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Unit 7 · Natural Selection Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Biology Unit 7 Visual Review

A topic-by-topic visual walkthrough of Natural Selection — how selection works, population genetics, Hardy–Weinberg, evidence of evolution, phylogeny, speciation, and the origin of life.

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TOPIC 7.1 Introduction to Natural Selection DARWIN'S FOUR OBSERVATIONS 1 · Variation Individuals in a population differ in their heritable traits. from mutation & meiosis 2 · Overproduction More offspring are born than can possibly survive. resources are limited 3 · Competition Individuals compete for food, mates, and space — a struggle. "struggle for existence" 4 · Survival Those with favorable traits survive & reproduce more. "survival of the fittest" Putting it together If heritable variation affects survival and reproduction, then over generations the favorable traits become MORE common in the population. This is natural selection — the mechanism of evolution Darwin proposed. Two key points that students often miss: • Selection acts on the individual, but populations evolve (individuals don't "choose" to change). • Only heritable traits can be selected — acquired traits (like a scar) are not passed on. Variation + overproduction + competition → the fittest survive and reproduce, so populations evolve. The Review Hub · AP Biology Unit 7 TOPIC 7.2 Natural Selection — Types THREE PATTERNS OF SELECTION Directional favors ONE extreme e.g., peppered moths Stabilizing favors the MIDDLE e.g., human birth weight Disruptive favors BOTH extremes can lead to two forms Fitness = reproductive success "Fitness" doesn't mean strength — it means how many fertile offspring an individual leaves. A trait that raises reproductive success has higher fitness. Adaptations & selective pressure An adaptation is a heritable trait that improves fitness in a given environment. The environment applies the selective pressure — predators, climate, food, disease. Selection can push toward one extreme, the middle, or both ends — always favoring the fittest. The Review Hub · AP Biology Unit 7 TOPIC 7.3 Artificial Selection Humans do the selecting In artificial selection (selective breeding), HUMANS — not the environment — choose which individuals reproduce, based on traits we find desirable. Over generations, those traits become exaggerated in the population. Examples: dog breeds from wolves, corn from teosinte, cattle bred for milk, and hundreds of crop varieties (broccoli, cabbage, kale, and cauliflower all descend from one wild mustard plant). Same mechanism, different selector The underlying process is identical to natural selection — heritable variation + differential reproduction. Only the source of the "pressure" differs (humans vs. nature). Why it's powerful evidence Darwin used artificial selection as evidence: if humans can reshape a species in just a few generations, nature — acting over millions of years — can produce all of life's diversity. A caution Breeding for one trait can reduce genetic diversity and accidentally concentrate harmful alleles — which is why many purebred animals suffer from inherited health problems. Artificial selection is natural selection with a human selector — proof that selection reshapes species. The Review Hub · AP Biology Unit 7 TOPIC 7.4 Population Genetics Evolution acts on populations An individual can't evolve — a POPULATION does, over generations. The gene pool is all the alleles of all the individuals in a population. We track evolution by measuring allele frequencies. Microevolution = change in allele frequency If the frequency of an allele shifts from one generation to the next, the population has evolved. That's the modern, genetic definition of evolution. Allele frequency = how common an allele is (as a fraction). Five mechanisms that change allele frequencies 1 · Natural selection — favorable alleles increase 2 · Mutation — introduces brand-new alleles 3 · Gene flow — migration moves alleles between populations 4 · Genetic drift — random chance (strong in small populations) 5 · Non-random mating — mate choice skews genotypes If NONE of these occur, allele frequencies stay constant — the Hardy–Weinberg baseline (Topic 7.5). Selection is the only one that reliably produces adaptation; the others change frequencies without "improving" fitness. Drift & gene flow are random with respect to fitness. Evolution is a change in allele frequencies in a population's gene pool over time. The Review Hub · AP Biology Unit 7 TOPIC 7.5 Hardy–Weinberg Equilibrium The two equations p + q = 1 p² + 2pq + q² = 1 What each term means p = frequency of the dominant allele q = frequency of the recessive allele p² = homozygous dominant · q² = homozygous recessive 2pq = heterozygous Tip: start from q² (the recessive phenotype) to find q. A null model: the 5 conditions for NO evolution Hardy–Weinberg describes a population that is NOT evolving. Allele frequencies stay constant only if ALL five hold: 1 · No natural selection 2 · No mutation 3 · No gene flow (no migration) 4 · No genetic drift (very large population) 5 · Random mating Real populations rarely meet all five — so the equations are a BASELINE. If the observed genotype frequencies differ from the predicted p², 2pq, q², then the population is evolving, and you can investigate why. Remember: p and q always add to 1, and the genotype frequencies always add to 1. p² + 2pq + q² = 1 is the "no evolution" baseline — deviations from it reveal that a population is evolving. The Review Hub · AP Biology Unit 7 TOPIC 7.6 Evidence of Evolution Fossil record Shows change over time and transitional forms (e.g., Tiktaalik, whale ancestors). Dated by rock layers & isotopes. Anatomy Homologous structures: same bones, different uses (whale fin, bat wing, human arm). Vestigial organs also count. Molecular biology Shared DNA & protein sequences. The more similar the sequence, the more recent the common ancestor. The strongest modern evidence. Embryology Vertebrate embryos look strikingly similar in early development (e.g., all have pharyngeal pouches) — hinting at shared ancestry. Biogeography The geographic distribution of species reflects evolutionary history — e.g., unique marsupials in isolated Australia; island species resembling the mainland. Homologous vs. analogous Homologous = shared ancestry (divergent evolution). Analogous = similar function, NO shared ancestor (convergent evolution — e.g., bird & insect wings). Only homology shows true relatedness. Fossils, anatomy, molecules, embryos, and biogeography all independently point to evolution. The Review Hub · AP Biology Unit 7 TOPIC 7.7 Common Ancestry All life shares a common ancestor Every living thing on Earth descends from a single common ancestor. The evidence is that all organisms share fundamental features that would be very unlikely to arise independently. The deeper two species share these features, the more recently they diverged from a common ancestor. Similarities that arose from a shared ancestor are called homologies. Universal genetic code Nearly all organisms use the SAME codons for the same amino acids. Shared machinery DNA/RNA, ribosomes, ATP, and core metabolic pathways are common to all cells. Conserved genes Some genes (like those for ribosomal RNA) are nearly identical across all domains. Molecular clocks Because mutations accumulate at a roughly steady rate, the number of DNA differences between two species estimates how long ago they diverged — used to build and date evolutionary trees. A universal genetic code and shared cellular machinery are strong evidence that all life is related. The Review Hub · AP Biology Unit 7 TOPIC 7.8 Continuing Evolution Evolution is happening right now Evolution isn't just ancient history — it's ongoing and observable, especially in organisms that reproduce quickly. Strong selective pressures can shift populations within a human lifetime. All life continues to evolve and shares a common set of processes for genetic change. Antibiotic resistance A few bacteria carry a resistance allele. Antibiotics kill the susceptible ones; the resistant survivors reproduce. Soon the whole population is resistant — a public-health crisis driven by natural selection. Other real-time examples • Pesticide resistance in insects • Herbicide resistance in weeds • Viruses (like influenza and HIV) evolving fast, which is why vaccines need frequent updates Why fast reproducers evolve fast Short generation times and huge population sizes mean many mutations and many rounds of selection per year. That's why bacteria and viruses can adapt to our drugs so quickly — making resistance a moving target. Evolution is ongoing — antibiotic & pesticide resistance are natural selection we can watch in real time. The Review Hub · AP Biology Unit 7 TOPIC 7.9 Phylogeny A PHYLOGENETIC TREE (CLADOGRAM) species A species B species C species D common ancestor each node = a shared ancestor · branch = lineage Reading a tree Each branch point (node) represents the most recent common ancestor of everything above it. The closer two tips branch, the more closely related they are. Shared derived characters Trees are built from traits shared by a group because they inherited it from a common ancestor. A trait that appears at a node is shared by all descendants above it. Key cautions The tips are all living today — none evolved "from" another. You can rotate branches at a node without changing the relationships. Molecular data now guides most trees. A phylogenetic tree maps evolutionary relationships — each node is a common ancestor. The Review Hub · AP Biology Unit 7 TOPIC 7.10 Speciation Speciation = the formation of a new species The biological species concept defines a species as a group that can interbreed and produce fertile offspring. Speciation happens when populations become REPRODUCTIVELY ISOLATED and their gene pools diverge. Allopatric speciation A physical barrier (river, mountain, ocean) splits a population geographically. The isolated groups adapt to different conditions and diverge until they can no longer interbreed. ("allo" = other place) The most common route — e.g., Galápagos finches. Sympatric speciation New species form WITHOUT geographic separation — while living in the same area. Causes include polyploidy (common in plants), differing habitats, or mate preferences. ("sym" = same place) Less common; a single generation can suffice in plants. Reproductive isolation barriers Prezygotic barriers prevent mating or fertilization (habitat, timing, behavior, mechanical, gametic). Postzygotic barriers act after a hybrid forms (hybrid inviability or sterility, e.g., a mule). Both keep gene pools separate. New species arise when populations become reproductively isolated — allopatric (barrier) or sympatric (same area). The Review Hub · AP Biology Unit 7 TOPIC 7.11 Variation in Populations Diversity = resilience Populations with MORE genetic variation are better able to survive environmental change, disease, and new predators — there's a higher chance some individuals carry a helpful allele. Low-diversity populations are fragile: a single disease can wipe them out (e.g., genetically uniform crops). Genetic drift Random change in allele frequencies from chance events. Has a MUCH bigger effect in small populations. Bottleneck & founder Bottleneck: a disaster slashes a population, shrinking diversity. Founder: a few individuals start a new, less-diverse colony. Gene flow Migration moves alleles between populations. It ADDS variation and makes populations more genetically similar. Where variation comes from The original source of all new alleles is mutation. Sexual reproduction then shuffles those alleles into new combinations (crossing over, independent assortment, random fertilization — Topic 5.2). More genetic variation = more resilience; drift, bottlenecks, and founder effects reduce it. The Review Hub · AP Biology Unit 7 TOPIC 7.12 Origins of Life on Earth ~4.6 byaEarth forms ~3.8 byafirst cells (prokaryotes) ~2.7 byaphotosynthesis → O₂ ~2.1 byaeukaryotes (endosymbiosis) Abiotic synthesis of monomers The early atmosphere + energy (lightning, UV, heat) could form simple organic molecules from inorganic ones. The Miller–Urey experiment produced amino acids this way, showing the building blocks of life can arise abiotically. RNA world & protobionts RNA likely came first: it can both store information AND catalyze reactions (as ribozymes) — so it could self- replicate before DNA and proteins. Membrane-bound droplets (protobionts) formed the first cell-like units. The hypothesized sequence Small organic molecules → polymers → self-replicating RNA → protobionts (membrane droplets) → the first true cells. Later, endosymbiosis gave rise to eukaryotes (Topic 2.10). These are well-supported hypotheses. Life likely began with abiotic organic molecules and an RNA world, leading to the first cells. The Review Hub · AP Biology Unit 7
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Spend 30 seconds per slide before clicking next. Look at the diagram, then ask yourself: "Could I draw this from memory and explain it?"

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This is great for review the night before the exam — fast, visual, and covers everything you need to remember about Unit 7's evolution content, the single most heavily weighted unit on the exam.