AP Biology: Unit 7, Speciation (Drill 31)

Question 1. The divergence of the East Bank and West Bank lizard populations is best described as an example of which speciation mechanism?

• A) Allopatric speciation, because a geographic barrier (the river) physically separated the populations and prevented gene flow, allowing independent divergence. ✓
• B) Sympatric speciation, because the two populations now occupy different ecological niches (different prey) within the same river valley region.
• C) Parapatric speciation, because the populations are adjacent and the river may allow occasional contact along its banks.
• D) Artificial selection, because researchers selectively recorded only the most divergent traits and thereby drove the populations apart.

Explanation: Correct answer: A. Allopatric speciation occurs when a geographic barrier physically separates a population, preventing gene flow, after which isolated populations diverge through independent natural selection, mutation, and genetic drift. The river formed ~15,000 years ago as a continuous barrier the lizards do not cross, and the populations have since diverged in coloration, call frequency, and diet. (B) is incorrect because sympatric speciation occurs within the same geographic area without a physical barrier; these populations are physically separated. (C) is incorrect because parapatric speciation involves populations with adjacent, partially overlapping ranges that can occasionally exchange genes; the passage states the populations have never been in contact. (D) is incorrect because no human-directed breeding has occurred; researcher observation does not constitute artificial selection.

Question 2. The observation that hybrid offspring develop but reach only ~60% of normal adult size and rarely survive to reproduce indicates which type of reproductive isolating mechanism?

• A) Habitat isolation, because the two populations prefer different prey items and microhabitats and rarely meet to mate.
• B) Temporal isolation, because the populations may have shifted their breeding seasons over 15,000 years.
• C) Post-zygotic isolation, because offspring form and develop but have substantially reduced fitness relative to either parent population. ✓
• D) Behavioral isolation, because individuals show reduced mating preference when housed together in the lab.

Explanation: Correct answer: C. Reproductive isolating mechanisms are classified as pre-zygotic (preventing fertilization) or post-zygotic (occurring after a zygote forms). Hybrid offspring that form and develop but rarely survive to reproduce represent reduced hybrid fitness that manifests after fertilization; this is post-zygotic isolation. (A) habitat isolation and (B) temporal isolation are both pre-zygotic; neither describes what happens to offspring after they form. (D) behavioral isolation is also pre-zygotic, referring to differences in mate recognition before mating. The passage actually notes individuals will occasionally mate in the lab, suggesting behavioral isolation is incomplete, making C the best answer.

Question 3. A researcher claims: "The difference in mating call frequency between the two populations is evidence of natural selection, not genetic drift." Which additional data would most strengthen this claim?

• A) Evidence that the call-frequency gene mutates faster in one population than the other.
• B) Evidence that the specific call frequency range of each population improves mate detection or communication efficiency in its local acoustic environment. ✓
• C) Evidence that both populations produce identical calls when raised under identical laboratory conditions.
• D) Evidence that the call-frequency differences disappear entirely when the same calibrated microphones are used to record both populations.

Explanation: Correct answer: B. To argue a trait diverged due to selection rather than drift, one must show the trait has a functional consequence, that it affects survival or reproduction differently in each environment. If the specific call frequency of each population demonstrably improves mate detection or communication in its local conditions (e.g., due to differences in ambient noise level or vegetation density), that provides evidence for a functional advantage, supporting selection over drift. (A) is incorrect because a higher mutation rate increases variation but does not show selection favors any particular frequency. (C) is incorrect because identical calls under lab conditions would suggest the differences are environmentally induced (phenotypic plasticity), weakening the case for genetic selection. (D) is incorrect because heritability of coloration is a separate trait and does not address call-frequency divergence.

Question 4. Based on the trait comparison table, a student argues that mating call frequency shows "the least divergence" between the two populations because both populations still produce calls. Which of the following most directly identifies the flaw in this reasoning?

• A) The reasoning is correct; because both populations still produce calls, this trait is essentially unchanged and shows the least divergence of any trait compared.
• B) The reasoning is incorrect; the fact that both populations produce calls does not mean the trait is unchanged, the call frequency ranges (3,400–3,800 Hz vs. 2,900–3,200 Hz) do not overlap, indicating the trait has diverged substantially. ✓
• C) The reasoning is correct; call frequency can only be meaningfully compared in terms of presence or absence, not numeric magnitude.
• D) The reasoning is incorrect because mating call frequency is not a heritable trait and therefore cannot diverge between isolated populations.

Explanation: Correct answer: B. The student's error is equating "both populations still produce calls" with "the trait has not diverged." Presence of a trait is not the same as similarity in that trait. The call frequency ranges of the two populations (East Bank: 3,400–3,800 Hz; West Bank: 2,900–3,200 Hz) do not overlap at all, the lowest East Bank frequency (3,400 Hz) is still higher than the highest West Bank frequency (3,200 Hz). The trait has clearly diverged; the student is only noting that the general behavior (producing calls) is shared. (A) is incorrect for the same reason the student's original claim is incorrect: shared presence of a behavior does not mean the behavior is unchanged. (C) is incorrect; call frequency is a quantitative trait that can be compared numerically, and that comparison reveals meaningful divergence. (D) is incorrect; mating call frequency is heritable in most animals, and heritable variation is precisely what allows populations to diverge under selection or drift.

Question 5. If the river were to permanently dry up and the geographic barrier were removed, what would be the most likely long-term outcome, and what biological concept does this illustrate?

• A) Because post-zygotic isolation has already developed, gene flow would be limited even in secondary contact; natural selection may further strengthen pre-zygotic barriers through reinforcement, as individuals that mate within their own population produce more surviving offspring. ✓
• B) The two populations would immediately merge into a single population through unrestricted gene flow, reversing all divergence.
• C) The populations would hybridize freely and produce a new polyploid species with twice the chromosome number.
• D) The less fit of the two populations would go extinct within a single generation due to competitive exclusion once the river barrier was removed, because natural selection would immediately eliminate every individual carrying the less advantageous traits, at every locus involved.

Explanation: Correct answer: A. Post-zygotic isolation (reduced hybrid fitness) limits gene flow even without a physical barrier, hybrids rarely survive to reproduce, so mating across populations yields few viable descendants. This substantially constrains gene flow in secondary contact. When two populations with incomplete reproductive isolation come into contact, selection can strengthen pre-zygotic barriers through reinforcement: individuals that mate within their own population leave more surviving descendants, so traits promoting within-population mate recognition are favored. (B) is incorrect because post-zygotic isolation prevents effective gene flow even without the river; the populations cannot simply merge. (C) is incorrect; polyploidy from hybrid offspring requires specific chromosomal events not described here, and reduced-fitness lizard hybrids do not become polyploid species. (D) is incorrect; competitive exclusion, if it occurs, unfolds over many generations, not one, and is not the primary concept illustrated.