3 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 3
Which observation is evidence that very different organisms share common ancestry?
Answer and reasoning
ABacteria, plants and animals use nearly the same genetic code for proteins.Correct The genetic code is nearly universal: the same codons specify the same amino acids in bacteria, plants and animals. This shared biochemical feature is best explained by inheritance from a common ancestor.
BBirds, bats and insects all have wings, which they use to fly through the air. A student who thinks any similarity shows close relatedness picks this. Insect wings and vertebrate wings evolved independently and have different structures, so shared wings do not show common ancestry of these groups.
CFish, whales and penguins all live in the ocean and mostly feed on smaller animals. A student who thinks shared environments make organisms alike, and therefore related, picks this. Sharing a habitat and diet says nothing about ancestry; these animals belong to different lineages.
DOrganisms with larger bodies have more DNA in each cell than smaller organisms. A student who thinks DNA amount follows size or complexity picks this. The amount of DNA per cell does not follow body size; the statement is false, and DNA amount does not show ancestry.
Carbon-14 has a half-life of about 5,730 years. A paleontologist finds a fossil bone in rock layers that are about 70 million years old. Which statement about dating this fossil is correct?
Answer and reasoning
AIt is dated by its depth in the rock, as every meter of rock took the same time to form. A student who thinks rock layers build up at a steady rate picks this. Layers form at very different rates, so depth alone does not give an age; the fossil is dated from volcanic layers around it, using isotopes with long half-lives.
BIts carbon-14 decayed with a shorter half-life, because the fossil holds so little carbon. A student who thinks half-life depends on the amount of material picks this. The half-life of carbon-14 is the same in every sample; the problem is that after millions of years almost none is left.
CIt can be dated from the carbon-14 in the volcanic rock layers just above and below it. A student who thinks carbon-14 dates any material picks this. Carbon-14 is used for the remains of once-living organisms up to about 50,000 years old; volcanic rock is dated with other isotopes.
DToo little carbon-14 would remain, so other isotopes in nearby volcanic rock are used.Correct 70 million years is over 12,000 half-lives of carbon-14, so essentially none remains. The fossil's age is found from the rock layers around it, by dating volcanic layers with isotopes that have much longer half-lives.
The figure shows the same 20-nucleotide segment of a gene in four hypothetical species, P, Q, R and S. Nucleotides that differ from species P are shown in bold and underlined. Which claim is best supported by the data?
Answer and reasoning
AP evolved from Q, because the two species have identical sequences in this gene segment. A student who thinks a very similar living species must be the ancestor of another picks this. Identical segments show that P and Q share a recent common ancestor; neither living species is the ancestor of the other.
BP and Q share a more recent common ancestor than either does with S, as S differs more.Correct P and Q have identical segments, while S differs from P at 7 positions (and from Q at the same 7). Fewer differences indicate a more recent common ancestor, so P and Q are more closely related to each other than either is to S.
CP and Q have stopped evolving, as this segment has not changed in either species. A student who equates evolution with change in one feature picks this. One unchanged 20-nucleotide segment does not show that a species has stopped evolving; other genes and allele frequencies can still change.
DS is the most advanced of the four species, because its segment has changed the most. A student who sees evolution as progress picks this. Differences from P show how long ago S's lineage split from P's, not how advanced S is; all four species are equally products of evolution.
Working Differences from P: Q 0; R 3 (positions 6, 15, 20); S 7 (positions 3, 6, 9, 15, 17, 19, 20). Q differs from S at the same 7 positions.
In preparation: 0 of 3 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
7.6.A.1 Evidence for evolution Fix
Evidence for evolution
Evolution is supported by data from many disciplines, including geographical, geological, physical, biochemical and mathematical data. Independent kinds of data that point to the same history make the case for it stronger.
Geographical (biogeographical) evidence
Data on where species live. For example, species on oceanic islands often most resemble species on the nearest mainland, as expected if they descended from ancestors that colonized the islands from there.
Biochemical evidence
Similarities in the molecules of organisms, such as a nearly universal genetic code, shared molecules such as DNA, RNA and ATP, and similar DNA and protein sequences, that indicate descent from common ancestors.
Students often think Similar environments directly produce similar organisms, so species that live in similar places resemble each other because of their surroundings. In fact No. An environment does not create traits in organisms. It can favor some existing variants by natural selection, but the traits a population can evolve depend on the variation inherited from its ancestors.
Students often think Organisms develop the traits they need for their way of life; the need causes the trait to appear. In fact No. New variation arises by random mutation and is not produced in response to need. Traits that help organisms survive and reproduce become common over generations by natural selection.
7.6.B.1 Extant and extinct organisms Fix
Extant and extinct organisms
Extant organisms are living today; extinct organisms are known from fossils. Molecular, morphological and genetic evidence from both adds to our understanding of evolution.
Fossil
Preserved remains or traces of an organism that lived in the past, usually found in sedimentary rock. Fossils show the forms of extinct organisms and, once dated, when they lived.
Dating by rock layers
In undisturbed sedimentary rock, lower layers formed before the layers above them, so fossils in lower layers are older. A fossil lying between two layers of known age is between those two ages.
Radiometric dating and half-life
Radioactive isotopes decay at a constant rate. The half-life is the time for half of the atoms of an isotope in a sample to decay, so after each half-life the amount remaining halves. Measuring how much of an isotope remains gives the age of a sample, such as a volcanic rock layer.
Carbon-14 dating
Living organisms take in carbon-14 along with other carbon. After death, the carbon-14 decays with a half-life of about 5,730 years, so the fraction remaining gives the time since death. It is useful only for remains up to roughly 50,000 years old, after which too little remains to measure.
Homologous structures
Structures in different species that share the same basic arrangement of parts because they were inherited from a common ancestor, even if they now have different functions, such as the forelimb bones of humans, bats and whales.
Vestigial structure
A structure that is greatly reduced compared with the corresponding structure in related species and has lost most or all of its ancestral function, such as small limb bones in a legless animal. It is evidence of descent from ancestors in which the structure was functional.
Students often think Individual organisms change into new forms during their lives, and this is how a species evolves. In fact No. Evolution is a change in the inherited characteristics of a population over generations. An individual can change during its life, but those changes are not passed on through its genes.
Students often think The half-life of an isotope depends on the size of the sample, so a sample with less material decays faster. In fact No. The half-life of an isotope is a fixed property of that isotope. A sample with less carbon has fewer carbon-14 atoms, but half of them still decay in about 5,730 years.
7.6.B.2 Sequence comparison Fix
Sequence comparison
Comparing the nucleotide sequence of a gene, or the amino acid sequence of a protein, among species. Species whose sequences differ less generally share a more recent common ancestor, because differences accumulate in each lineage after the lineages split.
Students often think When two variables are correlated, one of them must be causing the other, and either one can be taken as the cause without asking which change came first. In fact No. A correlation shows that two variables change together; it does not show which, if either, is the cause. In sequence data, the differences between two living species arose after their lineages split, so the time since divergence explains the number of differences, not the reverse.
Students often think When two living species are very similar, the one that looks more ancestral is the ancestor of the other, as in the idea that humans evolved from chimpanzees. In fact No. Two living species with very similar sequences share a recent common ancestor; neither is the ancestor of the other. Both lineages have continued to evolve since they split.
7 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 7
The bird species on a group of young volcanic islands far out at sea most closely resemble a bird species on the nearest mainland, although each island species has its own beak shape. Which explanation of this pattern is best supported by evolutionary theory?
Answer and reasoning
AMainland birds that flew to the islands changed during their lives into the island species. A student who thinks individuals evolve during their lives picks this. The colonists did not change into new species; their populations changed over many generations.
BEach island species developed the beak shape that it needed for the food on its island. A student who thinks traits appear because organisms need them picks this. Beak shapes changed because birds with beaks suited to the available food left more offspring, not because the need produced the beaks.
CThe island species descended from mainland birds that colonized the islands and then diverged.Correct Birds from the nearest mainland are the likeliest colonists of new islands. Their descendants inherited mainland features, and populations on different islands then diverged, for example in beak shape, giving several species that still resemble the mainland species.
DThe islands' climate is like the mainland's, so it produced the same kinds of birds there. A student who thinks environments directly produce similar organisms picks this. A climate cannot create birds; the island birds resemble mainland birds because they descended from them.
The diagram shows undisturbed rock layers in a cliff. The two volcanic ash layers were dated by radiometric dating. Fossil X belongs to a species with a more complex body plan than the species of fossil W. Which conclusion about the age of fossil X is best supported?
Answer and reasoning
AIt is about 55 million years old, from the carbon-14 that is still present in the fossil. A student who thinks carbon-14 dates fossils of any age picks this. After tens of millions of years no measurable carbon-14 remains; the fossil's age comes from the dated ash layers.
BIt is between 52 and 58 million years old, from the dated ash layers above and below it.Correct In undisturbed layers, lower layers are older. Fossil X lies in the shale above the 58-million-year-old ash and below the 52-million-year-old ash, so it is younger than 58 and older than 52 million years.
CIt is younger than fossil W, since its species had a more complex body plan than W's. A student who thinks complexity shows how recent a species is picks this. Fossil W lies above the 52-million-year-old ash and fossil X below it, so X is older than W whatever their body plans.
DIt is exactly 55 million years old, as it lies midway between the two dated ash layers. A student who thinks rock layers build up at a steady rate picks this. Deposition rates vary, so the fossil's position within the shale does not give an exact age; it shows only that X is between 52 and 58 million years old.
Working Superposition: fossil X is above the ash dated 58 million years and below the ash dated 52 million years, so 52 < age of X < 58 million years. Fossil W is above the 52-million-year ash, so W is younger than 52 million years and younger than X.
The diagram shows the forelimb bones of a human, a bat and a whale. Bones of the same kind are drawn in the same style. Which statement best describes what the diagram shows and how it provides evidence of common ancestry?
Answer and reasoning
AThe same kinds of bones, in the same order, serve different functions, as expected from shared ancestry.Correct Each limb has a humerus, then a radius and ulna, then carpals, then digits, although the limbs are used for grasping, flying and swimming. A shared plan used for different jobs is best explained by inheritance from a common ancestor: these are homologous structures.
BThe limbs have the same kinds of bones because all three limbs are used to do the same job. A student who thinks shared bones reflect a shared function picks this. The human arm grasps, the bat wing flies and the whale flipper swims; the bones are shared despite the different functions.
CThe human arm is the most advanced of the three, and the other two limbs are evolving toward it. A student who sees evolution as progress toward humans picks this. The three limbs are equally products of evolution, each suited to its own way of life, and none is evolving toward another.
DEach species built the same set of bones on its own because its way of life required it. A student who thinks needs produce traits picks this. Three very different ways of life would not produce the same set of bones independently; the shared plan was inherited.
A hypothetical species of burrowing lizard has tiny limb bones under its skin that are not used for movement. Closely related lizard species that live above ground have fully formed legs with the same bones. Which reasoning best connects these observations to common ancestry?
Answer and reasoning
AThe lizard's ancestors stopped using their legs, so each later generation was born with smaller ones. A student who thinks disuse causes inherited shrinkage picks this. Changes from use or disuse during an animal's life are not passed on in its DNA; reduction happens through the inherited variation that individuals pass on.
BThe bones are developing into new structures that the burrowing lizard is going to need in the future. A student who thinks evolution prepares for future needs picks this. Evolution cannot anticipate needs; the bones match the legs of relatives and are remnants of ancestral legs.
CThe lizard inherited limb bones from legged ancestors, and they were reduced over many generations.Correct The tiny bones match the leg bones of related species, so they are best explained as vestigial structures inherited from a legged common ancestor and reduced over many generations in the burrowing lineage.
DEach lizard's legs shrank during its own life as it adapted to burrowing through the soil. A student who thinks individuals evolve during their lives picks this. The limbs are tiny in every lizard of the species from birth; the reduction happened in the lineage over generations, not within each lizard's life.
For six pairs of hypothetical species, researchers used fossils to estimate how long ago each pair's lineages diverged. They also counted the amino acid differences between the two species' versions of a protein found in both. The graph shows the data. Which statement best describes the relationship shown?
Answer and reasoning
AAmino acid differences in the protein caused the two lineages to split apart. A student who takes a correlation as showing cause, without asking which change came first, picks this. The differences between the two species arose after their lineages split; the time since divergence explains the number of differences, not the reverse.
BThe pair with the fewest differences shows that one of its species is the ancestor of the other. A student who thinks a very similar living species must be the ancestor of the other picks this. Four differences show that the two species share a recent common ancestor; both lineages have changed since the split.
CThe protein data contradict the fossil dates, because the points do not lie exactly on a line. A student who expects data to match a relationship exactly picks this. The points scatter only a little around a rising trend; a strong trend despite scatter is agreement between the two kinds of data.
DPairs whose lineages diverged longer ago have more amino acid differences in the protein.Correct The points rise from 4 differences at 20 million years to 55 differences at 320 million years: the longer ago two lineages split, according to the fossils, the more differences their proteins have. The molecular and fossil data agree.
A biologist hypothesizes that a hypothetical rodent species, X, is more closely related to species Y than to species Z. Which investigation would best test this hypothesis?
Answer and reasoning
ACompare one gene's sequence in all three species, since differences build up over time.Correct Differences in DNA sequence accumulate after lineages split, so the pair of species with the fewest differences in a shared gene is likely to share the most recent common ancestor. Comparing the X–Y and X–Z differences tests the hypothesis.
BCompare the habitats of X, Y and Z, since closely related species live in similar places. A student who thinks similar environments produce similar species picks this. Distantly related species can share a habitat and close relatives can live in different ones, so habitat does not measure relatedness.
CMeasure the amount of DNA in a cell of each species, since related species have equal amounts. A student who thinks DNA amount reflects relatedness picks this. The amount of DNA can differ greatly between related species; relatedness is measured by comparing sequences.
DCompare the body sizes of X, Y and Z, since closely related species are similar in size. A student who thinks any similarity shows close relatedness picks this. Body size can be similar in distant relatives and different in close ones, so it is not a reliable test.
Researchers measured the thickness of fossil shells of a hypothetical snail lineage from rock layers of four ages and of shells of the living species descended from it. The graph shows the mean thickness of each sample; error bars show ±2 SE of the mean. Which conclusion is supported by the data?
Answer and reasoning
AShells were equally thick 4 million and 3 million years ago, as the error bars for those two samples overlap. A student who thinks overlapping error bars prove equal means picks this. The overlap (1.10 to 1.30 and 1.17 to 1.35 mm) means only that these data show no significant difference; the means could still differ.
BShells were likely thicker in each sample than in the sample before it, as each mean is larger. A student who treats any difference in means as real picks this. The bars for 4 and 3 million years ago overlap, and so do those for 1 million years ago and the living species, so those increases may be due to chance.
CIndividual snails thickened their shells during their lives as the lineage evolved over time. A student who thinks individuals evolve during their lives picks this. The data compare samples from different times; the change is in the lineage across generations, and the data say nothing about growth within one snail's life.
DShells were likely thicker 2 million years ago than 3 million years ago, as those error bars do not overlap.Correct The bars for 3 million years ago (1.17 to 1.35 mm) and 2 million years ago (1.44 to 1.60 mm) do not overlap, so the increase in mean thickness is likely to be a real difference, not sampling variation.
Working ±2 SE intervals: 4 Mya 1.10–1.30; 3 Mya 1.17–1.35; 2 Mya 1.44–1.60; 1 Mya 1.65–1.83; living 1.71–1.85 mm. Non-overlapping (likely different): 3 vs 2 Mya, 2 vs 1 Mya. Overlapping (no evidence of difference): 4 vs 3 Mya, 1 Mya vs living.
Compiled from the AP Biology Course and Exam Description (effective Fall 2025) and our question bank · Specialist review in progress. How these pages are made · Free, no account
