Assertion: Hardy-Weinberg principle explains the variations occurring in population and species over a number of generations.
Reason: Hardy-Weinberg principle is applicable only when genetic drift occurs.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that the Hardy-Weinberg principle explains the variations occurring in populations and species over generations. This is incorrect. The Hardy-Weinberg principle actually describes a model where allele frequencies remain constant in a population, meaning it does not account for variations or changes in allele frequencies over generations.

2. **Understanding the Reason**: The reason states that the Hardy-Weinberg principle is applicable only when genetic drift occurs. This statement is also incorrect. The Hardy-Weinberg principle applies when a population is in equilibrium, meaning there is no genetic drift, selection, mutation, or migration affecting allele frequencies.

3. **Evaluating Both Statements**:
- The assertion is false because the Hardy-Weinberg principle does not explain variations; it assumes stability in allele frequencies.
- The reason is false because the principle is not applicable when genetic drift occurs; rather, genetic drift disrupts the conditions required for Hardy-Weinberg equilibrium.

4. **Conclusion**: Since both the assertion and the reason are incorrect, the correct answer is that both statements are false.

### Final Answer:
Both the assertion and the reason are incorrect.

---