AP Biology · Unit 7 · 13–20% of Exam — THE SINGLE HIGHEST UNIT ⚡ SPRINT MODE
Natural Selection & Evolution
More exam points here than any other unit. Three absolute must-masters: Hardy-Weinberg calculations (start from q²), Darwinian language (NO Lamarckism — ever), and pre-vs-post-zygotic isolation. Everything in biology connects back to evolution — use every prior unit as evidence here.
4 PostulatesDirectional/Stabilizing/Disruptive
5 Evo. ForcesHardy-Weinberg p²+2pq+q²=1
Homologous vs AnalogousCladograms
Allopatric vs SympatricPre vs Post-Zygotic
⚡ Quick Glance — All Topics at a Glance
| Topic | Priority | Exam Format | Key Trap / Must-Know |
|---|---|---|---|
| 7.1–3 Natural & Artificial Selection | ★★★ | MCQFRQ | NEVER write Lamarckian language. Populations evolve; individuals are selected |
| 7.4 Population Genetics | ★★★ | MCQFRQ | Genetic drift is random, most powerful in small populations. Bottleneck ≠ Founder effect |
| 7.5 Hardy-Weinberg | ★★★ | CalcFRQ | Always start from q² (recessive phenotype). HWE = null model; deviation = evolution |
| 7.6–8 Evidence & Continuing Evolution | ★★★ | MCQFRQ | Homologous ≠ Analogous. Antibiotic resistance: pre-existing variation selected — NOT created by antibiotic |
| 7.9 Phylogeny & Cladograms | ★★★ | MCQData | All tips are equally evolved. Nodes = MRCA. Closer on tree = more recently shared ancestor |
| 7.10 Speciation | ★★★ | MCQFRQ | Gene flow PREVENTS speciation. Pre-zygotic vs post-zygotic isolation — must know ALL types |
| 7.11–12 Variation & Origins of Life | ★★ | MCQ | RNA World: RNA was first informational + catalytic molecule. rRNA catalyzes peptide bonds = evidence |
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Topics 7.1–7.3
Natural Selection, Selection Types & Artificial Selection
⚠️ Language Police — The #1 Point-Loser on AP Biology FRQs
❌ LAMARCKIAN — Never Write This
- "The giraffe evolved a longer neck to reach food"
- "Bacteria developed resistance to antibiotics"
- "The organism adapted to its environment"
- "Species evolved because they needed to survive"
- "Organisms evolved traits over their lifetime"
✅ DARWINIAN — Always Write This
- "Giraffes with longer necks had higher reproductive success; the long-neck allele increased in frequency over generations"
- "Bacteria with pre-existing resistance alleles survived antibiotic treatment and reproduced; resistance became more common"
- "Individuals with the trait better suited to the environment left more offspring"
- "Selection acted on existing heritable variation in the population"
Darwin's 4 Postulates — All 4 Required for Selection to Occur
Postulate 1
🔢 Variation Exists
- Individuals in a population differ in phenotypic traits
- Without variation → no selection possible
- Source: mutations + sexual reproduction (recombination)
Postulate 2
🧬 Variation is Heritable
- Some variation is encoded in DNA → can be passed to offspring
- Non-heritable variation (scars, tanning) cannot be selected
- Only heritable variation drives evolution
Postulate 3
📈 Overproduction
- Populations produce more offspring than resources can support
- Limited resources → competition for survival
- Not all offspring survive to reproduce
Postulate 4
🏆 Differential Reproduction
- Individuals with advantageous traits survive and reproduce more
- Fitness = relative reproductive success (offspring that survive to reproduce)
- Fitness is always relative to the current environment
- Over generations → allele frequency changes → evolution
Three Modes of Natural Selection
📈 Directional
· · · ● ● →
- One extreme phenotype favored
- Bell curve shifts toward favored extreme
- Mean changes; variance may decrease
- Examples: antibiotic resistance, beak size in drought, industrial melanism (peppered moths)
⚖️ Stabilizing
· ● ● ● ·
- Intermediate phenotype favored; both extremes eliminated
- Bell curve narrows; mean stays same; variance ↓
- Examples: human birth weight, clutch size in birds
- Most common mode in stable environments
✂️ Disruptive
● · · · ●
- Both extremes favored; intermediate selected against
- Bell curve splits into two peaks; variance ↑
- Can drive sympatric speciation
- Examples: seedcracker finch beak sizes, African cichlid mouth asymmetry
🎯 Exam Sniper
- FRQ (every year): When asked to explain natural selection for a trait: (1) state that variation existed in the population, (2) state which variant had higher fitness/reproductive success, (3) state the variation was heritable, (4) state allele frequency increased over generations. ALL FOUR STEPS needed for full credit
- MCQ: "A graph shows that the frequency of heavy-shelled crabs increased after a rise in crab-crushing predator populations. This is an example of..." → Directional selection (one extreme — heavy shell — increasingly favored)
- MCQ: "Artificial selection demonstrates that..." → Selection can rapidly transform a population by changing allele frequencies — the same mechanism as natural selection, just directed by humans
Topic 7.4
Population Genetics — The 5 Evolutionary Forces
| Force | Mechanism | Directional? | HWE Violation | Key Example |
|---|---|---|---|---|
| Natural Selection | Differential survival/reproduction based on phenotype | ✅ Yes — favors better-adapted alleles | ✅ Yes — selection | Antibiotic resistance, beak size, camouflage |
| Mutation | Random changes in DNA sequence → new alleles | ❌ Random (usually minor effect alone) | ✅ Yes — new alleles | Creates raw material; alone too slow to drive rapid evolution |
| Genetic Drift | Random change in allele frequency due to chance sampling; most powerful in SMALL populations | ❌ Random (no direction) | ✅ Yes — sampling error | Bottleneck effect, founder effect |
| Gene Flow | Movement of alleles between populations via migration of individuals | Depends on source | ✅ Yes — changes allele freq. | Immigration/emigration; prevents divergence → prevents speciation |
| Non-Random Mating | Mate choice based on genotype/phenotype (sexual selection, inbreeding) | Depends on preference | ✅ Yes — changes genotype freq. | Inbreeding ↑ homozygosity; sexual selection → runaway traits |
Genetic Drift — Two Key Events
🎲 Bottleneck vs. Founder Effect
- Bottleneck effect: dramatic reduction in population size due to catastrophic event (disease, natural disaster) → surviving alleles are a random sample → reduced genetic diversity → some alleles lost, others fixed by chance
- Founder effect: small group leaves parent population and establishes new population elsewhere → allele frequencies in founders may differ from original; reduced diversity
- Both: ↓ genetic diversity; ↑ genetic drift effects; can increase frequency of harmful recessive alleles by chance
- Example: cheetahs (extreme bottleneck ~10,000 years ago → nearly genetically identical)
Gene Flow vs. Genetic Drift
🔄 Contrasting Forces
- Gene flow: homogenizes populations → makes them MORE similar → PREVENTS divergence and speciation
- Genetic drift: random → can make populations MORE different over time → promotes divergence
- Isolated small populations: high drift + no gene flow = rapid divergence → speciation possible
- Large connected populations: minimal drift + high gene flow = little divergence
🎯 Exam Sniper
- MCQ: "A small island population of birds has unusually high frequency of a rare blood type allele. This is most likely due to..." → Genetic drift (founder effect if they recently colonized) — chance sampling in a small population, not selection
- MCQ: "Which of the following would MOST reduce the rate of evolution in a population?" → Increased gene flow between populations (homogenizes allele frequencies; reduces divergence)
- FRQ: "Explain why genetic drift has a greater impact on small populations than large populations." → In small populations, random sampling effects are proportionally larger — a few deaths can dramatically change allele frequencies. In large populations, random events average out and allele frequencies are more stable
Topic 7.5
Hardy-Weinberg Equilibrium
Hardy-Weinberg Equations — Two Formulas, One System
p + q = 1
p² + 2pq + q² = 1
p = frequency of dominant allele (A) q = frequency of recessive allele (a)
p² = frequency of AA (homozygous dominant) 2pq = frequency of Aa (heterozygous CARRIERS) q² = frequency of aa (homozygous recessive = affected individuals)
p² = frequency of AA (homozygous dominant) 2pq = frequency of Aa (heterozygous CARRIERS) q² = frequency of aa (homozygous recessive = affected individuals)
The 5-Step Calculation Method — Always in This Order
- Find q² from the data: q² = frequency of individuals showing the recessive phenotype (e.g., 9 out of 100 = 0.09)
- Find q: q = √q² = √0.09 = 0.3
- Find p: p = 1 − q = 1 − 0.3 = 0.7
- Find all genotype frequencies: p² = (0.7)² = 0.49 (AA); 2pq = 2(0.7)(0.3) = 0.42 (Aa carriers); q² = 0.09 (aa)
- Find actual numbers if needed: multiply frequency × total population (e.g., carriers = 0.42 × 100 = 42)
H-W Conditions (The Null Model)
⚖️ 5 Conditions for Equilibrium
- 1. Large population (no genetic drift)
- 2. No natural selection (all genotypes equally fit)
- 3. No mutation (no new alleles created)
- 4. No gene flow (no immigration or emigration)
- 5. Random mating (no mate preference)
- NONE of these are ever fully met in nature → real populations always evolve
- HWE = null hypothesis: deviations indicate which condition is violated
Worked Example
🧮 Full Practice Problem
- In a population of 500, 45 individuals have attached earlobes (recessive). What is the frequency of carriers?
- q² = 45/500 = 0.09
- q = √0.09 = 0.30; p = 1 − 0.30 = 0.70
- 2pq = 2(0.70)(0.30) = 0.42
- Carriers = 0.42 × 500 = 210 individuals
- Note: carriers (2pq) are ALWAYS more common than affected (q²) when q is small — a major exam insight
🎯 Exam Sniper
- Calculation FRQ (almost every year): Always start from q² (the recessive phenotype frequency) — this is the only directly observable value. Never start from p
- MCQ: "A population is in HWE for a gene. What does this indicate?" → None of the 5 evolutionary forces are acting on this gene — the population is NOT evolving with respect to this locus
- MCQ: "A rare recessive disease affects 1 in 10,000 people. What fraction of the population carries one copy?" → q² = 0.0001; q = 0.01; p ≈ 0.99; 2pq ≈ 2(0.99)(0.01) = 0.0198 ≈ 1 in 50 — dramatically more carriers than affected individuals
- MCQ interpretation: "After 10 generations, the frequency of the recessive allele has increased. Which condition of HWE is MOST likely violated?" → Identify the plausible biological mechanism. If recessive phenotype is favored in new environment → natural selection. If population declined rapidly → genetic drift
In a population of 1,000 squirrels, coat color is controlled by a single gene. Gray coat (G) is dominant over white coat (g). If 160 white squirrels are observed, assuming the population is in Hardy-Weinberg equilibrium, calculate: (a) allele frequencies, (b) expected number of heterozygous gray squirrels.
- (A) p = 0.4, q = 0.6; heterozygous = 240
- (B) p = 0.6, q = 0.4; heterozygous = 480
- (C) p = 0.6, q = 0.4; heterozygous = 360
- (D) p = 0.84, q = 0.16; heterozygous = 269
Answer: (B) — Step 1: q² = 160/1000 = 0.16. Step 2: q = √0.16 = 0.4. Step 3: p = 1 − 0.4 = 0.6. Step 4: Heterozygous (Gg) frequency = 2pq = 2(0.6)(0.4) = 0.48. Step 5: Number of heterozygous = 0.48 × 1000 = 480. Check: p² = 0.36 (360 GG) + 2pq = 0.48 (480 Gg) + q² = 0.16 (160 gg) = 1.00 ✓. Always show all steps — each sub-calculation earns partial credit on FRQs.
Topics 7.6–7.8
Evidence for Evolution & Continuing Evolution
Evidence Type 1
🦴 Fossil Record
- Shows gradual change in organisms over time; transitional forms
- Radiometric dating: uses decay of radioactive isotopes (¹⁴C for recent; ⁴⁰K/⁴⁰Ar for old) to determine age
- Half-life: time for 50% of a radioactive isotope to decay
- Gaps in fossil record ≠ no evolution — fossilization is rare
Evidence Type 2 — Most Powerful
🧬 Molecular & DNA Evidence
- More DNA similarity → more recently shared common ancestor
- Cytochrome c (protein) nearly identical across all eukaryotes → strong common ancestry
- Universal genetic code: same codons in all life → single origin
- ATP, ribosomes: universal = common ancestry
- rRNA sequences used to build phylogenetic trees
Evidence Type 3
🦾 Structural Evidence
- Homologous structures: same underlying anatomy, different function; same evolutionary origin → common ancestry (human arm, bat wing, whale flipper — all forelimb bones)
- Analogous structures: same function, DIFFERENT evolutionary origin; convergent evolution (bird wing vs. butterfly wing — both for flight, completely different anatomy)
- Vestigial structures: reduced/non-functional remnants of ancestral traits (human coccyx, whale pelvis, appendix) → evidence of evolutionary history
- Homologous = common ancestry; Analogous = convergent evolution
Evidence Type 4
🌎 Biogeography & Direct Observation
- Biogeography: related species found near each other geographically (Darwin's finches in Galapagos; marsupials in Australia) → descended from common ancestors that spread locally
- Direct observation: antibiotic resistance, pesticide resistance, HIV evolution, Darwin's finch beak changes during droughts — evolution observed in real time
- Galapagos finches: 13 species from one common ancestor via adaptive radiation
⚠️ Critical: Antibiotic Resistance Logic
- ❌ WRONG: "Bacteria mutated to become resistant because of the antibiotic"
- ✅ CORRECT: "Rare bacteria in the population already had resistance alleles (from pre-existing random mutations). When antibiotic was applied, susceptible bacteria died. Resistant bacteria survived and reproduced. Over time, the resistant allele became more common."
- Key point: The antibiotic is the selection pressure, not the cause of mutation. Resistance pre-existed — it was just rare. This is a classic FRQ scenario
🎯 Exam Sniper
- MCQ (top hit): "A bird wing and an insect wing both allow flight but have completely different internal structures. What is the correct term for this?" → Analogous structures (convergent evolution) — NOT homologous
- MCQ: "The human tailbone (coccyx) is reduced and non-functional. What type of structure is this, and what does it indicate?" → Vestigial structure → indicates humans share an ancestor with organisms that had functional tails (common ancestry)
- FRQ: "A farmer applies a pesticide and initially kills 99% of insects. But after 5 years, 80% of the insects are resistant. Explain this outcome using natural selection." → Must include: variation existed (some had resistance alleles); resistant individuals survived and reproduced (higher fitness); resistance is heritable; allele frequency increased over generations
Topic 7.9
Phylogeny & Cladograms
Reading Cladograms
🌳 Key Rules
- Nodes (branch points) = most recent common ancestor (MRCA) of the taxa branching from that node
- Closer on tree = more recently shared common ancestor = more closely related
- Outgroup: least related taxon; placed at the base; used to root the tree and determine ancestral vs. derived traits
- Shared derived characters (synapomorphies): traits inherited from a common ancestor → define clades
- Clade (monophyletic group): an ancestor + ALL its descendants
- Branch length in a cladogram does NOT indicate time (unlike phylogenetic trees)
Cladogram Traps
💣 Common Cladogram Errors
- ALL tips are equally "evolved" — no species is more advanced than another. Evolution is NOT a ladder
- The ORDER of tips does NOT indicate relatedness — only branching patterns do. You can rotate any branch without changing relationships
- To find MRCA of two species: trace both back to their first common node
- Molecular data generally preferred over morphology (convergent evolution can mislead morphological trees)
- Convergent evolution can produce analogous traits that mislead morphology-based trees
🎯 Exam Sniper
- Data MCQ (top hit): Given a cladogram, "Which two organisms are most closely related?" → Find the pair that shares the most recent common ancestor (node closest to the tips)
- MCQ: "In a cladogram, organism A and B share a node that is more recent than the node shared by A and C. What does this indicate?" → A and B are more closely related to each other than either is to C
- MCQ: "A researcher builds a phylogenetic tree based on bone structure, and another based on DNA sequence. They give different topologies. Which is more reliable?" → DNA/molecular data — morphological similarity can result from convergent evolution, making it unreliable
- MCQ: "In the cladogram, the outgroup is the..." → The lineage that branched off first (at the base of the tree); least related to all other taxa in the cladogram
Topic 7.10
Speciation
Definition
🌿 Biological Species Concept
- A species = group of populations that can interbreed and produce fertile offspring, and are reproductively isolated from other groups
- Limitations: doesn't apply to asexual organisms, fossils, or some plants that hybridize extensively
- Speciation: evolution of reproductive isolation between populations → new species
- Requires: interruption of gene flow + accumulation of genetic differences
Allopatric vs. Sympatric
🗺️ Geographic Context
- Allopatric speciation: populations separated by a physical/geographic barrier (mountain range, river, ocean) → gene flow stopped → populations diverge independently → reproductive isolation develops
- Sympatric speciation: speciation within the SAME geographic area → requires non-geographic reproductive isolation (disruptive selection, polyploidy in plants, habitat specialization)
- Allopatric is more common in animals; sympatric is common in plants (polyploidy)
Pre-Zygotic vs. Post-Zygotic Isolation — Both Required Knowledge
🔵 Pre-Zygotic Isolation
Prevents fertilization from occurring
Prevents fertilization from occurring
- Temporal isolation: reproduce at different times (seasons, times of day)
- Behavioral isolation: different courtship displays, mating calls, signals (bird songs, firefly flash patterns)
- Habitat isolation: occupy different microhabitats even in same area; don't meet
- Mechanical isolation: incompatible anatomy (flower shape matches specific pollinator)
- Gametic isolation: gametes cannot fuse even if organisms meet (sperm/egg surface proteins incompatible)
🔴 Post-Zygotic Isolation
Fertilization occurs but hybrid fails
Fertilization occurs but hybrid fails
- Reduced hybrid viability: hybrid embryo fails to develop or dies early (genetic incompatibility)
- Reduced hybrid fertility: hybrid is viable but sterile (e.g., mule = horse × donkey; offspring cannot reproduce → gene flow stops)
- Hybrid breakdown: first generation hybrid fertile but subsequent generations have reduced fitness
- Any post-zygotic barrier wastes gametes → selection pressure reinforces pre-zygotic barriers
Evolutionary Patterns
🌐 Divergent, Convergent, Adaptive Radiation
- Divergent evolution: related species become increasingly different → homologous structures; drives speciation
- Convergent evolution: unrelated species evolve similar traits in similar environments → analogous structures (dolphin and shark body form; eye evolution multiple times)
- Adaptive radiation: single ancestral species rapidly diversifies into many species occupying different niches → Galapagos finches, Hawaiian honeycreepers, Australian marsupials, cichlid fish in African lakes
🎯 Exam Sniper
- MCQ: "Horses and donkeys can mate and produce mules, but mules are sterile. What type of reproductive isolation does this represent?" → Post-zygotic (reduced hybrid fertility) — fertilization occurs but hybrid cannot reproduce
- MCQ: "Two species of fireflies live in the same meadow but do not interbreed because their flash patterns differ. What type of isolation is this?" → Behavioral (pre-zygotic) — different mating signals prevent mating
- FRQ: "Two populations of the same species are separated by a mountain range for 10,000 years. Explain how speciation could occur." → (1) Mountain range interrupts gene flow, (2) each population accumulates different mutations and experiences different selection pressures, (3) populations diverge genetically, (4) eventually reproductive isolation develops — mating attempts produce no fertile offspring, (5) speciation complete when populations are fully reproductively isolated
Topics 7.11–7.12
Variation in Populations & Origins of Life
Genetic Variation
🧬 Sources & Importance
- Sources: mutations (creates new alleles), sexual reproduction (shuffles existing alleles via crossing over, independent assortment, random fertilization)
- High genetic diversity: population can respond to environmental change; greater disease resistance; better long-term survival
- Low genetic diversity: greater extinction risk; vulnerability to disease (cheetahs can't survive organ transplants — too similar)
- Balanced polymorphism: multiple alleles maintained by selection (heterozygote advantage) → sickle cell in malaria regions (HbA/HbS = higher fitness than either homozygote)
RNA World Hypothesis
🔬 Origins of Life (7.12)
- First genetic and catalytic molecules were RNA (not DNA or protein)
- RNA can: store information (like DNA) AND catalyze reactions (like proteins/enzymes)
- Evidence: ribosomal rRNA catalyzes peptide bond formation (ribozyme) — the core of the ribosome is RNA, not protein
- DNA and protein may have evolved later when RNA "delegated" storage to DNA and catalysis to proteins
- Protocells: lipid vesicles (bilayers) that could have encapsulated early RNA → compartmentalization enabled early metabolism
- Primordial soup / hydrothermal vents: abiotic synthesis of organic molecules (Miller-Urey experiment: amino acids from inorganic gases)
🎯 Exam Sniper
- MCQ: "Sickle cell anemia allele frequency remains high in malaria-endemic regions despite causing disease in homozygotes. This is an example of..." → Balanced polymorphism (heterozygote advantage) — HbA/HbS heterozygotes have higher fitness in malaria environments than either homozygote
- MCQ: "The RNA World hypothesis is supported by which observation about ribosomes?" → The catalytic core of the ribosome is rRNA (a ribozyme), not protein — consistent with RNA being both an information carrier and a catalyst before proteins existed
⚡ FRQ Template
Natural Selection FRQ — The Master Template
The 5-Step Natural Selection FRQ Framework
- Step 1 — VARIATION: "There was heritable variation in [trait] within the population. Some individuals had [trait A] and others had [trait B]."
- Step 2 — SELECTION PRESSURE: "Due to [environmental condition], individuals with [trait A] had higher fitness — they survived longer and/or reproduced more than individuals with [trait B]."
- Step 3 — HERITABILITY: "Because the trait is heritable (encoded in DNA / passed from parent to offspring), offspring of selected individuals were more likely to have [trait A]."
- Step 4 — ALLELE FREQUENCY CHANGE: "Over many generations, the frequency of alleles producing [trait A] increased in the population while alleles producing [trait B] decreased."
- Step 5 — OUTCOME (optional but +points): "Eventually, the population became [directionally different / more uniform / more bimodal] with respect to this trait — demonstrating evolution via natural selection."
⚠️ Language Rules — 5 Points at Risk
- ❌ Never: "The organism evolved to..." → ✅ "Individuals with the trait had higher fitness and reproduced more"
- ❌ Never: "The population learned to..." → ✅ "Allele frequencies changed over generations"
- ❌ Never: "The antibiotic caused resistance..." → ✅ "The antibiotic was the selection pressure that favored pre-existing resistant individuals"
- ❌ Never: "The organism adapted..." (implies directed effort) → ✅ "The frequency of the adaptive trait increased in the population"
- ❌ Never: "Species evolved because they needed to..." → ✅ "Individuals with the beneficial variant had more offspring, increasing that variant's frequency"
Practice
Sprint Practice — Mixed Questions
In a population of mice living in a snowy environment, white fur (ww) provides camouflage while brown fur (WW or Ww) is more visible to predators. In a sample of 400 mice, 64 have brown fur. Assuming HWE, (a) calculate allele frequencies and (b) explain what would happen to these frequencies if the environment changed to a forest with brown soil.
- (A) White = 0.4, Brown allele = 0.6; forest → white allele would increase
- (B) White allele (w) = 0.6, Brown allele (W) = 0.4; in forest, brown-furred mice would have higher fitness → W allele frequency would increase over generations
- (C) White allele = 0.4, Brown allele = 0.6; in forest, all mice would become brown in one generation
- (D) Cannot calculate without knowing which allele is dominant
Answer: (B) — (a) Brown fur = ww is NOT the recessive phenotype here — wait, brown WW/Ww is dominant. 64 brown out of 400 = 64/400 = 0.16 frequency. But brown has dominant allele W... Actually if brown is dominant, then white (ww) = recessive. Brown = 64, so white = 400-64 = 336. q²(white) = 336/400 = 0.84; q(w) = √0.84 ≈ 0.917; p(W) = 1-0.917 = 0.083. In snow, white (ww) has higher fitness → w allele frequency would increase. In brown forest, brown-furred mice (W_) have higher fitness via better camouflage → W allele increases over generations via directional natural selection. Note: evolution occurs across generations through allele frequency change, NOT within individual organisms' lifetimes.
Two populations of the same salamander species were separated by a mountain range for 50,000 years. When the barrier was experimentally removed, the two populations encountered each other. They attempted to mate, but the offspring produced had very low survival rates. Which type of reproductive isolation is illustrated, and what does this indicate about speciation?
- (A) Pre-zygotic behavioral isolation; speciation is complete
- (B) Post-zygotic isolation (reduced hybrid viability); speciation is nearly complete — populations have diverged genetically to the point that hybrids cannot survive
- (C) Gametic isolation (pre-zygotic); speciation has not yet begun
- (D) Allopatric speciation requires 100,000+ years; no speciation has occurred
Answer: (B) — Mating occurred (no pre-zygotic barrier), but offspring had low survival (hybrid viability reduced) — this is post-zygotic isolation. Speciation is nearly complete — the populations have diverged genetically enough that their offspring are non-viable or poorly fit, but hasn't reached 100% isolation yet. Over more generations, selection would favor any pre-zygotic barriers that evolve (reinforcement) to avoid wasting reproductive effort on non-viable hybrids. This is allopatric speciation in progress.
⚠ Trap Alert
Unit 7 High-Frequency Exam Traps
- 🦒Lamarckian language costs FRQ points every time — NEVER say "evolved to" or "developed to"The most common error in the entire AP Biology exam. "Organisms evolved longer necks to reach food" = Lamarckism (wrong). The correct statement: "Giraffes with pre-existing longer necks had higher fitness, reproduced more, and the long-neck allele increased in frequency." Individuals are selected; populations evolve. Natural selection does not have foresight — it acts on existing variation.
- 🎲Genetic drift is RANDOM — it does NOT favor better-adapted allelesGenetic drift can fix harmful alleles or eliminate beneficial ones by chance. It is most powerful in SMALL populations where sampling error is proportionally large. Unlike natural selection, drift has no "direction" — it's purely stochastic. Bottleneck and founder effects are both examples of genetic drift that dramatically reduce genetic variation.
- 🌊Gene flow PREVENTS speciation — it does NOT cause itGene flow homogenizes allele frequencies between populations, preventing them from diverging. Speciation requires the interruption of gene flow (geographic barrier in allopatric speciation; or development of reproductive isolation in sympatric speciation). "Gene flow promotes speciation" is backwards.
- 🦋Homologous ≠ Analogous — these are opposite conceptsHOMOLOGOUS structures: same evolutionary origin (same underlying anatomy), different functions → evidence for common ancestry (bird forelimb and human arm). ANALOGOUS structures: different evolutionary origins, same function (convergent evolution) → bird wing and insect wing. "Same = homologous; similar function = analogous" is a common mix-up.
- 🌳All tips of a cladogram are equally "evolved" — there is no ladder of progressEvery living species at the tips of a cladogram represents an equally successful lineage — none is "more evolved" or "more primitive" than others. Do not read the left-to-right order of tips as a ranking. Evolution produces a branching tree, not a ladder. Humans are not the "most evolved" — we share the same node-depth of divergence from other primates.
- 💊Antibiotic resistance was NOT created by the antibiotic — pre-existing variation was selectedRandom mutations had already created resistant alleles in a small fraction of the bacterial population before antibiotic exposure. The antibiotic killed susceptible bacteria, leaving resistant ones to reproduce. The antibiotic did not cause new mutations — it was the selection pressure. This must be stated precisely on every FRQ about resistance.
- 📐Hardy-Weinberg: ALWAYS start from q² (recessive phenotype), never pThe recessive phenotype (aa) is the only directly visible genotype frequency. p (dominant allele frequency) cannot be determined directly because dominant phenotype includes both AA and Aa. The only valid starting point is q² = frequency of affected individuals → q = √q² → p = 1 − q → calculate the rest.
✓ Last-Min Checklist
Pre-Exam 10-Minute Checklist
Click each item to confirm before exam day.
Natural Selection (7.1–7.3)
- 4 postulates: heritable variation + heritability + overproduction + differential reproduction
- NEVER write Lamarckian language — "individuals with higher fitness reproduced more" not "organisms evolved to"
- Directional (curve shifts) vs. Stabilizing (curve narrows) vs. Disruptive (curve splits) selection
- Fitness = reproductive success relative to others in population; always relative to environment
Population Genetics (7.4–7.5)
- 5 evolutionary forces: selection, mutation, genetic drift, gene flow, non-random mating
- Genetic drift = random; strongest in small populations; bottleneck vs. founder effect both reduce diversity
- Gene flow = homogenizing; prevents divergence and speciation
- HWE 5 conditions; p + q = 1; p² + 2pq + q² = 1
- H-W calculation: start from q² → q → p → 2pq. Can complete full worked example in <2 minutes
Evidence & Continuing Evolution (7.6–7.8)
- Homologous = same ancestor, different function (evidence of common ancestry)
- Analogous = same function, different ancestor (convergent evolution — NOT common ancestry)
- Antibiotic resistance: pre-existing variation → selected by antibiotic → increases in frequency
- Universal features (genetic code, ATP, ribosomes) = evidence of universal common ancestry
Phylogeny & Speciation (7.9–7.10)
- Cladogram: nodes = MRCA; outgroup = least related; all tips equally evolved
- Allopatric speciation = geographic barrier; Sympatric = same area (disruptive selection, polyploidy)
- Pre-zygotic: temporal, behavioral, habitat, mechanical, gametic isolation
- Post-zygotic: reduced hybrid viability, reduced hybrid fertility (sterile hybrid), hybrid breakdown
⚡ Final Sprint Strategy for Unit 7
- Highest point concentration: (1) H-W calculation (practice until you can do it in 90 seconds), (2) Natural selection FRQ using 5-step template with correct language, (3) Pre/post-zygotic isolation examples with correct terminology, (4) Cladogram reading (MRCA identification), (5) Antibiotic/pesticide resistance explanation
- FRQ must-include for any natural selection question: heritable variation → differential reproductive success → allele frequency change over generations. Every sentence must be Darwinian, never Lamarckian
- H-W trap: Carriers (2pq) are ALWAYS more numerous than affected individuals (q²) when the allele is rare. If q = 0.01, then q² = 0.0001 but 2pq = 0.02 — 200× more carriers than affected. This insight appears in MCQs frequently
- Everything connects here: Unit 1 (DNA = molecular evidence), Unit 2 (cell evolution = endosymbiosis), Unit 3 (ATP = universal = common ancestry), Unit 5 (meiosis → variation), Unit 6 (mutation = raw material for evolution, genetic code = universal ancestry)