An inter-breeding population of finches became separated geographically, forming two isolated groups. Each group then became subect to different selective pressures. One group was then introduced into the habitat of the other.
Which one of the following would determiner wheather they now formed two district species?

Options

A. They had been separated for more than three milion years.
B. They failed to produce fertile `F_(1)` hybrids.
C. They showed marked differences in the shape of their beaks.
D. Their plumage had become markely different.

Explanation

To determine whether the two groups of finches have formed two distinct species after being geographically separated and subjected to different selective pressures, we need to focus on the concept of reproductive isolation. Here’s a step-by-step breakdown of the reasoning:

### Step-by-Step Solution:

1. **Understanding Speciation**:
- Speciation occurs when populations of the same species become so different that they can no longer interbreed successfully. This can happen due to geographical isolation and different selective pressures.

2. **Geographical Isolation**:
- The finches were separated geographically, which means they could not interbreed. This isolation allows for different evolutionary paths based on the unique environmental pressures each group faces.

3. **Different Selective Pressures**:
- Each group of finches was subjected to different selective pressures in their respective environments. Over time, these pressures can lead to adaptations that make the two groups increasingly different from one another.

4. **Reintroduction of Groups**:
- When one group of finches is introduced into the habitat of the other, we need to assess whether they can interbreed successfully.

5. **Testing for Reproductive Isolation**:
- The key factor that determines if they are now distinct species is whether they can produce fertile offspring together. If they can interbreed but produce sterile offspring, or if they cannot interbreed at all, this indicates reproductive isolation.

6. **Conclusion**:
- Therefore, the determination of whether they have formed two distinct species hinges on their ability to interbreed and produce fertile offspring. If they cannot, then they are considered two distinct species.

### Final Answer:
The factor that determines whether the two groups of finches now form two distinct species is whether they can produce fertile offspring when interbred.

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