Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Evolution occurs in a population when its allele frequencies change over time. For example, the frequency of the A allele might change from 0.7 to 0.8. If that happens, evolution has occurred. What causes allele frequencies to change? The answer is forces of evolution. Answer: evolution
Output:
What occurs in a population when its allele frequencies change over time?