Assertion: Species is a genetically closed system.
Reason: Because the reproductive isolation constitutes the most important boundary between different species.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Species is a genetically closed system." This means that members of a species share a common gene pool and do not exchange genes with members of other species.

2. **Understanding the Reason**:
- The reason given is that "reproductive isolation constitutes the most important boundary between different species." Reproductive isolation refers to mechanisms that prevent different species from interbreeding. This can include behavioral differences, temporal isolation (different mating seasons), or physical barriers.

3. **Examples of Reproductive Isolation**:
- Consider the example of lions (Panthera leo) and tigers (Panthera tigris). Although they belong to the same genus (Panthera), they are different species and do not mate in nature due to reproductive isolation.

4. **Implications of Reproductive Isolation**:
- If two different species were to mate, they might produce hybrids. However, these hybrids are often sterile (like mules, which are hybrids of horses and donkeys) and cannot reproduce, which reinforces the concept of species as a genetically closed system.

5. **Conclusion**:
- Both the assertion and the reason are true. The assertion that species are genetically closed systems is supported by the reason that reproductive isolation is a critical factor in maintaining the boundaries between species.

### Final Answer:
Both the assertion and the reason are true, and the reason correctly explains the assertion.

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