Closely related morphologically similar, reproductively isolated sympatric populations are designated as

Options

A. Clones
B. Demes
C. Sibling species
D. Clines

Explanation

To answer the question regarding closely related morphologically similar, reproductively isolated sympatric populations, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Terms**:
- **Morphologically Similar**: This means that the organisms in question have similar physical characteristics.
- **Reproductively Isolated**: This indicates that these populations do not interbreed, even if they are in the same geographic area.
- **Sympatric Populations**: These are populations that live in the same geographic area and thus have the potential to interact.

2. **Identifying the Relationship**:
- The question specifies that we are looking for populations that are closely related. This implies that they share a common ancestor but have diverged in some way.

3. **Analyzing the Concept of Sibling Species**:
- Sibling species are defined as two or more species that are morphologically similar and are capable of interbreeding but do not do so due to reproductive isolation mechanisms. They are often found in the same geographic area (sympatric).

4. **Conclusion**:
- Given the definitions and relationships, the term that best fits the description of closely related morphologically similar, reproductively isolated sympatric populations is **sibling species**.

5. **Final Answer**:
- Therefore, the answer to the question is **sibling species**.