Evolution and speciation · USMLE Step 1

Which condition must be met for populations to undergo speciation?

  1. Continuous gene flow between the two populations
  2. Reproductive isolation that prevents interbreeding
  3. Identical environmental pressures acting on both groups
  4. Large population size that minimizes any genetic drift
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Correct answer: Reproductive isolation that prevents interbreeding

Speciation requires reproductive isolation, so that gene flow is limited or absent — allowing genetic divergence and formation of distinct species.

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