What does the biological species concept use as the primary criterion for determining species boundaries?

Gene flow is used as the primary criterion for determining species boundaries

Explanation:

It is the movement of "genetic variation" from one group population to the other, in case the two population will belong to the same allele or frequency the rate of gene flow will be much higher. It generally occurs during migration, the loss or the addition of the people can easily alter the frequencies of the gene pool, when there is no other evolutionary mechanism being operated. The most simple example that can be easily understood is the transfer of the pollens from one flower to another flower by the honeybees