Question

1. How might the fate of two hybridizing populations—that is, whether or not they persist as distinct populations—depend on the kinds of isolating barriers that reduce gene exchange between them?

Answer 1

Population refers to the group of organisms belonging to a species organisms. A population will have several subpopulations. Organisms which are belonging to to every subpopulation can be considered as a separate breed or variety. The crossing between the organisms of such different subpopulations can be considered as hybridization.
Hybridization is possible between the organisms of two different subpopulations only when there is no barrier between them. If there is a barrier which can prevent the crossing between the subpopulations then hybridization may not be becoming possible.
If the barriers are not able to to prevent that hybridization between the organisms of subpopulations then the crossing between the subpopulations is possible.
F crossing between the organisms of subpopulations takes place then and the gene transfer takes place from one population to the other population.
If the crossing between the organisms of subpopulations is prevented then, the gene flow or transfer also is prevented.
If the hybridization between the organisms of populations between different breeds is prevented by isolating barriers, then the populations remain completely separated from each other. This may further cause reproductive isolation due to the severe genomic changes that take place in each subpopulation even up to the extent of change in the number of their chromosomes and structures.
This will cause the formation of new species of organisms from the already existing species.