Step 1: Understanding the Question:
This question tests knowledge of population genetics and evolutionary mechanisms.
Step 2: Detailed Explanation:
Gene flow is the mechanism by which genes move from one population to another.
It is also called gene migration.
Mutation (B): Change in DNA sequence.
Natural selection (C): Differential survival of individuals.
Gene migration (D): Also means gene flow.
Thus, the correct answer is gene flow.
Step 3: Final Answer:
Thus, gene flow is the movement of genes between populations, which corresponds to option (A).
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