6 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 6
A cladogram based only on skull shape hypothesizes that two hypothetical rodent species, X and Y, share a more recent common ancestor with each other than either does with a third species, Z. Which investigation would best test this hypothesis?
Answer and reasoning
ANone is possible, since no one observed the ancestors of X, Y and Z diverging. A student who thinks past events cannot be studied scientifically picks this. The cladogram predicts patterns in evidence that exists today, such as DNA sequences and fossils, so it can be tested without observing the split.
BCompare the overall body size and fur color of X, Y and Z in the wild. A student who thinks species that look alike overall are the closest relatives picks this. Size and color can be similar in species that are not closely related and do not separate shared derived characters from ancestral ones.
CMeasure the branch lengths on the skull-based cladogram to see when X and Y split. A student who thinks a cladogram's branch lengths show time picks this. Branch lengths on a cladogram are not drawn to a time scale, and reading the skull-based cladogram again cannot test it; an independent kind of evidence is needed.
DCompare DNA sequences of several genes in X, Y, Z and an out-group species.Correct A cladogram is a hypothesis that can be tested with independent evidence. DNA sequences provide many characters that were not used to build the skull-based cladogram; if X and Y share derived sequence features not found in Z or the out-group, the hypothesis is supported.
A student draws a phylogenetic tree and a cladogram for the same four species. Which statement correctly describes a difference between the two kinds of diagram?
Answer and reasoning
AA cladogram's branch lengths typically show how many years have passed since each split. A student who reads every branching diagram as drawn to a time scale picks this. A cladogram's branch lengths are not scaled to time at all; a phylogenetic tree calibrated by fossils or a molecular clock can show time.
BA cladogram is built from DNA data, whereas a phylogenetic tree is built from fossils. A student who thinks the kind of data decides the kind of diagram picks this. Both can be built from morphological data or from DNA and protein sequences; they differ in whether time or amount of change is shown.
CA phylogenetic tree is a proven record of descent, whereas a cladogram is a hypothesis. A student who thinks a phylogenetic tree is a proven record picks this. Phylogenetic trees and cladograms are both hypotheses about relationships, revised as new evidence is found.
DA phylogenetic tree can show time since divergence; a cladogram shows only branching order.Correct Branch lengths on a phylogenetic tree can be scaled to time or to amount of change, calibrated by fossils or a molecular clock. A cladogram shows the order in which lineages branched, not a time scale or how different the groups are.
The table shows whether four traits, W–Z, are present (+) or absent (−) in four species of hypothetical lizards, A–D, and in an out-group species, O. Which trait is a shared derived character indicating that species A and B share a more recent common ancestor with each other than with species C or D?
Answer and reasoning
ATrait WCorrect Trait W is absent in the out-group, so it is derived, and among A–D only A and B have it. It most likely arose once, in the common ancestor of A and B, after their lineage split from the lineage leading to C and D.
BTrait X A student who takes any shared derived trait as evidence that two species are closest relatives picks this. Trait X is derived, but all four species A–D have it, so it groups A–D together without singling out A and B.
CTrait Y A student who treats any trait two species share as evidence of close relationship picks this. A and B both have trait Y, but so does the out-group, so having Y is the ancestral condition; its absence in C and D is the derived state, which groups C with D, not A with B.
DTrait Z A student who groups species by traits they both lack picks this. A and B lack trait Z, but so does the out-group, so lacking it is the ancestral condition; trait Z is a derived trait shared by C and D.
On a phylogenetic tree or cladogram, what does a node at which two lineages branch apart represent?
Answer and reasoning
AA living species from which the species at the tips of the branches evolved A student who thinks living species are ancestors of other living species picks this. Living species are at the tips; the ancestor at a node is a population that existed in the past.
BThe appearance of a new trait in the species that branch from that node A student who confuses nodes with the marks for traits picks this. Gains and losses of traits are marked on branches; a node stands for the ancestor in which lineages split, which need not coincide with a new trait.
CThe last ancestor shared by the lineages that branch from that nodeCorrect Each node marks a speciation event, in which an ancestral lineage split into two. The node therefore represents the most recent common ancestor of the groups that descend from it.
DThe point at which one lineage became more advanced than the other A student who sees evolution as progress picks this. A node records a split into two lineages; neither lineage is more advanced, and both continue to evolve.
A paleontologist hypothesizes that an extinct species, F, known only from fossil skeletons, is more closely related to living species G than to living species H. Which observation would best support this hypothesis?
Answer and reasoning
AF and G share a skeletal feature that H lacks but the out-group has. A student who treats any shared feature as evidence of close relationship picks this. Because the out-group has the feature, it was most likely present in the common ancestor of F, G and H; F and G simply kept it, and its absence in H is a loss in H's lineage, so it does not show that F is closer to G than to H.
BF's fossils are older than any fossils of G, so F is the ancestor of G. A student who thinks older fossil species are ancestors of later species picks this. An older fossil may belong to a side branch; age alone does not show descent or that F is closer to G than to H.
CF and G are similar in overall body size, and both are larger than H. A student who reads relatedness from overall similarity picks this. Body size can be similar in species that are not closely related; it does not separate shared derived characters from other similarities.
DF and G share a skeletal feature that is absent in both H and the out-group.Correct A feature absent in the out-group is derived. If F and G share it but H lacks it, it most likely arose once, in a common ancestor of F and G alone. This shared derived character, scored from fossil and living skeletons, supports the hypothesis.
Cladograms based on body features once placed whales outside the group of hoofed mammals that includes hippos, pigs and cows. DNA sequence data from many genes later indicated that hippos are the closest living relatives of whales, and the cladograms were redrawn. Which statement best relates these findings to how biologists use phylogenetic trees and cladograms?
Answer and reasoning
AThe earlier cladograms were hypotheses, and new DNA evidence led biologists to revise them.Correct The earlier cladograms were hypotheses based on the body features then studied. When DNA from many genes consistently supported a different grouping, the hypothesis was revised, which is how phylogenetic trees and cladograms are meant to be used.
BThe conflict between the two cladograms shows that whales share no common ancestors with land mammals. A student who thinks disagreement among trees counts against common descent picks this. Both cladograms assume common descent; they differ only in which group shares the most recent common ancestor with whales.
CAdding the DNA evidence made the new cladogram a proven fact that will not need further revision. A student who thinks a tree becomes fixed once it is well supported picks this. The new cladogram is still a hypothesis that could be revised again if new evidence required it.
DThe new cladogram shows that whales evolved from hippos, the closest living relatives of whales. A student who thinks one living species evolved from another picks this. Whales and hippos share a common ancestor; neither living group is the ancestor of the other.
In preparation: 0 of 6 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
7.9.A.1 Phylogenetic trees and cladograms as hypotheses Fix
Phylogenetic trees and cladograms as hypotheses
Branching diagrams that propose how lineages are related by common descent. Each is a hypothesis: it predicts patterns in further evidence, such as other DNA sequences or new fossils, and so can be tested.
Students often think Evolutionary relationships cannot be tested scientifically because no one observed the past events. In fact Yes. A tree predicts patterns in evidence that exists today, such as shared derived characters, DNA and protein sequences, and fossils. New data can support or contradict those predictions.
7.9.A.2 Phylogenetic tree Fix
Phylogenetic tree
A branching diagram of hypothesized evolutionary relationships whose branch lengths can show time or the amount of change, calibrated by fossils or a molecular clock.
Cladogram
A branching diagram that shows the hypothesized order in which lineages branched, based on shared characters. It does not show a time scale or how different the groups are.
Molecular clock
The use of DNA or protein sequence differences, which accumulate at an approximately steady rate in many genes, to estimate how long ago lineages diverged; the rate is calibrated with fossils of known age.
Students often think The lengths of the branches on a cladogram show how much time has passed since each split. In fact No. A cladogram shows only the order of branching; its branch lengths are not drawn to a time scale and do not show how different the groups are. A phylogenetic tree calibrated by fossils or a molecular clock can show time.
Students often think Cladograms are built from DNA data and phylogenetic trees from fossils, so the kind of data decides the kind of diagram. In fact No. Either can be built from morphological traits of living or fossil species or from DNA and protein sequences. They differ in what they show: a phylogenetic tree can show time or amount of change; a cladogram does not.
7.9.A.3 Out-group Fix
Out-group
The lineage in a phylogenetic tree or cladogram that is least closely related to the rest of the organisms. A trait it shares with some members of the in-group was probably present in their common ancestor, so comparison with the out-group shows which traits are ancestral and which are derived.
Derived trait
A trait that was gained or lost during evolution within the group being studied, so its state differs from the ancestral state seen in the out-group.
Shared derived character
A derived trait present in two or more lineages. It indicates that they inherited it from a common ancestor in which it arose, and so it is informative for building phylogenetic trees and cladograms.
Ancestral character
A trait that was already present in an older common ancestor, including that of the out-group. A trait that is ancestral within a group does not show which members of the group are most closely related.
Molecular data
Comparisons of DNA and protein sequences. Each aligned position is a separate character, so molecular data provide many characters and typically give more accurate and reliable evidence of relationships than morphological traits.
Students often think The lineage that branches off first, or the out-group, is the most primitive and has changed least since the common ancestor. In fact No. The lineages leading to all living species have been evolving for the same length of time since their common ancestor. An early-branching lineage may have changed as much as any other.
Students often think The out-group is the ancestor from which the other species on the tree evolved. In fact No. The out-group is a living lineage that shares a more distant common ancestor with the other species. It has been evolving for as long as they have since that ancestor, so it is a relative, not an ancestor.
7.9.B.1 Node Fix
Node
A branch point on a phylogenetic tree or cladogram. It represents the most recent common ancestor of the lineages that branch from it, and a speciation event in which one lineage split into two.
Most recent common ancestor
The latest ancestor shared by two lineages. Of two species, the one that shares the more recent common ancestor with a third species is more closely related to it.
Sister groups
Two lineages that branch from the same node and so share a common ancestor not shared with any other group on the tree.
Students often think The fewer nodes there are between two species on a tree, the more closely related the two species are. In fact Not necessarily. Relatedness is judged by how recent the most recent common ancestor of two species is, shown by the node where their lineages meet. The number of nodes between two tips depends on how many other lineages branched off along the way.
Students often think Species whose tips are drawn next to each other on a tree are the most closely related. In fact Not necessarily. Branches can be rotated around any node without changing the relationships, which changes the order of the tips. Relatedness is read from the nodes where lineages meet.
7.9.B.2 Morphological data Fix
Morphological data
Comparisons of body structures, which can be scored in living species and in fossil species, so extinct species can be placed on trees from their fossil remains.
Students often think A fossil species that lived earlier than related living species is their ancestor. In fact Not necessarily. Many fossil species belong to side branches that share an ancestor with living species. A fossil species is placed on a tree by its shared derived characters, not by its age alone.
Students often think Two species that differ from a reference species by the same amount are each other's closest relatives. In fact Not necessarily. Each percentage compares one species with the reference species only. Two species can differ from the reference by the same amount yet differ from each other at many sites, so their relationship to each other must be judged from a direct comparison.
7.9.B.3 Revision of trees Fix
Revision of trees
Trees and cladograms are revised when new evidence, such as sequences of more genes or newly discovered fossils, supports different relationships from those first proposed.
Students often think A phylogenetic tree is a proven record of descent, and once strong evidence supports a tree it is settled for good. In fact No. Phylogenetic trees and cladograms are hypotheses based on the evidence available. Even well-supported trees can be revised when new evidence, such as more DNA sequences or new fossils, supports different relationships.
Students often think When two trees for the same species disagree, the conflict shows that the species may not share common ancestry at all. In fact No. Competing trees all assume that the species share common ancestors; they disagree only about which groups share the most recent common ancestor. Resolving such disagreements with more evidence is how trees are refined.
8 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 8
The phylogenetic tree shows the hypothesized evolutionary relationships among five living species of a hypothetical genus of beetles, A–E. Which statement is supported by the tree?
Answer and reasoning
ASpecies A is more closely related to species D than to species C. A student who reads relatedness from which tips are drawn next to each other picks this. A's tip is beside D's, but A shares the node joining A, B and C with C, which is more recent than the node A shares with D.
BSpecies D is more closely related to species C than it is to A. A student who counts nodes picks this: two nodes lie between D and C but three lie between D and A. D's most recent common ancestor with C and with A is the same node, so D is equally closely related to both.
CSpecies C is more closely related to species B than to D.Correct C's lineage meets B's at the node that joins C to the pair A and B. C meets D only at the older node that joins D to A, B and C. C's most recent common ancestor with B is therefore more recent than its most recent common ancestor with D.
DSpecies E is the ancestor from which species A, B, C and D evolved. A student who reads the tree as a ladder in which the first-branching living species gave rise to the rest picks this. E is a living species at the tip of its own branch; E and the other four share a common ancestor at the root.
The table shows whether four traits are present (+) or absent (−) in an out-group species, O, and in four related species, P–S, of a hypothetical group of animals. Assuming that each trait was gained or lost as few times as possible, which of the numbered cladograms best represents the relationships among the five species?
Answer and reasoning
ACladogram 1 A student who groups species because they both lack traits picks this. O and P both lack traits 2 and 3, but the out-group lacks them too, so that absence is the ancestral condition. Pairing O with P also contradicts O's role as the out-group, the lineage least closely related to all the others.
BCladogram 2Correct Traits 1, 2 and 3 are present in nested sets of species, P–S, Q–S and R and S, so the branching order is O, then P, then Q, with R and S as sister species. S lacks trait 4, which O and the other species have, so trait 4 was most likely lost once in S's lineage. This cladogram needs only four changes in all.
CCladogram 3 A student who thinks living species evolved from one another picks this. P, Q, R and S are living species, each at the tip of its own branch; a cladogram shows their common ancestors as nodes, not as a line of living species.
DCladogram 4 A student who thinks a species lacking a trait must have branched off before it evolved picks this. S has traits 1, 2 and 3, so placing S first would need trait 2 and trait 3 each to change twice; one loss of trait 4 in S's lineage explains the data with fewer changes.
The phylogenetic tree shows the hypothesized relationships among four living species of hypothetical rodents, J–M, and an extinct species, N, known only from fossils. The tree is calibrated with fossils of known age. Which statement is supported by the tree?
Answer and reasoning
ASpecies K and M have a more recent common ancestor than N and M, as their node is younger.Correct The node that joins K to the L–M pair is at about 25 million years ago, and the node that joins N to the K–L–M group is at about 35 million years ago. The node shared by K and M is therefore about 10 million years younger: their common ancestor lived more recently.
BSpecies J is the most primitive of the living species, since its lineage branched first. A student who thinks the first lineage to branch off is primitive picks this. J's lineage has been evolving for the same 45 million years since the root as the lineage leading to each of the other living species.
CSpecies N, a fossil species, is the ancestor from which species K, L and M evolved. A student who assumes that fossil species are the ancestors of living ones picks this. N is at the end of its own branch, which stops about 20 million years ago; N shares a common ancestor with K, L and M at the node about 35 million years ago but is not that ancestor.
DSpecies M has evolved more than J, because more branch points lie along its lineage. A student who thinks more branch points mean more evolution picks this. Every living lineage has had the same 45 million years to change since the root; nodes record splits into separate lineages, not amounts of change.
Biologists studying a group of hypothetical land snails find that a cladogram based on shell shape conflicts with a cladogram based on the DNA sequences of several genes. Which statement identifies the cladogram that should usually be given more weight and gives a valid reason?
Answer and reasoning
AThe shell cladogram, because shell shape can be observed and measured directly. A student who trusts what can be seen over molecular evidence picks this. Being visible does not make a trait reliable; a few shell features give few characters to compare.
BThe DNA cladogram, because the genes provide hundreds of characters that can be compared.Correct Each aligned nucleotide position is a separate character, so the genes provide far more characters than a few shell features, and the result does not hang on a handful of traits. This is why molecular data typically give more accurate and reliable evidence than morphological traits.
CThe DNA cladogram, because a tree built from DNA is proven and will not need revision. A student who thinks a well-supported tree is a proven record picks this. Molecular data usually give more reliable evidence, but a cladogram built from DNA is still a hypothesis that can be revised as more genes or species are added; the valid reason is the large number of characters compared.
DThe shell cladogram, because shell complexity shows which of the snails are more advanced. A student who sees evolution as progress toward complexity picks this. No living snail is more advanced than another; each lineage has evolved for the same time since their common ancestor.
The cladogram shows the hypothesized relationships among three species of hypothetical frogs, K, L and M, and an out-group, O. Marks show where two derived traits arose. A student claims that K and L are more closely related to each other than either is to M. Which reasoning best supports the claim?
Answer and reasoning
AK and L share trait 1, which arose before the lineage leading to M split off. A student who uses any shared trait as evidence of close relationship picks this. Trait 1 is also present in M, so it is ancestral within this group of three and cannot show that K and L are each other's closest relatives.
BK and L share trait 2, which arose after the lineage leading to M had split off.Correct Trait 2 is a shared derived character of K and L: it arose in their common ancestor after that lineage split from M's. Only K and L share this ancestor, so they are more closely related to each other than either is to M.
CK and L have more derived traits than M, so K and L are the most evolved of the three. A student who sees evolution as progress picks this. Having more derived traits does not make a species more evolved, and the number of traits is not what groups K with L; the shared derived trait 2 is.
DM branched off first, so M is the ancestor from which both K and L later evolved. A student who thinks an early-branching living species is the ancestor of the others picks this. M is a living species at the tip of its own branch; M, K and L share an ancestor at the node where M's lineage split off.
Researchers studying four species of hypothetical beetles, P–S, compare 20 traits in each species. They also score the same traits in a fifth species, T, which they include as an out-group. Which statement best justifies including species T?
Answer and reasoning
AT is the ancestor of species P–S, so its traits show what their common ancestor looked like. A student who thinks the out-group is the ancestor of the other species picks this. T is a living species that has been evolving for as long as P–S since their common ancestor; it is a relative, not the ancestor.
BT is unrelated to species P–S, so the traits it shares with them probably arose by chance. A student who thinks the out-group is unrelated to the species studied picks this. The out-group must be related to P–S, only less closely than they are to each other, or its traits could not show which traits are ancestral.
CT is a more distant relative, so a trait it shares with some of P–S is likely ancestral, not derived.Correct The out-group is the lineage least closely related to the other species, so a trait it shares with some of P–S was most likely present in their common ancestor. Comparison with T separates ancestral traits from derived ones, and only shared derived traits are informative for grouping P–S.
DT is the most primitive of the five species, so it has changed least since they diverged. A student who thinks the least closely related lineage is primitive picks this. T has been evolving for as long as P–S since they shared an ancestor and may have changed as much as any of them.
A cladogram based on body form hypothesized that two hypothetical fish species, R and S, share a more recent common ancestor with each other than either does with a third species, T. The table shows the number of nucleotide differences between each pair of species in three genes. Which conclusion about the hypothesis is best supported by the data?
Answer and reasoning
AThe data do not support it; in all three genes R differs least from T, so revise it.Correct In each gene, R and T differ at the fewest sites (12, 9 and 15), while S differs from both R and T at many more (37 to 51). Three independent genes agree that R and T are the closest relatives, so the body-form hypothesis should be revised.
BThe data do not affect it, since body form is more reliable evidence than DNA sequences. A student who trusts body form over molecular data picks this. Molecular data typically give more reliable evidence than morphological traits, and here three genes agree with each other against the body-form hypothesis.
CThe data show that R, S and T do not share common ancestry, since the two kinds of evidence conflict. A student who thinks conflicting evidence undermines common ancestry picks this. Both kinds of evidence assume that the three species share ancestors; they disagree only about which two are most closely related.
DThe data prove that R and T are sister species, so the new cladogram is now a settled fact. A student who treats strong support as proof picks this. A cladogram revised to group R with T is still a hypothesis that further evidence could change.
Working Compare the pairwise differences gene by gene. Gene 1: R and T 12, R and S 42, S and T 44. Gene 2: R and T 9, R and S 37, S and T 39. Gene 3: R and T 15, R and S 51, S and T 50. In every gene R and T differ at the fewest sites, and S differs from R and from T by similar, much larger numbers. The molecular data therefore support R and T, not R and S, as the most closely related pair, so the body-form hypothesis is not supported and should be revised. The revised cladogram remains a hypothesis.
A gene was sequenced in five hypothetical species of fungi, A–E. The graph shows the percentage of nucleotide sites in this gene at which each of species B–E differs from species A. Which statement is supported by the data?
Answer and reasoning
AB differs from A at the fewest sites, so B is a species that evolved from species A. A student who thinks one living species can be the ancestor of another picks this. A and B are both living species; a small difference indicates a recent common ancestor, not that B descended from A.
BE differs from A at the most sites, so E is probably the most evolved of the five species. A student who sees evolution as progress picks this. E's larger difference indicates an older common ancestor with A; the lineages of A and E have evolved for the same time since then, and neither is more evolved.
CB differs from A at the fewest sites, so B is probably A's closest relative among B–E.Correct B's sequence differs from A's at 2% of sites, fewer than C (7%), D (7%) or E (12%). Because sequence differences accumulate in lineages after they split, the smallest difference suggests that A and B share the most recent common ancestor of these pairs.
DC and D differ from A at the same percentage of sites, so C and D are each other's closest relatives. A student who groups species because they differ from a reference species by the same amount picks this. Each value compares one species with A only; C and D could differ from each other at many sites, and the graph does not compare them directly.
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
