Evolution Study Guide

📖 Core Concepts Evolution – Change in heritable traits of populations over generations; driven by genetic variation. Genotype vs. Phenotype – Genotype = full DNA makeup; phenotype = observable traits after genotype interacts with environment. Fitness – Expected contribution of an individual’s genes to future generations (proportion of gene copies passed on). Natural Selection – Differential survival/reproduction of individuals due to trait differences; sorts existing variation. Genetic Drift – Random changes in allele frequencies, strongest in small effective populations. Gene Flow (Migration) – Transfer of alleles between populations, homogenizing genetic differences. Mutation – Ultimate source of new alleles; can be point, duplication, or larger chromosomal changes. Speciation – Process where one lineage splits into two reproductively isolated lineages (allopatric, peripatric, parapatric, sympatric). 📌 Must Remember Four evolutionary mechanisms: mutation, gene flow, genetic drift, natural selection. Types of selection: Directional → shift trait mean. Disruptive → favor extremes, create two peaks. Stabilizing → favor intermediate, reduce variance. Effective population size (Ne) determines drift speed; smaller Ne = faster drift. Hitchhiking (selective sweep) – Linked neutral alleles rise with a beneficial allele. Adaptation vs. Exaptation – Adaptation = trait shaped by current function; exaptation = trait co‑opted for a new function. Biological Species Concept – Groups that can interbreed and are reproductively isolated from others. Common descent – All life shares a last universal common ancestor (3.5–3.8 Ga). 🔄 Key Processes Mutation → Allele Generation DNA replication error → point mutation or duplication → new allele at a locus. Sexual Reproduction & Recombination Independent assortment + homologous recombination → random allele combos in gametes. Natural Selection Cycle Variation → Differential fitness → Change in allele frequencies → New variation (via mutation, recombination). Speciation (Allopatric) Geographic isolation → independent drift/selection → reproductive barriers → two species. Genetic Hitchhiking Beneficial allele ↑ → nearby linked alleles ↑ (low recombination) → possible fixation of neutral/deleterious alleles. 🔍 Key Comparisons Directional vs. Stabilizing vs. Disruptive Selection Directional: ↑ trait mean (e.g., larger beaks). Stabilizing: ♂ trait mean, ↓ variance (e.g., birth weight). Disruptive: ↑ extremes, ↓ intermediates (e.g., beak size in two seed types). Genetic Drift vs. Natural Selection Drift: random, size‑dependent, can fix neutral or deleterious alleles. Selection: non‑random, favors alleles that increase fitness. Allopatric vs. Sympatric Speciation Allopatric: physical barrier separates populations. Sympatric: same area; requires strong assortative mating or niche differentiation. ⚠️ Common Misunderstandings “Evolution = individuals changing” – Evolution acts on populations, not on single organisms. “More offspring = higher fitness” – Fitness is measured by gene transmission to future generations, not raw offspring number. “Genetic drift only occurs in tiny populations” – Drift occurs in all populations; its effect is simply weaker in large ones. “All vestigial structures are useless” – Some retain minor functions or can re‑appear as atavisms. 🧠 Mental Models / Intuition “Evolutionary forces as a tug‑of‑war” – Mutation adds new rope (alleles), selection pulls the rope toward higher fitness, drift nudges the rope randomly, gene flow ties ropes between islands. “Hitchhiking train” – A beneficial allele is the locomotive; nearby neutral alleles are the cars that get taken along the track (chromosome). 🚩 Exceptions & Edge Cases Mutation bias – Certain mutations occur more frequently (e.g., transitions > transversions) and can bias genomic composition even without selection. Gene flow vs. selection – High gene flow can swamp local adaptation; low gene flow can allow divergent selection to drive speciation. Exaptation – Traits originally evolved for one purpose may later be co‑opted (e.g., feathers for thermoregulation → flight). 📍 When to Use Which Predict allele change → Use selection coefficient if selection strong; use drift formulas (e.g., $p{t+1}=pt \pm \frac{1}{2Ne}$) for small populations. Identify cause of trait distribution → Directional selection → shift in mean; stabilizing → narrow distribution; disruptive → bimodal distribution. Choose speciation model – Geographic isolation present → Allopatric; strong hybrid disadvantage without physical barrier → Parapatric; niche/behavioral isolation within same area → Sympatric. 👀 Patterns to Recognize High variance + two peaks → Disruptive selection. Low variance centered on mean → Stabilizing selection. Rapid allele frequency change in small population → Genetic drift dominant. Cluster of linked alleles rising together → Selective sweep/hitchhiking. Presence of homologous structures with different functions → Exaptation or deep homology. 🗂️ Exam Traps “More offspring = higher fitness” – Wrong if offspring have low survival or reproduction. Confusing gene flow with mutation – Gene flow moves existing alleles; mutation creates new ones. Assuming all vestigial structures are non‑functional – Some retain minor roles; atavisms show latent potential. Selecting “genetic drift” for a large, well‑mixed population – Drift effect is minimal; look for selection or gene flow instead. Choosing “allopatric speciation” when no geographic barrier is described – Must pick parapatric or sympatric based on context.