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AP Biology · Unit 3 · Evolution and Natural Selection

Evolution: How the Exam Actually Asks It

Evolution is the biggest single unit on the AP exam (the College Board calls it Unit 7; it is 13 to 20 percent of the test), and it is the unit where students lose points on questions they could have answered in eighth grade. The biology is not hard. The wording is a minefield. One word turns a true sentence about populations into a false sentence about individuals.

What This Unit Looks Like on the Exam

This is our Unit 3. The College Board's Unit 7 runs from Darwin to phylogenetics: natural selection, the sources and removers of variation, Hardy Weinberg, the evidence for evolution, cladograms, speciation, extinction, and the origin of life. Your class added the Lamarck debate and the Shmoigle lab up front, which turns out to be the right order, because almost every trap in this unit is a Lamarckian sentence wearing modern vocabulary.

Three kinds of questions show up again and again:

Spot the false clause. Four descriptions of natural selection, genetic drift, or fitness. Three contain one clause that gives an organism a goal, a need, or the ability to change itself. Find it.
The scenario with no data. Mice on sand, moths on soot, bacteria in a hospital. A population moves or the environment changes. You must decide what was already there, what got selected, and which way the frequency goes.
The data dump. Genotype counts in two years. An amino acid difference table. A cladogram with four traits marked. The math is small. The reading order is everything.

This page gives you the 15 ideas you need, the five moves you make on every question, and one fully dissected example of each question type. The Shmoigle Hunt page maps every rule of the lab onto these concepts.

Zoom out

The 15 Core Concepts of Unit 3

Every Unit 3 question is secretly about one or two of these. When a question feels new, your first job is to figure out which number it really is.

Early Earth and where organic molecules came fromEarth formed about 4.6 billion years ago, was too hostile for life until about 3.9, and the oldest fossils are about 3.5 billion years old. The Miller Urey experiment showed that the gases of the early atmosphere plus an energy source (sparks for lightning) produce amino acids and other organic molecules from inorganic ones. Organics may also have arrived on meteorites. Either way, the building blocks came before the builders.

Asked as: what Miller Urey demonstrated and what it did not (it made molecules, not life), and the trap that puts oxygen in the early atmosphere.

RNA world, first cells, endosymbiosisRNA was most likely the first genetic material because it can both store information and catalyze reactions (ribozymes), including its own copying. A membrane around it changed the chemistry inside, then DNA took over as the stable master copy and proteins took over catalysis. Prokaryotes came first. Eukaryotes came from one prokaryote engulfing another and keeping it (endosymbiosis), which is why mitochondria and chloroplasts have their own circular DNA and ribosomes.

Asked as: why RNA and not DNA first, the order of the steps, or the evidence for endosymbiosis.

Lamarck versus DarwinLamarck: an organism changes during its life through use and disuse, then passes the acquired change to its offspring. Stretch your neck, your kids get long necks. Darwin: the variation is already there at birth. Giraffes born with longer necks survive and reproduce more, and their offspring inherit the long neck. The difference is one question: did the trait exist before the environment tested it? Darwin says yes. The exam agrees with Darwin.

Asked as: never by name. It shows up as a wrong answer that says an organism "developed" or "acquired" a trait because it "needed" one. Any clause where an individual changes its own heritable traits is Lamarck, and it is red.

The four conditions of natural selectionVariation exists in the population. More offspring are born than can survive, so there is a struggle for existence (competition). Individuals with beneficial traits survive and reproduce more, so those traits become more common. The traits are heritable. Run that for many generations with a steady supply of new variation and you get adaptations, and eventually new species.

Asked as: a scenario missing one condition (a trait that is not heritable, a population with no variation) and you have to say why selection cannot act. Also as the giveaway describe on an FRQ.

Fitness is reproductive successFitness means leaving more surviving offspring than others in the population. Not strength, not speed, not living longest. "Survival of the fittest" is shorthand for the organisms best adapted to the current environment surviving to reproduce. A giraffe that reaches the leaves, survives, and breeds is fit. A giraffe that is huge and dies childless is not.

Asked as: which organism has the highest fitness in a table, and the trap always rewards the biggest or strongest one. Count offspring.

Selection acts on existing variation, the environment decidesAntibiotics do not create resistant bacteria; a few resistant mutants already existed and the drug killed everyone else. Soot did not darken the peppered moths; dark moths already existed and, once the trees were dark, birds stopped eating them. The environment chooses which existing phenotype wins. Change the environment and the winner changes.

Asked as: "what happens to the frequency of X when the environment changes," and as the trap that says the mutation "arose in response to" the change.

The three selection graphsDraw the population's trait distribution before (red) and after (blue). Directional: the whole curve shifts toward one extreme (giraffes get taller). Stabilizing: the middle is favored, the curve gets narrower (human birth weight). Disruptive: both extremes are favored, the curve splits into two humps (shell colors on a two colored beach). The one that can start speciation is disruptive.

Asked as: here is a before and after curve, name the mode. Or here is a scenario, predict the curve.

Artificial selectionSame mechanism as natural selection, different selector. Humans decide which individuals breed, based on traits humans want, so those traits become more common. Dogs, livestock, corn. The pug skull next to the wolf skull is the same number of generations of selection pointed somewhere nature never would have.

Asked as: a breeding program, and the question is whether a trait that humans chose for has to be good for the organism's survival. It does not.

Populations evolve, individuals do notNatural selection acts on individuals: this one survives, that one does not. But evolution is a change in the population: the gene pool is all the alleles in the population, and microevolution is a change in allele frequencies in that gene pool over generations. An individual never evolves. It is born with its alleles and dies with them.

Asked as: the single most common false clause in the unit. "The lizards evolved longer legs" is fine. "Each lizard evolved longer legs" is red.

What adds variation: mutation, gene flow, sexMutation is the only source of brand new alleles: a change in DNA can change an amino acid, which can change a protein's shape and function, which can change the phenotype and therefore fitness. Gene flow is individuals or gametes moving between populations, which brings alleles in and makes populations more alike. Sexual reproduction (crossing over, independent assortment, random fertilization) shuffles existing alleles into new combinations without making new ones.

Asked as: "Which process introduces a new allele?" Only mutation does. Sex rearranges, gene flow relocates.

What removes variation: nonrandom mating, drift, selectionNonrandom mating (sexual selection): traits that attract mates get passed on more, so their alleles rise and others fall. Genetic drift: allele frequencies change by chance, not fitness, and the effect is large in small populations. Founder effect: a few individuals start a new colony. Bottleneck: a disaster shrinks the population regardless of who was fit (cheetahs). Natural selection: advantageous alleles rise, the rest fall, and which alleles count as advantageous depends on the environment.

Asked as: a story about a population, and you must name which of the three it is. The word "chance" or "random" points to drift. The word "advantage" points to selection.

Hardy WeinbergA hypothetical population that is not evolving. Five conditions: very large population (no drift), no migration (no gene flow), no mutation, random mating, no natural selection. If all five hold, allele frequencies stay the same forever. p is the dominant allele frequency, q the recessive, p + q = 1. Genotype frequencies: p² (homozygous dominant), 2pq (heterozygous), q² (homozygous recessive), and p² + 2pq + q² = 1. The one number you can read from a phenotype count is q², because only homozygous recessives show the recessive trait. Take the square root to get q.

Asked as: 1 in 400 has the recessive condition, find the carriers. Or, genotype counts in two years, is the population evolving. If the frequencies moved, it is.

The evidenceFossils: today's organisms descend from ancestral ones, and the series shows the trend (the horse). Anatomy: homologous structures are the same underlying structure from a common ancestor doing different jobs (human arm, cat leg, whale flipper, bat wing); analogous structures are different structures doing the same job because of similar environments (bird wing, insect wing); vestigial structures are leftovers with little or no function (whale hip bones). Embryology: related species develop alike (every vertebrate embryo has gill pouches). Molecules: the genetic code is universal, so sequences can be compared, and closer relatives have fewer differences. Biogeography: related species live where geography put them (four flightless birds on four southern continents).

Asked as: which observation supports common ancestry, and the trap is a structure that does the same job but is built differently.

Cladistics, phylogeny, and evolutionary trendsA phylogeny is the evolutionary history of a group built from living species, fossils, and molecular data. A clade is a common ancestor plus every descendant. A node is the most recent common ancestor of the branches above it. Derived characters (synapomorphies) are shared traits that appeared once and are inherited by everything after that point. The outgroup branches first and shares the fewest derived traits. Molecular data beats morphology because convergent evolution makes unrelated species look alike. Divergent evolution gives homologous structures; convergent gives analogous ones; coevolution is two species shaping each other (flower and pollinator, predator and prey).

Asked as: build the tree from a difference table (fewest differences branch together), read a trait on a tree (everything above the mark has it), or explain why DNA and body shape disagree (trust the DNA).

Speciation and extinctionA species is a group that can interbreed and produce fertile offspring. New species form when populations stop sharing genes and evolve apart. Allopatric: a geographic barrier. Sympatric: same place, isolated some other way. Prezygotic barriers stop a zygote from forming (geographic, ecological, temporal, behavioral, mechanical, gametic). Postzygotic barriers act after (reduced hybrid viability, reduced hybrid fertility like the 63 chromosome mule, hybrid breakdown). Hybrid zones are where it is still being decided. Adaptive radiation: one ancestor, many new environments, many species fast (finches, honeycreepers, Manhattan rats). Gradualism versus punctuated equilibrium is about pace. Extinction is background (ongoing, one species at a time) or mass (many at once), and diversity is speciation rate minus extinction rate.

Asked as: classify a barrier as pre or post zygotic, or explain why two populations on either side of a new river are expected to diverge. No gene flow is the answer.

The math that rides along Hardy Weinberg is the only formula in the unit and it is on the equation sheet. What is not on the sheet: the recessive phenotype frequency is q², not q. Students lose the point by reporting q² as the allele frequency or by forgetting to square root. Say it every time: "that number is q squared."

The strategy

The Five Moves You Make on Every Question

Same five moves as Units 1 and 2. Same five moves as the SAT. In class you learned to read an AP question in three layers: background and context, the prompt, the answers. That is the same thing with the pieces named differently, so the table maps them.

What you did in class	What this course calls it
Background and context: find key words from class, phenomena from nature, and distracting content unique to the question	Dissect, and sort each clause: blue for class content, amber for scenario detail, nothing for filler
The prompt: find the keywords that connect it to class, note your first instinct before reading answers	Close read the stem, find the verb, say the answer in your own words before you look
The answers: phrases that are constant in every answer, unique words that make one 100 percent right or less than 100 percent correct	Hunt for right and wrong, shade the unique clauses, one red kills the choice
Do not skim the first few times	Scrutinize the survivors, then zoom in on the one word

In this order, every time

Close read the stem. Find the verb. "Describe" wants a fact. "Explain" wants a mechanism. "Predict" wants a direction: the frequency goes up or down. "Best supported" means the data decide, not your memory.
Dissect. Split every sentence into clauses. Each clause is its own claim and gets judged on its own.
Hunt for right and wrong. Shade each clause green (true) or red (false). In this unit, red usually means an individual changed, a trait appeared because it was needed, or fitness got confused with strength.
Scrutinize the survivors. If two choices have no red, one of them is answering a different question. Reread the verb.
Zoom in, zoom out. Zoom in on the one word: "each" versus "the population," "developed" versus "already had," "q" versus "q squared." Zoom out and name which of the 15 concepts this question is really about.

The Unit 3 version of the rule

An answer choice is 100% right or 100% wrong. In this unit the single false clause is usually a subject or a motive. Who changed: the population or the individual? Why did the trait appear: because it was needed, or because it was already there and the environment picked it? A choice can get the mechanism, the example, and the vocabulary perfect, and one Lamarckian clause makes it false.

When and how

The Three Question Types and What Each One Wants

Type	What it looks like in Unit 3	What actually wins
1. Spot the false clause	Four descriptions of natural selection, drift, fitness, or a Hardy Weinberg condition.	Clause by clause. Check every subject (individual or population) and every motive (needed, in order to, so that). Zero red wins.
2. Scenario, no data	A population moves, an environment changes, a drug is introduced. "Which best explains" or "predict."	Sort class words from scenario words. Decide what was already present, what the environment now favors, which frequency rises. Build the chain, then shade.
3. Data dump	Genotype counts, an amino acid difference table, a cladogram, a before and after curve.	Stem, text, table, figure. Compute p and q before you read a choice. Find the fewest differences before you draw a branch.

Quick filter for all three Ask: "Does this choice let anything evolve on purpose?" If an organism changes itself, wants something, or gets a mutation because it needed one, stop reading. It is out.

Worked example 1: Spot the false clause

How Natural Selection Produces an Adaptation

Lowest level question type, highest number of careless misses. Every choice below uses correct vocabulary. Three of them smuggle in Lamarck. Read the question plain first. Decide. Then turn on the X ray.

Question 1

Which of the following best describes how natural selection leads to an adaptation in a population?

AIndividuals that need a trait to survive develop it during their lifetimes, and because the trait improves survival it is passed to their offspring.

BHeritable variation already exists in the population, individuals with the most beneficial variant leave more offspring, and the variant becomes more common over generations.

CA beneficial mutation arises in response to a change in the environment, and the mutation then spreads because the strongest individuals carry it.

DVariation exists in the population, and the individuals with the trait best suited to the environment survive longer, so each individual gradually evolves toward that trait.

Move 1: Close read the stem The verb is describes. Fact check. The target is natural selection producing an adaptation in a population. Before you read a choice, say the four conditions in your own words: "Variation exists. More are born than survive. The ones with the better trait reproduce more. The trait is inherited." Every clause gets checked against that.

Move 5 (early): Zoom out This is Concept 4 (the four conditions) with Concept 3 (Lamarck), Concept 5 (fitness), and Concept 9 (populations, not individuals) as the distractors. The exam builds the wrong answers by keeping three true clauses and swapping one subject or one motive.

Green: factually correct Red: factually wrong

A Individuals that need a trait to survive develop it during their lifetimes, and because the trait improves survival it is passed to their offspring.

Wrong

This is Lamarck's giraffe. "Need" and "develop it during their lifetimes" are the two words that give it away. A trait an individual builds during its life (muscles, a tan, a stretched neck) is not in its DNA and is not inherited. Ties to Concept 3. Trap: the individual changes itself.

B Heritable variation already exists in the population, individuals with the most beneficial variant leave more offspring, and the variant becomes more common over generations.

Correct

Variation first, and it is heritable. Then differential reproduction, which is the definition of fitness. Then the population level result: the variant's frequency rises. Nobody wants anything, nobody changes themselves, and the thing that changes is the population. Zero red.

C A beneficial mutation arises in response to a change in the environment, and the mutation then spreads because the strongest individuals carry it.

Wrong

Mutations are random with respect to need. The dark fur allele in rock pocket mice existed before the lava flow; the lava made it useful, it did not make it appear. And "strongest" is not fitness. The mutation spreads because its carriers leave more offspring, whether or not they are strong. Ties to Concept 6 and Concept 5. Trap: mutation on demand, then fitness is strength.

D Variation exists in the population, and the individuals with the trait best suited to the environment survive longer, so each individual gradually evolves toward that trait.

Wrong

Two perfect clauses, then "each individual evolves." Individuals do not evolve. They are born with their alleles and die with them. The population's allele frequencies shift because some individuals reproduce more than others. Ties to Concept 9. Trap: right process, wrong subject. This is the one that catches students who know the material, because the first two thirds are flawless.

Look at what just happened Choice D was 90 percent correct and 100 percent wrong. The exam reuses this trick in every evolution question it writes. Find the subject of every verb. If the subject is an individual and the verb is "evolves," "adapts," or "develops," the clause is red.

Worked example 2: Scenario, no data

Rock Pocket Mice Move to the Sand

You have seen this question in class. That is the point. You read it there in three layers: background, prompt, answers. Watch the five moves run on the same question and notice they land in the same places, with one extra step: the causal chain you say out loud before you touch the choices.

Question 2

Rock pocket mice (Chaetodipus intermedius) in the American Southwest can have light brown fur or dark brown fur. These mice live on a landscape that includes light sand and dark lava rock. Mice that are camouflaged in the landscape are less likely to be caught by owls, which are their main predators. Scientists determined that dark brown fur in rock pocket mice is caused by a mutation in a gene called Mc1r. Mice with dark brown fur have this mutation, whereas mice with light brown fur do not.

If a population made up of light and dark mice was moved from an area with light sand and dark rock to an area with only light sand, which of the following describes the most likely result?

AOver many generations, the percentage of the population with the Mc1r gene mutation will decrease.

BOver many generations, the percentage of the population with the Mc1r gene mutation will remain the same.

COver many generations, a new Mc1r gene variant that causes mice to have medium brown fur will arise and become more common in the population.

DOver many generations, the percentage of the population with the Mc1r gene mutation will increase.

Step 1: Close read the stem before anything else

Find the verb Describes the most likely result. This is a prediction wearing a describe verb. The answer has to say what happens to a frequency over generations, with a direction. Every choice starts with "over many generations," which is the constant phrase you learned to ignore. The unique words are "decrease," "remain the same," "new variant will arise," "increase." That is the whole question: which direction, or no direction, or a new allele from nowhere.

Step 2: Dissect the scenario into two piles

Blue: you learned this in class Amber: specific to this question

Rock pocket mice in the American Southwest can have light brown fur or dark brown fur. These mice live on a landscape that includes light sand and dark lava rock. Mice that are camouflaged in the landscape are less likely to be caught by owls, which are their main predators. Scientists determined that dark brown fur is caused by a mutation in a gene called Mc1r. Mice with dark brown fur have this mutation, whereas mice with light brown fur do not.

The passage handed you two gifts First: the mutation already exists in the population. Dark mice are walking around right now. Second: the last sentence ties the phenotype to the allele one to one. Dark fur means mutant allele, light fur means no mutant allele. That sentence exists so that "percentage with the mutation" and "percentage that are dark" are the same number. Keep both loaded.

Step 3: Translate the scenario into your notes

The question says	Your notes call it	Concept
"light brown fur or dark brown fur"	Variation exists in the population. Condition one.	4
"camouflaged mice are less likely to be caught by owls"	The selection pressure, and the environment decides which color is camouflaged.	6
"dark fur is caused by a mutation in Mc1r"	The trait is heritable, it is an allele. Condition four.	10
"moved to an area with only light sand"	Environment changed. Dark mice are now visible on every surface. The winner flipped.	6, 11
"percentage of the population with the mutation"	Allele frequency in the gene pool. This is microevolution, measured.	9

Zoom in: the phrase that decides the question

"Only light sand." In the old landscape both colors had somewhere to hide, so both alleles persisted. Take away the dark rock and there is no longer anywhere a dark mouse is camouflaged. The exam put "only" in that sentence for one reason: to make the direction unambiguous. When a scenario removes an option, it is telling you which way the arrow points.

Step 4: Run the causal chain

The population moves to all light sandtherefore
Dark mice are visible on every surface; light mice are camouflaged everywheretherefore
Owls catch dark mice more often, so dark mice survive and reproduce lesstherefore
Fewer copies of the Mc1r mutant allele make it into each new generationtherefore
The frequency of the mutation in the population falls, generation after generation. Directional selection toward light.

Step 5: Now shade the answers

A Over many generations, the percentage of the population with the Mc1r gene mutation will decrease.

Correct

The right unit of measurement (percentage of the population, which is allele frequency) and the right direction (down). That is link five of the chain, in the exam's words.

B Over many generations, the percentage of the population with the Mc1r gene mutation will remain the same.

Wrong

Frequencies stay the same only when none of the five Hardy Weinberg conditions are violated. The passage built a selection pressure on purpose: owls, camouflage, a landscape with one color. Selection is happening, so the frequency moves. Trap: Hardy Weinberg where the passage says there is selection (Concept 12).

C Over many generations, a new Mc1r gene variant that causes mice to have medium brown fur will arise and become more common in the population.

Wrong

Mutations do not show up because they would be handy. And even if one did, medium brown on pure light sand is worse camouflage than light brown, so it would not spread. Trap: mutation on demand (Concept 6), then a direction the environment would not reward. This is the choice that catches students who remember "mutation creates variation" and forget that it does so randomly.

D Over many generations, the percentage of the population with the Mc1r gene mutation will increase.

Wrong

Right unit, wrong arrow. This is what would happen if the population had moved onto dark lava. Students pick D when they remember "dark fur was advantageous" from the deck and forget to check which landscape this question is in. Trap: flipped direction (Concept 6). The environment decides, and this environment is light.

Why the chain matters Without the chain you are choosing between "increase" and "decrease" from memory of a slide. With the chain, the direction is forced by the word "only." Build the chain every time, even when the question looks familiar. Especially then.

Worked example 3: The data dump

Is the Lizard Population Evolving?

This is the Unit 3 data question: genotype counts, a formula on the equation sheet, and a conclusion that depends on whether you squared, square rooted, or did neither. Read it in the right order and do the arithmetic before you read a single choice.

The reading order for every data question

Stem first. Scroll to the actual question. Find the verb.
Text second. Sort class words from scenario words. Underline any constraint the passage hands you.
Table third. Title, headers, units. Compute p and q from the counts. Write them down.
Figure last. The figure is usually the table drawn. Confirm it agrees with your numbers.
State the trend and the mechanism in one line before you read a single answer choice.

Question 3

In a species of lizard, tail color is controlled by a single gene with two alleles. The allele for a blue tail (B) is dominant to the allele for a brown tail (b). Researchers sampled 1,000 lizards from an island population in Year 1 and again in Year 10. Between the two samples, a bird that hunts by sight was introduced to the island. The genotype of every sampled lizard was determined directly from its DNA. Counts are shown in Table 1 and the resulting allele frequencies are plotted in Figure 1.

Table 1. Genotype counts in two samples of 1,000 lizards

Sample	BB	Bb	bb	Total
Year 1	360	480	160	1,000
Year 10	490	420	90	1,000

Figure 1. Allele frequencies in the two samples

Allele frequencies calculated from the genotype counts in Table 1.

Which of the following conclusions is best supported by the data?

AThe population is in Hardy Weinberg equilibrium, because the frequency of the dominant allele is greater than the frequency of the recessive allele in both samples.

BThe population is evolving, because the frequency of the b allele decreased from 0.40 to 0.30, which is consistent with selection by a visual predator against brown tailed lizards.

CThe population is evolving, because the frequency of the b allele decreased from 0.16 to 0.09, which shows that brown tailed lizards developed blue tails to avoid the predator.

DThe population is not evolving, because the total number of lizards sampled was the same in both years and the dominant phenotype remained the most common.

Step 1: Stem first

Find the verb, then the target "Best supported by the data." The data decide. Any choice that reaches past the table into a story the table does not tell is out, even if the story is good biology. And "conclusion" about a population over time in this unit means one thing: did the allele frequencies change?

Step 2: Text second, sorted into piles

Blue: from class Amber: from this question only

In a species of lizard, tail color is controlled by a single gene with two alleles. Blue (B) is dominant to brown (b). Researchers sampled 1,000 lizards in Year 1 and again in Year 10. Between the two samples, a bird that hunts by sight was introduced. The genotype of every lizard was determined directly from its DNA.

The passage handed you two gifts "Determined directly from its DNA" means you have real genotype counts, not phenotype counts. You do not need to square root anything to get q; you can count alleles. And "a bird that hunts by sight" is the mechanism, planted in advance, for whichever direction the data turn out to go. Keep both loaded.

Step 3: Table third, and do the arithmetic now

Count alleles, then divide by 2,000 Every lizard carries two alleles, so 1,000 lizards hold 2,000. Year 1: B alleles = 2(360) + 480 = 1,200, so p = 1,200 / 2,000 = 0.60, and q = 1 − 0.60 = 0.40. Check against Hardy Weinberg: p² = 0.36 (360 expected BB), 2pq = 0.48 (480 Bb), q² = 0.16 (160 bb). The Year 1 population matches the equilibrium prediction exactly. Year 10: B alleles = 2(490) + 420 = 1,400, so p = 0.70 and q = 0.30. The frequency of b dropped by a quarter in nine years.

Step 4: Figure last

Confirm, do not discover The figure plots 0.60 and 0.40, then 0.70 and 0.30. It agrees with your arithmetic. It adds nothing you did not already compute, which is normal. The figure is there to catch the student who skipped the table.

Step 5: One line before the answers

Say this, then read the choices

"The b allele went from 0.40 to 0.30. Allele frequencies changed, so the population is evolving. A sighted predator showed up in between, so the likely cause is selection against the brown phenotype." That is Concept 12 for the test and Concept 6 for the mechanism. Now every choice gets judged against that sentence.

Step 6: Shade

A The population is in Hardy Weinberg equilibrium, because the frequency of the dominant allele is greater than the frequency of the recessive allele in both samples.

Wrong

The green clause is true and irrelevant. Equilibrium means the frequencies do not change between generations. They changed. Which allele is bigger has nothing to do with it; a population at 0.9 and 0.1 can be in equilibrium and one at 0.5 and 0.5 can be evolving. Trap: true fact, wrong test (Concept 12).

B The population is evolving, because the frequency of the b allele decreased from 0.40 to 0.30, which is consistent with selection by a visual predator against brown tailed lizards.

Correct

The conclusion, the number that supports it (allele frequencies, not genotype frequencies), and a mechanism the passage planted. "Consistent with" is the right strength: the data show the change; the predator is the likely cause, not a proven one. Zero red.

C The population is evolving, because the frequency of the b allele decreased from 0.16 to 0.09, which shows that brown tailed lizards developed blue tails to avoid the predator.

Wrong

0.16 and 0.09 are q², the bb genotype frequencies, not the allele frequencies. Right conclusion, wrong number. Then the last clause hands you Lamarck: a brown lizard does not grow a blue tail. Brown lizards got eaten and left fewer offspring. Trap: right frequency, wrong symbol (Concept 12), then the individual changes itself (Concept 3).

D The population is not evolving, because the total number of lizards sampled was the same in both years and the dominant phenotype remained the most common.

Wrong

Both green clauses are true and neither one is the test. Sample size is a design choice. Which phenotype is most common says nothing about whether frequencies moved. Evolution is measured in allele frequencies, and those changed. Trap: true destination denied by the wrong evidence. Sample size and "most common" are the two numbers the exam dangles when it wants you to skip the arithmetic.

Bonus: the calculation the exam usually attaches

Question 3b

In a different, much larger lizard population on the mainland, 9 percent of lizards have brown tails. Assuming the population is in Hardy Weinberg equilibrium, what percentage of the population is heterozygous?

A9 percent

B16 percent

C42 percent

D49 percent

Zoom out, then plug in Brown is recessive, so "9 percent have brown tails" is a phenotype count, which means it is q², not q. Say it: "that number is q squared." q = √0.09 = 0.30. p = 0.70. Heterozygotes = 2pq = 2(0.70)(0.30) = 0.42, or 42 percent. Choice C. The traps: A reports q² again, B skipped the square root and used 0.09 as q (2 × 0.91 × 0.09 ≈ 0.16), D is p². Square root first. Then 2pq.

What to watch for

The Six Traps Unit 3 Sets

✗The individual changes itself. "Each lizard evolved," "the mice developed dark fur," "the bacteria adapted during treatment." Individuals are born with their alleles and die with them. Populations evolve. Find the subject of the verb.

✗Mutation on demand. "A mutation arose in response to," "in order to survive," "so that they could." Mutations are random with respect to need. The environment selects among variants that already exist.

✗Fitness is strength. Biggest, fastest, longest lived, most aggressive. Fitness is offspring that survive to reproduce. The table with a column for number of offspring is the whole answer.

✗Right frequency, wrong symbol. A phenotype percentage is q², not q. A genotype frequency is not an allele frequency. Hardy Weinberg questions are lost on a missing square root, not on biology.

✗Same job, same ancestor. A bird wing and an insect wing both fly. They are analogous, built from different parts, and they say nothing about common ancestry. Ask what it is made of, not what it does. When DNA and body shape disagree, trust the DNA.

✗Chance called selection, or selection called chance. A storm kills half the population at random: drift, not selection, no matter how the survivors look. Owls eat the visible ones: selection, not drift, no matter how small the population. The word "random" or "chance" points to drift. The word "advantage" points to selection.

✓The survivor: the population changes, nobody wanted anything, the variation was already there, the frequency has a direction, and the arithmetic used the right symbol.

The golden rule

An answer is 100% right or 100% wrong. One red clause makes the whole choice wrong, and in this unit the red clause is usually a subject or a motive. Before you read a choice, say which way the frequency goes and why. Then find the only choice where the population, not the individual, changes for that reason.

Ready to Practice?

10 questions for this unit: 3 spot the false clause, 3 scenarios, 4 data. Mark up the question, write what you need, hunt for red, then pick. Three scores per question.

Start the Unit 3 practice quiz Open the Shmoigle Hunt lab