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IB Biology HL A3.1.5 Difficulties in Distinguishing Species Questions

Evaluate how gradual divergence, geographic contact and incomplete reproductive isolation can make species boundaries partly arbitrary.

Syllabus
First assessment 2025
Course
Biology HL
Level
HL

Exam points

  • Interpret gradual divergence in spatial or population evidence as related populations become increasingly different.
  • Explain how geographic separation can limit gene flow even when populations remain theoretically able to interbreed.
  • Judge whether populations should be treated as separate species when geographic barriers may be insufficient for reproductive isolation.

IB Biology HL A3.1.5 Difficulties in Distinguishing Species Questions question 1

[Maximum number: 3]

Evolution Canyon is a steep sided rocky valley near Mount Carmel in Israel. There are populations of fruit flies (Drosophila melanogaster) living on the north facing and south facing slopes of the canyon. Fruit flies are sensitive to drought conditions and die if they become desiccated.

Twenty samples of fruit flies were collected from both sides of the canyon. They were subjected to the same conditions of drought stress and the life span (in hours) of the adult flies was measured.

Figure for Question IB Biology HL A3.1.5 Difficulties in Distinguishing Species Questions question 1 — IB Biology HL

Discuss whether the populations on the two slopes could evolve into two species.

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