Mechanisms of Evolution

Evolution: change in allele frequencies in a population over generations. Natural selection (Darwin, 1859): differential survival and reproduction based on heritable variation — the ONLY mechanism that produces adaptation. Types: directional (shifts mean), stabilizing (reduces variance), disruptive (favors extremes), sexual selection (mate choice and competition). Genetic drift: random changes in allele frequencies — stronger in small populations. Founder effect: small group colonizes new area with reduced genetic diversity. Bottleneck effect: population crash eliminates alleles. Gene flow: migration of alleles between populations — homogenizes. Mutation: ultimate source of genetic variation — typically 10⁻⁸ to 10⁻⁹ per bp per generation in humans. Most mutations neutral; beneficial mutations rare but drive adaptation. Non-random mating: assortative mating (like with like) changes genotype frequencies without changing allele frequencies. Hardy-Weinberg equilibrium: p² + 2pq + q² = 1 — baseline when no evolution occurs (requires large population, random mating, no selection/mutation/migration).

Speciation and Macroevolution

Species concepts: biological (Mayr — reproductive isolation), phylogenetic (distinct lineage), morphological (distinct features). Allopatric speciation: geographic barrier separates populations → independent evolution → reproductive isolation. Example: Darwin's finches on Galápagos Islands. Sympatric speciation: new species arise without geographic separation — polyploidy in plants (instant speciation via genome doubling), host race formation in insects. Parapatric speciation: adjacent populations with partial barrier — hybrid zones. Ring species: chain of populations around a barrier — adjacent populations interbreed, but endpoints cannot. Reproductive isolation: prezygotic (habitat, temporal, behavioral, mechanical, gametic) and postzygotic (hybrid inviability, sterility, breakdown). Adaptive radiation: rapid diversification into diverse ecological niches — cichlid fishes (700+ species in Lake Victoria in <15,000 years), Hawaiian silverswords. Mass extinctions: "Big Five" — Ordovician, Devonian, Permian (95% species), Triassic, Cretaceous (dinosaurs). Current: 6th mass extinction — extinction rate 100-1,000× background. Macroevolutionary patterns: punctuated equilibrium (Gould & Eldredge) vs. gradualism, convergent evolution, coevolution.

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Molecular Evolution and Evo-Devo

Molecular clock: mutations accumulate at roughly constant rate → calibrate divergence times. Human-chimpanzee divergence: ~6-7 million years ago (1.2% DNA sequence difference). Neutral theory (Kimura, 1968): most molecular evolution is driven by genetic drift of neutral mutations, not natural selection. Nearly neutral theory (Ohta): slightly deleterious mutations also important, especially in small populations. Molecular phylogenetics: DNA/protein sequences reconstruct evolutionary relationships — 16S rRNA (prokaryotes), cytochrome c, mitochondrial DNA. Horizontal gene transfer (HGT): widespread in prokaryotes — complicates tree-of-life (more like a "web of life"). Evo-devo (evolutionary developmental biology): how changes in developmental genes produce morphological evolution. Hox genes: master regulators of body plan — conserved across animals from flies to humans. Mutations in regulatory elements (enhancers, promoters) drive morphological evolution more than changes in protein-coding genes. Deep homology: ancient regulatory circuits shared across distantly related organisms — eyes, limbs, hearts use similar genetic toolkits despite independent origins. Modularity: semi-independent body parts can evolve independently — basis of evolutionary innovation. Gene duplication (Ohno, 1970): creates raw material for evolution — one copy maintains function, the other can diverge.

Human Evolution

Hominid timeline: Sahelanthropus tchadensis (~7 Mya) → Ardipithecus (~4.4 Mya) → Australopithecus afarensis (Lucy, ~3.2 Mya) → Homo habilis (~2.5 Mya) → Homo erectus (~1.9 Mya — first out of Africa) → Homo heidelbergensis (~600 Kya) → Homo sapiens (~300 Kya, Jebel Irhoud, Morocco). Key adaptations: bipedalism (oldest evidence ~6-7 Mya), stone tools (~3.3 Mya, Lomekwi), brain expansion (400 cm³ → 1,400 cm³ over 3 million years), language (debated timing, ~100-300 Kya). Out of Africa: modern humans migrated from Africa ~70-100 Kya, replacing or absorbing archaic populations. Interbreeding: 1-4% Neanderthal DNA in non-African populations, up to 6% Denisovan DNA in Melanesians (Svante Pääbo, Nobel 2022). Neanderthals (Homo neanderthalensis): lived 400-40 Kya in Europe and western Asia, made tools, buried dead, possibly had language. Denisovans: known primarily from DNA — a finger bone in Denisova Cave (Siberia) revealed a distinct hominin group. Recent evolution in humans: lactase persistence (LP, ~8,000 years ago), malaria resistance (sickle cell HbS), high-altitude adaptation (Tibetans — EPAS1 gene from Denisovans). Human genetic diversity: greatest in Africa (source population), decreases with distance from Africa (serial founder effects).

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Evolutionary Biology: The History and Mechanisms of Life's Diversity