Argumentation
Two populations of frogs live in the same pond but breed during different seasons. Which mechanism most directly reduces gene flow between the populations?
- AA prezygotic barrier caused by temporal isolationcorrect
- BA postzygotic barrier caused by hybrid sterility after mating
- CA decrease in reproductive isolation
- DA mutation that makes all frogs genetically identical
Why A is correct
Breeding at different times is temporal isolation, a prezygotic barrier because it prevents mating or fertilization from occurring.
The stem asks students to explain a specific mechanism, not just recall a unit term: "Two populations of frogs live in the same pond but breed during different seasons. Which mechanism most directly...". The correct choice is supported by Breeding at different times is temporal isolation, a prezygotic barrier because it prevents mating or fertilization from occurring. The tempting alternatives such as postzygotic barrier caused by hybrid sterility after mating or decrease in reproductive isolation because they share a habitat fail because they do not produce the same outcome described in the stem.
Concept Explanation · with figure
A continuous population of lizards is split when a mountain range rises between two regions. After many generations, the separated populations differ genetically. Which model best explains this pattern?
- AThe mountain range increases gene flow by preventing migration.
- BGeographic separation by itself identifies the exact genetic mechanism causing reproductive isolation.
- CThe two populations must remain genetically identical because they were once continuous.
- DVicariance can split a population and reduce gene flow, allowing divergence.correct
Why D is correct
Vicariance occurs when a geographic barrier divides a population. Reduced gene flow between separated populations can allow genetic divergence over generations.
The evidence in the prompt matters before the topic label: "A continuous population of lizards is split when a mountain range rises between two regions. After many...". The supported answer is "Vicariance can split a population and reduce gene flow, allowing divergence" because Reduced gene flow between separated populations can allow genetic divergence over generations. Distractors such as mountain range increases gene flow by preventing migration or Geographic separation by itself identifies the exact genetic... either ignore the stated pattern or name a related concept without matching the evidence.
Argumentation · with figure
Two related orchid species grow in the same forest and flower at the same time, but their flowers fit different pollinator body sizes. Which claim is best supported by the data?
- AThe plants are temporally isolated because they flower in different seasons.
- BHybrid offspring must be sterile because pollen transfer is high between species.
- CThe two species are geographically isolated
- DStructural mismatch can create mechanical isolation before fertilizationcorrect
Why D is correct
The orchids grow in the same forest and flower at the same time, so the low cross-species pollen transfer is best explained by mechanical fit differences rather than geographic or temporal isolation.
The evidence in the prompt matters before the topic label: "Two related orchid species grow in the same forest and flower at the same time, but their flowers fit different...". The supported answer is "Mechanical isolation can reduce cross-species pollen transfer when flower and..." because The orchids grow in the same forest and flower at the same time, so the low cross-species pollen transfer is best explained by mechanical fit differences rather than geographic or.... Distractors such as plants are temporally isolated because they flower in different seasons or Hybrid offspring must be sterile because pollen transfer is high... either ignore the stated pattern or name a related concept without matching the evidence.