___________ is the condition for speciation.

Answer

### Step-by-Step Solution:

1. **Understanding Speciation**: Speciation refers to the process through which new species arise from existing species. This involves changes in the genetic makeup of populations over time.

2. **Role of Isolation**: For speciation to occur, populations of the same species must become isolated from each other. This isolation can be due to geographical barriers, environmental changes, or other factors that prevent individuals from mating.

3. **Reproductive Isolation**: Once populations are isolated, they may undergo evolutionary changes. Over time, these changes can lead to reproductive isolation, which means that even if the populations come back into contact, they can no longer interbreed successfully.

4. **Conclusion**: The key condition for speciation is reproductive isolation. This ensures that the genetic differences that have accumulated in the isolated populations are significant enough to prevent interbreeding, thus leading to the formation of new species.

5. **Final Answer**: The blank in the question can be filled with "reproductive isolation."