AP Biology Unit 7: Natural Selection and Evolution Review
The first time I mapped the eight AP Biology CEDunits against their exam weights, Unit 7's number stood out: 13 to 14 percent. That is roughly 1 in 8 points on the exam dedicated to natural selection and evolution. Yet in most AP Bio prep guides I have worked through, Unit 7 gets half the page space of Cellular Energetics or Gene Expression. That gap is one reason Hardy-Weinberg problems catch students unprepared on the FRQ section, where the same five-step algebra pattern shows up almost every year.
What Does AP Biology Unit 7 Cover?
Unit 7 covers how populations change over time through natural selection, genetic drift, and gene flow, and how those changes produce new species across geological timescales. The College Board groups the content under six broad topics in the AP Biology CED: natural selection mechanisms, Hardy-Weinberg equilibrium, evidence for evolution, speciation, phylogenetic trees, and evolutionary fitness.
The unit builds on Unit 5 (Heredity) and Unit 6 (Gene Expression). Without understanding how alleles segregate and how mutations arise, the population-level math in Unit 7 has no foundation. That connection appears explicitly on some FRQ questions, which link a mutation in Unit 6 territory to its selective effects in Unit 7 territory.
Unit 7 Exam Weight and High-Yield Topics
| Unit 7 Topic | What the AP Exam Tests |
|---|---|
| Natural selection types | Identify directional, stabilizing, or disruptive selection from a described scenario or graph |
| Hardy-Weinberg equilibrium | Calculate allele frequencies (p, q) and genotype frequencies (p², 2pq, q²) from population data |
| Evidence for evolution | Explain how fossil record, biogeography, comparative anatomy, and molecular data support common ancestry |
| Speciation | Distinguish allopatric from sympatric speciation; identify which conditions promote each |
| Phylogenetic trees | Interpret cladograms: common ancestors, sister taxa, synapomorphies, and where to place new organisms |
| Evolutionary fitness | Define relative fitness and explain how selection changes allele frequencies over generations |
Source: AP Biology Course and Exam Description (College Board)
What Are the Three Types of Natural Selection?
Natural selection occurs when individuals with certain heritable traits reproduce more successfully than others in their environment. The trait, not the individual, increases in frequency across generations as those individuals pass more copies of their genes to the next generation. Three patterns describe how this process reshapes a population's trait distribution over time.
Directional, Stabilizing, and Disruptive Selection
Directional selection shifts the population mean toward one extreme. If larger beak size helps birds crack harder seeds during a drought, the average beak size in that population increases each generation. The distribution curve moves to the right, and the left tail (small beaks) shrinks.
Stabilizing selection favors intermediate phenotypes and removes both extremes. Human birth weight is the classic textbook example: infants born too small face developmental challenges, while unusually large infants face delivery complications. The distribution narrows around the middle without shifting left or right.
Disruptive selection does the opposite of stabilizing. It favors both extremes and eliminates intermediates, producing a bimodal distribution. Disruptive selection sometimes drives speciation when the two subpopulations become reproductively isolated.
What Biological Fitness Means in AP Biology
Fitness in biology measures reproductive success relative to other genotypes in the same population. A genotype with a fitness of 1.0 is the reference; a genotype with fitness 0.8 produces 20% fewer surviving offspring per generation. Fitness is not about strength, speed, or any absolute trait value. An allele that dramatically reduces survival can still spread if it increases the number of surviving offspring.
AP Biology questions sometimes describe a scenario where a "weaker" organism reproduces more. Students who conflate fitness with strength misclassify this as directional selection toward weakness, rather than recognizing that reproductive rate drives allele frequency change, not physical robustness.
How Do You Solve a Hardy-Weinberg Problem?
Hardy-Weinberg equilibrium describes a population where allele frequencies stay constant from one generation to the next. The two equations that define it are p + q = 1 (where p and q are the frequencies of the two alleles at a locus) and p² + 2pq + q² = 1 (where each term gives the expected frequency of a genotype). When those conditions hold, the population is not evolving at that locus.
The equations appear simple, but AP FRQ questions test them with population data where you must work backward from observable phenotypes to allele frequencies. The recessive phenotype (individuals showing the recessive trait, genotype bb) is the only genotype you can identify directly from phenotype alone. That makes q² your starting point every time.
The Five Hardy-Weinberg Assumptions
A population maintains Hardy-Weinberg equilibrium only when all five conditions hold simultaneously. Each condition corresponds to an evolutionary force that, when present, changes allele frequencies:
Sufficiently large population size
Eliminates genetic drift. In small populations, random chance causes allele frequencies to fluctuate even without selection. The Hardy-Weinberg model assumes an infinitely large population where chance events average out.
Random mating
No sexual selection or assortative mating (where individuals choose mates based on phenotype). If mates are chosen non-randomly, heterozygote frequencies deviate from the 2pq prediction.
No mutation
New alleles introduced by mutation would change allele frequencies over generations. The Hardy-Weinberg model holds allele frequencies constant across time, so mutation must be absent or negligible.
No migration or gene flow
Individuals entering or leaving the population carry alleles that shift the population's allele frequency. A closed population with no immigration or emigration is required.
No natural selection
All genotypes must have equal fitness. If one genotype produces more offspring than another, its allele increases in frequency, violating equilibrium.
Hardy-Weinberg Worked Example: Step by Step
Problem: In a population of 500 beetles, 80 show the recessive phenotype (brown body color, genotype bb). Assuming Hardy-Weinberg equilibrium, what are the allele frequencies p and q, and what percentage of the population is heterozygous?
Step 1: Find q²
The recessive phenotype (bb) directly gives you q². There are 80 brown beetles out of 500 total, so q² = 80 ÷ 500 = 0.16.
Step 2: Find q
Take the square root of q². q = √0.16 = 0.4. The recessive allele makes up 40% of all alleles in the population.
Step 3: Find p
Since p + q = 1, solve for p. p = 1 − 0.4 = 0.6. The dominant allele makes up 60% of all alleles.
Step 4: Calculate all genotype frequencies
BB (p²) = 0.6² = 0.36. Bb (2pq) = 2 × 0.6 × 0.4 = 0.48. bb (q²) = 0.16. Check: 0.36 + 0.48 + 0.16 = 1.00.
Step 5: Answer the question
Heterozygous frequency = 2pq = 0.48. Of 500 beetles, 500 × 0.48 = 240 are expected to be heterozygous (Bb). The answer is 48% or 240 beetles.
Every Hardy-Weinberg FRQ starts the same way: find q² from the recessive phenotype frequency, take the square root to get q, subtract from 1 to get p. If the question gives you a dominant phenotype frequency instead, subtract from 1 to get q² first, then proceed. Students lose points by trying to start from p, which requires knowing the dominant homozygote frequency, information the question almost never provides directly.
What Is the Evidence for Evolution?
Four independent lines of evidence support the theory of evolution through common descent. AP Biology questions ask you to match each type of evidence to a specific claim about evolutionary relationships, or to evaluate which evidence best supports a given hypothesis.
| Evidence Type | What It Shows | AP Biology Example |
|---|---|---|
| Fossil record | Change in populations over geological time; transitional forms connecting extinct and living species | Tiktaalik links fish and tetrapods; horse evolution series shows directional change in limb structure |
| Biogeography | Geographic distribution of species reflects descent from common ancestors, not independent origin | Darwin's finches: 14 species from one ancestral population that colonized the Galapagos |
| Comparative anatomy | Homologous structures (same bones, different functions) indicate shared ancestry; analogous structures indicate convergent evolution | Forelimbs of humans, bats, whales, and horses share the same bone arrangement despite different functions |
| Molecular evidence | DNA sequence similarity correlates with evolutionary closeness; the genetic code is nearly universal | Humans and chimpanzees share approximately 98-99% of their DNA coding sequence |
All four evidence types appear in AP Biology MCQ and FRQ questions
Molecular evidence has become the strongest single line of evidence since the 1980s because DNA sequences can be compared across all living organisms with mathematical precision. The AP Biology student resources page includes practice FRQs where students must interpret molecular phylogenies derived from DNA sequence comparisons.
How Does Speciation Work?
Speciation produces new species when populations accumulate enough genetic differences that they can no longer interbreed to produce fertile offspring. The College Board tests two primary mechanisms: allopatric speciation (geographic separation) and sympatric speciation (divergence within the same geographic range).
Allopatric vs Sympatric Speciation
Allopatric Speciation
- •Physical barrier separates one population into two
- •Geographic isolation prevents gene flow
- •Each subpopulation evolves independently under different selective pressures
- •Reproductive isolation develops over time as byproduct of divergence
- •Most common mechanism documented in animals
- •Examples: Darwin's finches (island isolation), cichlid fish (lake barriers)
Sympatric Speciation
- •No geographic barrier; divergence occurs within same range
- •Ecological niche differentiation drives separation
- •Polyploidy (chromosome duplication) is a common mechanism in plants
- •Assortative mating within subgroups reduces gene flow
- •Better documented in plants and insects than in vertebrates
- •Example: apple maggot flies that shifted host from hawthorn to apple trees
How Do You Read a Phylogenetic Tree?
A phylogenetic tree (or cladogram) shows evolutionary relationships among organisms based on shared derived characters. Internal nodes represent common ancestors. Branch tips represent the taxa being compared. The topology of the tree, meaning which branches connect where, carries all the information; branch length is often not meaningful in AP Biology cladograms unless the question specifically states otherwise.
To find the most recent common ancestor of two taxa, trace both branches back toward the root until they meet at a node. That node represents the ancestor shared by those two taxa and no others (unless other taxa also branch from that node). Two taxa that share a more recent common ancestor are more closely related, regardless of how similar they look.
Synapomorphies vs Plesiomorphies
A synapomorphy is a shared, derived character: a trait that arose in a common ancestor and is inherited by all descendants of that ancestor, but that the outgroup does not share. Synapomorphies define clades and mark internal nodes on cladograms. The amniotic egg is a synapomorphy shared by all reptiles and mammals but absent in fish and most amphibians.
A plesiomorphy is an ancestral character shared by all members of a group including the outgroup. Having a vertebral column is a plesiomorphy for the group that includes sharks, frogs, lizards, cats, and humans. It tells you nothing about relationships within the group because all members have it.
When asked to justify that two organisms are more closely related to each other than either is to a third organism, cite the synapomorphy they share that the third organism lacks. That shared derived character, visible as the branch point connecting only the two organisms, is your evidence. A shared plesiomorphy does not justify a closer relationship.
How Is Unit 7 Tested on the AP Biology Exam?
Unit 7 accounts for 13-14% of the 60-question MCQ section, translating to roughly 8-9 questions. FRQ questions also draw from Unit 7 material in most years. For a quick reference to formulas and concepts across all eight units, see the AP Biology cheat sheet. The patterns that appear most consistently in Unit 7:
- Hardy-Weinberg calculations: Given a population size and recessive phenotype count, calculate allele frequencies, genotype frequencies, and expected numbers of each genotype. Sometimes asked in reverse (given allele frequencies, predict how many individuals show the recessive phenotype).
- Hardy-Weinberg violation identification: Describe a scenario and ask which of the five equilibrium conditions is being violated, and whether allele frequencies will increase, decrease, or remain unchanged.
- Cladogram interpretation: Read a provided cladogram and answer questions about common ancestors, sister taxa, and the placement of new organisms based on character data.
- Selection type identification: Given a graph showing a population's trait distribution before and after selection, classify the selection type and predict the effect on the mean and variance.
- Evidence for evolution: Match a described finding (fossil, DNA comparison, anatomical structure) to the type of evidence it represents.
Connecting Unit 7 to Units 5 and 6
Hardy-Weinberg equilibrium sits at the intersection of Units 5, 6, and 7. The alleles in the p and q equations come from Mendelian inheritance (Unit 5). Those alleles exist because mutations created them in the first place (Unit 6). Natural selection then changes their frequencies based on reproductive success (Unit 7). AP FRQ questions sometimes span all three units in a single prompt.
From Unit 5 (Heredity), bring forward your understanding of allele segregation during meiosis, which is the mechanism that produces the genotype frequencies Hardy-Weinberg predicts. Genetic recombination in meiosis also generates the variation selection acts on. From Unit 6, bring the understanding that point mutations, insertions, and deletions create new alleles. Without mutation, the gene pool would be static, and natural selection would have no raw material to work with.
For broader AP Biology context, see the AP Biology resource hub, which links to CED documents, practice resources, and unit-specific guides. The AP Biology difficulty overview covers overall exam strategy, while the AP Biology practice questions post includes Unit 7 problems you can work through before the exam.
Estimate Your AP Biology Score
Once you finish reviewing Unit 7, run your current knowledge through the score predictor to see how your unit-level preparedness translates to an estimated AP Biology score. The tool accounts for the relative weighting of each unit, including Unit 7's 13-14% share.
AP Score Predictor
Enter your practice test scores and unit-by-unit confidence ratings to get an estimated AP Biology score. Useful for identifying which units need more time before exam day.
Key Takeaways
- Unit 7 covers 13-14% of the AP Biology exam, spanning natural selection types, Hardy-Weinberg equilibrium, evidence for evolution, speciation, and phylogenetic tree interpretation.
- Hardy-Weinberg problems always start from q² (the recessive phenotype frequency). Find q, subtract from 1 to get p, then calculate p², 2pq, and q². Verify they sum to 1.00.
- Three types of natural selection shift population distributions in different directions: directional (shifts mean), stabilizing (narrows variation), and disruptive (splits into two peaks).
- Allopatric speciation requires a geographic barrier; sympatric speciation produces new species through ecological divergence or polyploidy within the same geographic range.
- In cladograms, synapomorphies (shared derived characters) define clades and justify grouping organisms together. Plesiomorphies (ancestral characters shared with outgroups) do not support specific relationships.
- Biological fitness measures reproductive success relative to other genotypes, not physical strength. A lower-fitness genotype produces fewer surviving offspring per generation, causing its allele frequency to decline under selection.
- Unit 7 connects to Unit 5 (meiosis and allele segregation as the mechanism Hardy-Weinberg predicts) and Unit 6 (mutations as the source of variation selection acts on).
