Evolution and speciation · USMLE Step 1

Which of the following best describes parapatric speciation?

  1. Speciation driven by complete geographic separation of populations
  2. Speciation across an environmental gradient in adjacent populations
  3. Speciation within a uniform environment lacking any physical barriers
  4. Speciation that occurs exclusively in flowering plant lineages
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Correct answer: Speciation across an environmental gradient in adjacent populations

Parapatric speciation occurs when populations are contiguous but inhabit different ecological niches or environmental gradients, leading to reduced gene flow and gradual divergence despite geographic proximity.

Difficulty: Medium Question 18 of 20

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