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About This Drill
AP Biology — Unit 7 — Speciation — Drill 31 is a practice drill. It contains 5 original questions created by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.
Examine a case of population divergence in lizards separated by a geographic barrier. Analyze trait differences, reproductive isolation evidence, and the role of selection and isolation in speciation to evaluate claims about evolutionary processes.
Passage
Two populations of a ground-dwelling lizard inhabit opposite sides of a wide, fast-moving river in a semi-arid region. The river formed approximately 15,000 years ago due to tectonic activity and increased rainfall, creating a continuous barrier that the lizards do not cross. The populations — referred to as East Bank and West Bank — are estimated to have been similar in all traits at the time of separation.
Today the populations differ as follows:
| Trait | East Bank | West Bank |
|---|
| Body coloration | Pale sandy brown | Darker reddish-brown |
| Mating call frequency (Hz) | 3,400–3,800 | 2,900–3,200 |
| Primary prey item | Ground beetles | Moth larvae |
| Hybrid fitness (lab crosses) | Hybrids develop but reach only ~60% of normal adult body size and rarely survive to reproductive maturity |
The two populations have never been observed in contact in the wild. In laboratory trials, individuals occasionally mate when housed together, but hybrid offspring rarely survive to reproduce.
Questions in This Drill
- The divergence of the East Bank and West Bank lizard populations is best described as an example of which speciation mechanism?
- 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 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?
- 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?
- 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?