Final answer:
When observed and expected do not match, it is likely due to either genetic drift or natural selection, indicating a violation of Hardy-Weinberg equilibrium.
Step-by-step explanation:
When observed and expected do not match, it suggests a violation of Hardy-Weinberg equilibrium. This means that the allele frequencies in the population are changing over time, which is usually due to factors such as genetic drift, gene flow, or natural selection.
In the context of the provided options, if observed and expected do not match, it is likely due to either genetic drift (option A) or natural selection (option B). Gene flow (option D) could also be a factor if it results in changes in the allele frequencies. However, the most likely explanation is a violation of Hardy-Weinberg equilibrium (option C).