--- title: "Directional Selection — AP Bio Definition & Exam Guide" description: "Directional selection shifts a population's average trait toward one extreme. Learn how it shows up in DDT resistance, peppered moths, and AP Bio Unit 7." canonical: "https://fiveable.me/ap-bio/key-terms/directional-selection" type: "key-term" subject: "AP Biology" --- # Directional Selection — AP Bio Definition & Exam Guide ## Definition Directional selection is a type of natural selection in which individuals at one extreme of a trait's range have higher fitness, shifting the population's average phenotype toward that extreme over generations (AP Bio 7.2). ## What It Is Directional selection is [natural selection](/ap-bio/unit-7/natural-selection/study-guide/Nc1t327OihZEnIVHHYtC "fv-autolink") that pushes a population toward one end of a [trait](/ap-bio/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV "fv-autolink")'s range. Picture a bell curve of trait values, like plant height or beak size. When the environment favors one extreme (say, taller plants or thicker beaks), individuals on that side survive and reproduce more. Generation after generation, the whole curve slides in that direction. The average value of the trait literally moves. This fits straight into EK 7.2.A.1, which says natural selection acts on **[phenotypic variation](/ap-bio/key-terms/phenotypic-variation "fv-autolink")** already present in a population. Selection doesn't create new traits, it just favors some of the variants that already exist. An environment changes (EK 7.2.A.2), that change applies a selective pressure, and the variants that happen to fit best leave more offspring (EK 7.2.A.3). Two classic AP examples are pure directional selection: DDT resistance in insects and dark-colored peppered moths spreading as pollution darkened tree trunks. ## Why It Matters Directional selection lives in [Unit 7](/ap-bio/unit-7 "fv-autolink"): Natural Selection, specifically Topic 7.2. It's the most intuitive demonstration of learning objective [AP Bio](/ap-bio "fv-autolink") 7.2.A: phenotypic variation matters because selection acts on it, and a changing environment (AP Bio 7.2.A.2) can shift which phenotype wins. It also ties to AP Bio 7.2.B, since molecular variation (like an enzyme that breaks down a pesticide) is exactly the kind of variation directional selection can amplify. Understanding this term is how you connect a fuzzy idea like 'evolution happens' to a concrete, measurable prediction: the population mean will move. ## Connections ### Stabilizing and Disruptive Selection (Unit 7) These three are the same idea aimed in different directions. Stabilizing selection favors the middle and squeezes the curve, disruptive selection favors both extremes and splits it, and directional selection favors one extreme and slides the whole curve over. Knowing which pattern a graph shows is a common MCQ skill. ### [Phenotypic Variation (Unit 7)](/ap-bio/key-terms/phenotypic-variation) Directional selection has nothing to act on without preexisting [variation](/ap-bio/unit-6/mutations/study-guide/WIuGA11Yy2RsVq8JpSnt "fv-autolink"). If every plant were the same height, herbivores eating tall plants couldn't shift anything. Variation is the raw material; selection is the editor. ### [Mutation (Unit 6 to Unit 7)](/ap-bio/key-terms/mutation) [Mutation](/ap-bio/key-terms/mutation "fv-autolink") is the original source of new alleles, like the one coding for enhanced cytochrome P450 enzymes that break down DDT. Directional selection then sweeps that rare allele to high frequency. Mutation supplies, selection chooses. ### Adaptive Radiation (Unit 7) When directional selection acts on isolated populations facing different environments, each can shift toward a different optimum, eventually producing many species from one ancestor. The Galápagos finches are the textbook payoff of this process scaled up over time. ## On the AP Exam On MCQs, you'll get a scenario and have to name the selection type. The pattern is always the same: an environment changes, and one extreme of a trait becomes more common. Herbivores eating only the tallest wildflowers, dark moths increasing as pollution darkens trees, mosquitoes with DDT-resistance alleles surging, these all point to directional selection. Watch for the reversal twist too: when air quality improved in England, light moths increased again, which is still directional selection, just aimed the other way. On the 2021 short free-response about Galápagos finches, you'd use directional selection to argue that a changed food supply favored one beak-size extreme and shifted the population mean. Your job is usually to predict the outcome after several generations and explain it using variation plus a selective pressure. ## Directional Selection vs Stabilizing Selection Both shift what's common, but in opposite ways. Directional selection favors ONE extreme and moves the average toward it (taller plants survive, so the population gets taller). Stabilizing selection favors the AVERAGE and selects against both extremes, so the mean stays put and the variation shrinks (think birth weight in humans). If the population's average is moving, it's directional; if it's staying centered and getting narrower, it's stabilizing. ## Key Takeaways - Directional selection shifts a population's average phenotype toward one extreme of the trait's range. - It requires preexisting phenotypic variation to act on; selection edits variation, it doesn't create it (EK 7.2.A.1). - A change in the environment applies the selective pressure that decides which extreme is favored (EK 7.2.A.2). - Classic AP examples are DDT resistance in insects and dark-colored peppered moths in polluted areas. - If the population mean is moving, it's directional selection; if it's holding center and narrowing, it's stabilizing. ## FAQs ### What is directional selection in AP Bio? It's a type of natural selection where individuals at one extreme of a trait (like the tallest plants or fastest-metabolizing insects) have higher fitness, so the population's average trait value shifts toward that extreme over generations. It maps to Topic 7.2 and learning objective AP Bio 7.2.A. ### Is directional selection the same as stabilizing selection? No. Directional selection moves the population average toward one extreme, while stabilizing selection favors the middle value and selects against both extremes, keeping the average the same but reducing variation. The quickest test: if the mean is moving, it's directional. ### Are peppered moths an example of directional selection? Yes. When pollution darkened tree trunks, dark moths were better camouflaged and increased in frequency, shifting the population toward the dark extreme. When air quality laws cleaned up the trees, light moths rebounded, which is directional selection running in the opposite direction. ### Does directional selection create new traits? No. It only favors variation that already exists in the population. New [alleles](/ap-bio/key-terms/allele "fv-autolink") come from mutation; directional selection just increases the frequency of whichever existing variant fits the environment best, like the DDT-resistance allele in mosquitoes. ### How do I tell directional from disruptive selection on the exam? Directional selection favors ONE extreme and slides the whole curve one way. Disruptive selection favors BOTH extremes at once and splits the curve into two peaks while selecting against the average. 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