AP Biology: Unit 7, Phylogeny & Common Ancestry (Drill 32)

Question 1. Based on the cladogram described in the passage, which pair of species shares the most recent common ancestor?

• A) Species B and Species E
• B) Species C and Species D
• C) Species D and Species E
• D) Species A and Species B ✓

Explanation: Correct answer: D. The branching order is E (outgroup) → D → C → (A + B). A and B form a terminal clade; they share a node not shared with any other species. The most recent common ancestor is the one branching off most recently, shared by the fewest taxa. (A) is incorrect; B and E are the most distantly related pair; E is the outgroup and B is in the most derived clade. (B) is incorrect; C and D are not in a terminal clade together. (C) is incorrect; D and E are both basal lineages but do not form a clade; E is the outgroup to all others.

Question 2. According to the data in Table 1, which trait is a shared derived trait (synapomorphy) that unites Species A, B, and C to the exclusion of D and E?

• A) Vascular tissue
• B) Seeds ✓
• C) Flowers
• D) Comparable chloroplast DNA sequences across all five species

Explanation: Correct answer: B. Seeds are present in A, B, and C but absent in D and E. In the cladogram, seeds evolved in the ancestor of the clade (C + A + B), making seeds a synapomorphy, a shared derived trait, of that clade. (A) is incorrect; vascular tissue is present in D as well, making it a synapomorphy of the broader clade (D + C + A + B), not just A + B + C. (C) is incorrect; flowers are only present in A and B, not C, flowers are a synapomorphy of the (A + B) clade only. (D) is incorrect; comparable chloroplast DNA sequences are shared across all five species in the study, making them useful for comparison but not a derived trait uniting only A + B + C.

Question 3. A student states: "Species E must have lost vascular tissue over time, since it evolved from a common ancestor that had it." Evaluate this claim using the data.

• A) The claim is not supported by the data. E is the outgroup, most likely diverging before vascular tissue evolved in the lineage leading to A–D; the most parsimonious interpretation is that E's lineage never possessed vascular tissue. ✓
• B) The claim is supported because all species share a common ancestor, and that ancestor must have had vascular tissue since most species in the table do.
• C) The claim is supported because Species E has chloroplast DNA, which is found only in plants with vascular tissue.
• D) The claim cannot be evaluated without knowing the fossil record of Species E.

Explanation: Correct answer: A. The most parsimonious interpretation minimizes evolutionary events. E is the outgroup; its lineage diverged before the traits uniting A–D evolved. Vascular tissue is a derived trait of the ingroup (D + C + A + B). The simplest explanation is that it evolved once, in the ancestor of that clade, after E diverged. Invoking a secondary loss in E requires an additional evolutionary event with no supporting evidence. (B) is incorrect; the most common character state in a group is not necessarily ancestral, character polarity is determined by outgroup comparison. (C) is incorrect; chloroplast DNA is present in non-vascular plants such as mosses and liverworts, not exclusively in vascular plants. (D) is incorrect; parsimony analysis of available trait data is sufficient to evaluate the most likely interpretation.

Question 4. The chloroplast DNA sequence data is used alongside morphological traits in this study. Which of the following best explains why chloroplast DNA is often useful for reconstructing plant phylogenies?

• A) Chloroplast DNA mutates faster than nuclear DNA, providing more variable sites for comparison at short evolutionary timescales.
• B) Chloroplast DNA is found only in flowering plants, making it a reliable marker for the most derived lineages.
• C) Chloroplast DNA codes for identical proteins in all plant species, so sequence differences directly reflect evolutionary divergence without any confounding variation.
• D) Chloroplast DNA is often maternally inherited in many angiosperms and generally recombines less than nuclear DNA, which can reduce some sources of conflicting phylogenetic signal and make it useful for tracing lineage relationships. ✓

Explanation: Correct answer: D. Chloroplast DNA (cpDNA) is often maternally inherited in many angiosperms, transmitted through the egg cell rather than pollen, and generally undergoes less recombination than nuclear DNA. These features can reduce the mixing of genetic lineages that nuclear recombination causes, providing a more consistent signal for reconstructing evolutionary history. cpDNA inheritance patterns are not universal across all plant groups. (A) is incorrect, cpDNA is often useful not because it mutates faster, but because its inheritance pattern and relatively limited recombination can simplify lineage tracing; in many plant lineages cpDNA evolves more slowly than nuclear DNA, though rates vary across lineages and genome regions. (B) is incorrect; chloroplasts are found in non-vascular plants, algae, and many other photosynthetic organisms, not exclusively in flowering plants. (C) is incorrect; the premise is false, cpDNA does not code for identical proteins in all plants, and if it did, sequence comparison would be uninformative for phylogeny.

Question 5. A sixth species (F) is discovered with the following data: chlDNA 93% identity to A, vascular tissue present, seeds present, flowers absent. Based on the cladogram described in the passage, where would Species F most likely be placed?

• A) As the sister taxon to Species E (outgroup)
• B) As the sister taxon to Species D
• C) Within the (A + B) terminal clade
• D) As the sister taxon to Species C, within the seed-plant clade but outside the flowering plant clade. ✓

Explanation: Correct answer: D. Species F has: vascular tissue (shared with D + C + A + B), seeds (shared with C + A + B; absent in D), and no flowers (absent in C, D, E). This profile places F in the seed-plant clade but outside the flowering plant clade, the same position as C. F's chlDNA similarity (93%) is also close to C's (94%), supporting this placement. (A) is incorrect; F has vascular tissue and seeds, both absent in E. (B) is incorrect; F has seeds and D does not; F must be more derived than D. (C) is incorrect; F lacks flowers, while A and B both have them; F cannot be nested inside the flowering plant clade.