Answer:
reduction of gene flow can occur due to geographical isolation too
or it can be dur to sympatric or allopatric speciation an occur.
where due to this the species are being behaviouraly isolated, and mad too reject mating due to certain reasons. Moreover mutations too can take palace causing for a genetic drift. Where the isolated species may undergo selection pressure too. So as an overall view the gene flow eventually decreases since the species diversity along with the genetic diversity decreases causing fare the reduction of the gene pool.
After many generations a new species will be formed. Resulting for speciation.
(refer to the AL biology text book under biodiversity related topics, it will be easy)