Introduction to Evolution & Population Genetics

Historical Development of Evolutionary Thought

• Carolus Linnaeus (1707-1778): Proposed that life originated on a tropical mountain 6,000 years ago; created the binomial nomenclature ($\text{Genus species}$) hierarchical classification system.
• Georges-Louis Leclerc, Comte de Buffon (1707-1788): Formulated Buffon's Law, observing that environmentally similar but geographically isolated regions possess distinct species of birds and mammals.
• Jean-Baptiste Lamarck: Hypothesized that organisms evolve upward toward human perfection through the inheritance of acquired characteristics.
• Alexander von Humboldt (1769-1859): Documented elevational zones and floristic belts in the Andes, pioneering the study of geographical and climatic influences on vegetation.
• Charles Lyell (1797-1875): "Father of Geology" who established that slow, continuous geological processes (uplift and erosion) profoundly change Earth over long timescales.
• Erasmus Darwin: Formulated early ideas that life descended from a single common ancestor and recognized artificial selection in animal breeding.
• Charles Darwin & Alfred Russel Wallace: Independently derived the theory of evolution via natural selection; jointly presented their findings in 1858 prior to Darwin's publication of On the Origin of Species in 1859.

Stages and Hypotheses of the Origin of Life

• Four Sequential Stages:
  1. Stage 1: Abiotic synthesis and accumulation of organic monomers (nucleotides and amino acids) forming a "prebiotic soup".
  2. Stage 2: Polymerization of monomers on clay soil surfaces to form DNA, RNA, and proteins.
  3. Stage 3: Enclosure of polymers within boundary membranes, forming protobionts (such as coacervates or liposomes) capable of self-replication.
  4. Stage 4: Emergence of chemical selection resulting in an RNA world, which later transitioned to a system using DNA for information storage and proteins for catalysis.
• Origin Hypotheses:
  • Reducing Atmosphere Hypothesis: Tested by Stanley Miller and Harold Urey (1953), who used electrical sparks to simulate lightning in a reducing mixture of $\text{CH}_4$, $\text{NH}_3$, $\text{H}_2$, and $\text{H}_2\text{O}$, producing amino acids.
  • Extraterrestrial Hypothesis: Assumes meteorites and comets (e.g., Comet 67P/Churyumov-Gerasimenko analyzed by the Rosetta/Philae mission) delivered organic molecules and water to Earth.
  • Deep-Sea Vent Hypothesis: Proposes organic synthesis occurred in thermal gradients between hot hydrothermal vent fluids ($\text{H}_2\text{S}$) and cold ocean water.

Geologic Timeline and Evolutionary Milestones

• Geologic Eons:
  • Hadean ($>4.0\,\text{BYA}$): High temperatures, frequent cosmic impacts, lack of free $\text{O}_2$.
  • Archaean ($4.0\text{--}2.5\,\text{BYA}$): Emergence of diverse anaerobic prokaryotic life ($3.8\text{--}3.5\,\text{BYA}$); cyanobacteria in stromatolites ($3.5\,\text{BYA}$) initiated atmospheric oxygenation.
  • Proterozoic ($2.5\,\text{BYA}\text{--}543\,\text{MYA}$): Appearance of unicellular eukaryotic cells ($1.8\,\text{BYA}$) via endosymbiosis, followed by multicellular eukaryotes ($1.5\,\text{BYA}$) and early invertebrates ($632\,\text{MYA}$).
  • Phanerozoic ($543\,\text{MYA}\text{--}\text{present}$): Comprises Paleozoic, Mesozoic, and Cenozoic eras, marked by major adaptive radiations and extinctions.

• Major Evolutionary Events:
  • Cambrian Explosion ($533\text{--}525\,\text{MYA}$): Rapid appearance of diverse animal body plans and first vertebrates ($520\,\text{MYA}$).
  • Land Colonization ($440\,\text{MYA}$): Plants and arthropods established terrestrial populations; seed plants and tetrapods appeared around $400\,\text{MYA}$.
  • Mesozoic Diversification: Dinosaurs and mammals appeared ($225\text{--}200\,\text{MYA}$), birds evolved ($160\,\text{MYA}$), and flowering plants arose ($135\,\text{MYA}$).
  • Hominid Origin: First hominids emerged approximately $7\,\text{MYA}$.
• Geologic Dating:
  • Radiocarbon (${}^{14}\text{C}$) Dating: Measures decay of ${}^{14}\text{C}$ to ${}^{14}\text{N}$ (half-life = $5,730\,\text{years}$) for organic material up to $50,000\text{--}60,000\,\text{years}$ old.
  • Long-term Radiometric Dating: Uses isotopes such as ${}^{40}\text{K}\rightarrow{}^{40}\text{Ar}$ (half-life = $1.3\,\text{billion years}$) and ${}^{238}\text{U}\rightarrow{}^{206}\text{Pb}$ (half-life = $4.5\,\text{billion years}$).
• Mass Extinctions: Five major mass extinction events recorded in Earth's history; the Cretaceous-Paleogene (K-Pg / K-T) boundary extinction ($65.5\,\text{MYA}$) was triggered by a $\approx 10\,\text{km}$ asteroid impact at the Chicxulub crater in Yucatán, Mexico.

Evidence of Biological Evolution

• Direct Observation: Real-time adaptive changes recorded in natural populations, such as Peter and Rosemary Grant's documented increase in Geospiza fortis beak depth on Daphne Major following the 1977 drought.
• Fossil Record: Sequential anatomical transitions shown in rock strata, including transitional forms like Tiktaalik roseae (linking fish to tetrapods) and structural shifts in equines adapting from forests to grasslands.

• Biogeography: Isolated island geography leads to endemic species differentiation (e.g., Island fox Urocyon littoralis evolving from the mainland gray fox Urocyon cinereoargenteus).
• Convergent Evolution: Distantly related lineages independently develop analogous traits due to similar ecological niches (e.g., elongated snouts and sticky tongues in giant anteaters and echidnas).
• Selective Breeding: Artificial selection by humans demonstrates rapid phenotypic transformation in domesticated animals and plants.
• Homologies:
  • Anatomical Homology: Shared structural frameworks derived from a common ancestor, such as the pentadactyl limb plan in mammals, reptiles, and birds.
  • Developmental Homology: Transient embryonic traits reflecting shared ancestry, such as pharyngeal gill ridges and bony tails in human embryos.
  • Molecular Homology: Universally shared biochemical mechanisms, cell structures, and conserved gene/protein sequences (e.g., $p53$ gene).
• Vestigial Structures: Anatomical remnants retaining little or no ancestral function (e.g., human coccyx, boa constrictor pelvic spurs, whale pelvic remnants).

Microevolution and Population Genetics

• Definition: Microevolution involves generation-to-generation changes in single gene allele frequencies within a population.
• Genetic Variation Sources: Mutations (e.g., single nucleotide polymorphisms or SNPs), gene duplications, exon shuffling, and horizontal gene transfer.
• Key Processes:
  • Natural Selection: Differential survival and reproduction of individuals possessing advantageous heritable traits.
  • Genetic Drift: Random fluctuations in allele frequencies that disproportionately affect small populations; includes the bottleneck effect (drastic population reduction) and founder effect (colonization by a small subgroup).
  • Gene Flow / Migration: Exchange of alleles between populations, reducing inter-population divergence while boosting local genetic diversity.
  • Nonrandom Mating: Selection of mates based on phenotype or genetic relation; shifts genotype frequencies without altering allele frequencies.
• Neutral Theory of Evolution: Assumes most genetic variation at the molecular level results from neutral mutations accumulating via genetic drift rather than directional natural selection.

Hardy-Weinberg Equilibrium

• Concept: A mathematical model describing a non-evolving population in which allele and genotype frequencies remain constant across generations.
• Equations:
  • Allele Frequencies:     $p + q = 1.0$
  • Genotype Frequencies:     $p^2 + 2pq + q^2 = 1.0$
  • Variable Definitions: $p$ = frequency of the dominant allele; $q$ = frequency of the recessive allele; $p^2$ = homozygous dominant genotype frequency; $2pq$ = heterozygous genotype frequency; $q^2$ = homozygous recessive genotype frequency.

Patterns of Natural and Sexual Selection

• Directional Selection: Shifts the phenotypic distribution toward one extreme (e.g., dark fur coloration in rock pocket mice inhabiting volcanic basalt).
• Stabilizing Selection: Favors intermediate phenotypes and reduces extremes (e.g., human birth weight, egg clutch size).
• Diversifying / Disruptive Selection: Favors two or more distinct extreme phenotypes in heterogeneous environments (e.g., metal tolerance in Agrostis capillaris).
• Balancing Selection: Maintains multiple alleles over time through mechanisms like heterozygote advantage (e.g., sickle-cell allele maintaining protection against malaria).
• Sexual Selection:
  • Intrasexual Selection: Direct competition among members of one sex (typically males) for access to mates or territories.
  • Intersexual Selection: Mate choice exerted by one sex (typically females), promoting elaborate display traits in males.

Speciation and Macroevolution

• Definition: Macroevolution refers to broader evolutionary changes that produce entire new species or higher taxonomic groups.
• Species Concepts:
  • Biological Species Concept: Defines a species as a group of interbreeding natural populations that are reproductively isolated from other groups.
  • Morphological Species Concept: Classifies species based on distinct, measurable structural traits with a heritable genetic basis.
  • Evolutionary Species Concept: Defines a species as a lineage that maintains its distinct identity and independent historical trajectory over time.
• Modes of Speciation:
  • Allopatric Speciation: Occurs when populations become geographically isolated from one another.
  • Sympatric Speciation: Occurs without geographic separation, often driven by adaptive radiation into vacant ecological niches (e.g., East African cichlids in Lake Malawi, Hawaiian honeycreepers).