What must occur within a gene pool before the formation of a new species?

The correct answer is- Reproductive isolation.

As per the biological species concept of Ernst Mayor, organisms that are capable of interbreeding and producing viable, fertile offspring belong to same species.

Within a gene pool, if organisms become reproductively isolated, they would be considered as two different species.

Thus, reproductive isolation must occur before formation of a new species.

answer; 60-100; beats per minute