AP Biology: Host-Pathogen Coevolution Practice Questions
Practice host-pathogen coevolution reasoning with original AP Biology multiple-choice questions. These items focus on reciprocal selection, genetic variation, evolutionary trade-offs, experimental design, and cautious interpretation of evidence.
Free original practice questions for study purposes. Open Exam Prep is an independent study resource and is not affiliated with, endorsed by, or sponsored by the makers of AP Exams.
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1. A bacterial surface protein evolves so that a host receptor recognizes it less effectively. Several generations later, a new receptor variant that binds the altered protein becomes more common in the host population. Which process best explains this sequence?
Explanation. The pathogen variant changes the selective environment for the host, and the host receptor variant changes the selective environment for the pathogen. This reciprocal selection is a central feature of coevolution.
2. A resistance allele protects plants during fungal outbreaks but slows growth when the fungus is absent. Why might both resistant and susceptible alleles persist across years?
Explanation. A resistance allele can be favored when infection is common yet selected against when its growth cost matters more. Variation through time or space can therefore maintain both alleles.
3. Two host genotypes carry similar pathogen loads, but one maintains higher survival and reproduction by limiting tissue damage. Which defense strategy is most directly illustrated?
Explanation. Tolerance reduces the fitness harm caused by a given pathogen load rather than necessarily reducing that load. Resistance, in contrast, acts by preventing infection or limiting pathogen abundance.
4. Why is the Red Queen hypothesis often used to describe host-pathogen coevolution?
Explanation. An adaptation in one population can reduce the relative fitness of genotypes in the other, creating ongoing selection for counteradaptation without guaranteeing a permanent advantage to either side.
5. Researchers suspect that pathogens are locally adapted to nearby host populations. Which experiment would provide the strongest test?
Explanation. A reciprocal infection design separates host source from pathogen source. Greater pathogen performance on sympatric than allopatric hosts would support local adaptation while replication and common conditions reduce confounding.
6. A pathogen strain that reproduces rapidly causes severe symptoms but often kills hosts before they contact new susceptible individuals. What evolutionary idea does this pattern illustrate?
Explanation. Rapid within-host reproduction may raise pathogen output, but excessive host damage can shorten the infectious period or reduce contact. Selection acts on total transmission success, not virulence alone.
7. A pathogen most effectively infects the commonest host genotype. A rare host genotype then increases in frequency because it is infected less often. Which mechanism best fits this change?
Explanation. When common host genotypes experience greater pathogen pressure, rare genotypes gain a temporary fitness advantage. As genotype frequencies change, the direction of selection can also change.
8. Host and pathogen phylogenies show several matching branch patterns. Which conclusion is most appropriate?
Explanation. Similar phylogenies can support a cospeciation hypothesis, but they are not conclusive by themselves. Host switching, missing lineages, extinction, and sampling can also produce or obscure matching patterns.
9. A plant immune protein detects a pathogen effector and triggers defense. A pathogen allele that alters the effector avoids detection but still performs its infection function. What is the most likely immediate consequence?
Explanation. If the altered effector preserves pathogen function while escaping recognition, it can improve infection success in hosts with the matching immune protein. Later host counteradaptation may change that advantage.
10. Which genomic observation would most strongly support repeated coevolutionary change at host-pathogen interaction genes?
Explanation. Repeated amino-acid change or other signatures of positive selection concentrated at interacting loci are consistent with cycles of adaptation and counteradaptation. The evidence is stronger when compared with genomic background patterns.
11. The same host resistance allele is common in one region but rare in another, and pathogen genotypes also differ between those regions. Which explanation is most consistent with a geographic mosaic of coevolution?
Explanation. Coevolutionary hot spots, cold spots, and movement of alleles among populations can produce a geographic mosaic. Local pathogen composition and ecological conditions can favor different host alleles.
12. Scientists passage a virus for many generations in one host genotype and observe improved viral reproduction in that genotype. What additional comparison best supports the claim that adaptation occurred specifically to that host genotype?
Explanation. A factorial comparison of evolved versus ancestral viruses across passage and alternative hosts can distinguish general laboratory adaptation from host-genotype-specific adaptation. Replicate lines also reduce the chance that one random event drives the result.
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FAQ
What is host-pathogen coevolution?
It is reciprocal evolutionary change in which host traits affect selection on pathogens and pathogen traits affect selection on hosts.
How are resistance and tolerance different?
Resistance reduces the chance or intensity of infection, while tolerance reduces the fitness damage caused by a given infection level.
Does coevolution always produce greater pathogen virulence?
No. Virulence evolves under trade-offs involving replication, transmission, host survival, immunity, and ecological conditions, so different outcomes are possible.