Natural selection Study Guide

📖 Core Concepts Natural Selection – Differential survival & reproduction of individuals because of fitness differences tied to observable traits. Fitness – Expected reproductive success of an individual (or genotype) measured by number of viable offspring, not lifespan. Heritable Variation – Genetic differences (mutations, recombination, chromosomal changes) that can be passed to offspring and affect fitness. Selection Types – Stabilizing, directional, disruptive (by trait effect); purifying vs. balancing (by genetic diversity); viablity vs. fecundity (by life‑stage). Units of Selection – Gene, individual, group – each level can be the target of selection under different scenarios. Sexual vs. Ecological Selection – Competition for mates (sexual) vs. competition for resources (ecological). Speciation – Requires reproductive isolation; allopatric separation and Bateson‑Dobzhansky‑Muller incompatibilities are classic mechanisms. --- 📌 Must Remember Fitness (w): \( w = \frac{\text{average number of offspring of genotype}}{\text{population mean offspring number}} \). Selection coefficient (s): change in fitness of a genotype, \( w = 1 - s \). Directional selection shifts the population mean toward the favored extreme. Stabilizing selection reduces variance around the optimum; eliminates extremes. Disruptive selection favors both extremes → can split a population. Balancing selection (e.g., heterozygote advantage) keeps multiple alleles at intermediate frequencies. r‑selection: many offspring, low parental care; K‑selection: few offspring, high care. Fisherian runaway: positive feedback between trait expression and mate preference → exaggerated traits. Selective sweep: rapid fixation of a beneficial allele drags linked neutral alleles (genetic hitchhiking). Mutation–selection balance: equilibrium frequency of deleterious allele \( q \approx \sqrt{\frac{\mu}{s}} \) (where \( \mu \) = mutation rate). Neutral theory: most molecular variation is neutral; drift dominates allele frequency changes. --- 🔄 Key Processes Variation Generation – Mutation + recombination → new alleles. Inheritance – Alleles transmitted from parents to offspring (Mendelian segregation). Struggle for Existence – Competition/cooperation for limited resources creates differential survival. Differential Reproduction – Individuals with higher fitness produce more offspring → allele frequency change. Fixation or Loss – Beneficial alleles increase (possible sweep); deleterious alleles are removed (purifying selection). Sexual selection workflow Trait display (male) → Female preference → Higher mating success → Trait frequency rises → Potential runaway. Speciation via allopatry Geographic split → Independent mutations → Accumulating incompatibilities → Reproductive isolation when contact re‑occurs. --- 🔍 Key Comparisons Natural vs. Artificial Selection – Natural: no intentional direction; driven by environment. Artificial: human‑directed breeding. r‑selection vs. K‑selection – r: high fecundity, low survival; K: low fecundity, high parental investment. Stabilizing vs. Directional vs. Disruptive – Stabilizing: keeps trait near optimum; Directional: pushes mean one way; Disruptive: favors extremes, can split population. Gene vs. Individual vs. Group Selection – Gene: alleles act to increase own transmission; Individual: whole organism fitness; Group: traits beneficial to the group’s survival (rare). Sexual vs. Ecological Selection – Sexual: mate competition/choice; Ecological: resources, predation, climate. --- ⚠️ Common Misunderstandings “Survival of the fittest” = strongest – Fitness is reproductive output, not physical strength. All mutations are harmful – Most are neutral or have very small effects; some are beneficial. Stabilizing selection eliminates variation – It removes only extreme variants; the population still retains genetic variation around the optimum. Speciation always needs geographic isolation – Sympatric and parapatric speciation also occur (e.g., strong disruptive selection). Balancing selection only means heterozygote advantage – Frequency‑dependent selection is another major mechanism. --- 🧠 Mental Models / Intuition “Fitness Landscape” – Imagine a 3‑D hill where height = fitness; natural selection pushes the population uphill (toward peaks). “Arms Race” – Two climbers (e.g., bacteria vs. antibiotics) constantly pulling the rope higher; each adaptation spurs a counter‑adaptation. “Runaway Train” – In Fisherian sexual selection, a trait and the preference for it are coupled; once the train gains speed, it’s hard to stop. “Genetic Hitchhiking” – Like a car (neutral allele) caught in a fast‑moving traffic jam (beneficial sweep) that carries it along. --- 🚩 Exceptions & Edge Cases Gene‑level selection can dominate when kin selection or selfish genetic elements (e.g., meiotic drive) act. Balancing selection may maintain deleterious alleles (e.g., sickle‑cell) because heterozygotes have a net advantage. Selective sweeps can be “soft” (multiple beneficial mutations arise) rather than a single hard sweep. r/K selection is a spectrum, not a strict dichotomy; many species show mixed strategies. Sexual dimorphism can evolve without strong sexual selection if ecological niches differ between sexes. --- 📍 When to Use Which Predicting trait change → Use directional or disruptive selection models based on environmental pressure. Estimating allele frequency under deleterious load → Apply mutation–selection balance formula. Assessing genetic diversity patterns → Look for signatures of balancing selection (high heterozygosity) vs. purifying selection (low variation). Identifying cause of rapid trait evolution → Consider sexual selection (runaway) or arms‑race dynamics. Choosing a speciation explanation → If geographic barrier present → allopatric; if strong disruptive selection without barrier → sympatric (Bateson‑Dobzhansky‑Muller). --- 👀 Patterns to Recognize High variance + extreme phenotypes → possible disruptive selection. Stable mean, reduced variance → stabilizing selection. Allele frequency spikes + surrounding low diversity → recent selective sweep. Maintenance of multiple alleles at intermediate frequencies → balancing selection (heterozygote advantage or frequency‑dependence). Rapid trait exaggeration correlated with mate preference → Fisherian runaway sexual selection. --- 🗂️ Exam Traps Confusing “survival” with “fitness” – Remember fitness = reproductive success, not just longevity. Assuming all selection is natural – Questions may ask you to identify artificial or sexual selection cues. Mixing up r‑ vs. K‑selected traits – r = many, cheap offspring; K = few, costly offspring. Attributing every high‑frequency allele to positive selection – Could be a neutral drift effect or linked hitchhiking. Thinking balancing selection only involves heterozygote advantage – Frequency‑dependent selection is equally valid. Believing speciation always requires physical separation – Look for evidence of reproductive isolation mechanisms instead. ---