral species and all its descendants; used to construct cladograms.

  • Ancestral Trait (Plesiomorphy / Symplesiomorphy): A shared trait that originated prior to the most recent common ancestor of the group.

  • Homoplasy: A shared structural feature that arose independently through convergent evolution or evolutionary reversal rather than shared ancestry.

  • Homologous Trait: A structure derived from a shared common ancestor regardless of its modern function.

• Taxonomic Groupings:

  • Monophyletic Group (Clade): Consists of an ancestral species and all of its descendants.

  • Paraphyletic Group: Consists of an ancestral species and some, but not all, of its descendants (e.g., traditional class Reptilia, which excludes birds).

  • Polyphyletic Group: Consists of distantly related species but excludes their most recent common ancestor.

• Tree Construction Principles:

  • Maximum Parsimony: Assumes that the phylogenetic tree requiring the fewest evolutionary events (the fewest evolutionary changes or appearances of shared derived characters) is the most likely hypothesis.

  • Maximum Likelihood: Identifies the tree that is most likely to have produced a given set of DNA sequence data, based on statistical models of evolutionary rates.

  • Phylogenetic Bracketing: A technique used to predict features of extinct ancestors by evaluating shared traits among their living descendants (e.g., inferring parental care, egg brooding, and vocalization in extinct dinosaurs based on shared traits in living crocodilians and birds).

• Gene Trees vs. Species Trees:

  • Orthologous Genes: Homologous genes found in different species resulting from a speciation event. They diverge only after speciation occurs.

  • Paralogous Genes: Homologous genes resulting from gene duplication within a single genome/species. They evolve new functions within a species.

• Horizontal Gene Transfer & The Ring of Life:

  • Horizontal Gene Transfer (HGT): The primary transfer of genetic material between distantly related organisms across domain boundaries (e.g., via viral infection, plasmid exchange, or endosymbiosis).

  • Ring of Life Hypothesis: Suggests that eukaryotes arose through an endosymbiotic fusion between an ancestral bacterium and an archaean. Consequently, the base of the Tree of Life may be better represented as an interconnected Ring of Life.