[A]: The establishment of reproductive isolation is an event of biological significance.
[R]: Because in absence of reproductive isolation they can merge back into single popu- lation

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

To solve the question, we need to analyze the assertion and reason provided:

**Assertion (A)**: The establishment of reproductive isolation is an event of biological significance.

**Reason (R)**: Because in the absence of reproductive isolation, they can merge back into a single population.

### Step-by-Step Solution:

1. **Understanding Reproductive Isolation**:
- Reproductive isolation refers to mechanisms that prevent different species from interbreeding. This can occur through various means such as temporal isolation, behavioral isolation, mechanical isolation, etc.
- The significance of reproductive isolation lies in its role in maintaining the distinctiveness of species and preventing gene flow between them.

2. **Evaluating the Assertion**:
- The assertion states that the establishment of reproductive isolation is biologically significant. This is true because it allows for the evolution of distinct species by preventing interbreeding and maintaining genetic differences.

3. **Evaluating the Reason**:
- The reason states that without reproductive isolation, species can merge back into a single population. This is also true because if two different species can interbreed, they can exchange genes and potentially lose their distinct identities over time.

4. **Linking Assertion and Reason**:
- The reason provided explains why the assertion is true. If reproductive isolation is not established, the distinct species can interbreed, leading to a merging of populations, which undermines the significance of having separate species.

5. **Conclusion**:
- Since both the assertion and reason are true, and the reason correctly explains the assertion, the correct answer is option A: both assertion and reason are true, and the reason is the correct explanation for the assertion.

### Final Answer:
The correct option is **A**: Both assertion and reason are true, and the reason is the correct explanation for the assertion.