Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: New species arise in the process of speciation. Allopatric speciation occurs when some members of a species become geographically separated. They then evolve genetic differences. If the differences prevent them from interbreeding with the original species, a new species has evolved. Sympatric speciation occurs without geographic separation. Answer: allopatric speciation
Output:
What occurs when some members of a species become geographically separated?