Question

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

• A. They had been separated for more than three million 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 markedly different

Answer 1

The correct option is B They failed to produce fertile $F_1$ hybrids

Once inter-breeding population of finches, now have failed to produce fertile $F_1$ hybrids, this means they have now formed two distinct species.

So, the correct option is 'Option B'.