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About This Drill
AP Biology — Unit 7 — Population Genetics — Drill 30 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.
Apply Hardy-Weinberg equations to calculate allele and genotype frequencies, then analyze how introducing selection disrupts equilibrium predictions. This drill requires multi-step math and conceptual reasoning about population genetics.
Passage
A population of beetles (Tribolium castaneum) is maintained in a laboratory. The beetles exhibit a heritable difference in body color: red (dominant, R) versus black (recessive, rr). A researcher establishes that the population is currently in Hardy-Weinberg equilibrium for this locus. A census of 500 beetles reveals the following phenotype counts:
Table 1. Beetle phenotype counts (n = 500).
| Phenotype | Count |
|---|
| Red (R_) | 320 |
| Black (rr) | 180 |
The researcher then introduces a selective pressure: black beetles in the population are 30% less likely to survive to reproductive maturity per generation than red beetles. All other Hardy-Weinberg assumptions (no mutation, random mating, no migration, large population) remain in effect except for natural selection.
Questions in This Drill
- Based on the phenotype data in Table 1, what is the frequency of the recessive allele (r) in this population?
- Based on your calculation of allele frequencies, what is the expected frequency of heterozygous (Rr) beetles under Hardy-Weinberg equilibrium?
- After one generation of selection against black beetles (30% lower survival), which change in the population is most consistent with the principles of natural selection?
- A student argues: "Because the r allele is protected in heterozygotes, selection will never completely eliminate it from the population." Evaluate this claim.
- If heterozygous beetles (Rr) had slightly higher reproductive success than either homozygote, this would be an example of which evolutionary mechanism, and what would be the long-term outcome?