Here's why:

* Gene flow is the transfer of genetic material from one population to another. This can happen through various mechanisms like migration, pollen dispersal, or the movement of seeds.

* Species are defined as groups of organisms that can interbreed and produce fertile offspring. This means that individuals from different populations within the same species can potentially interbreed and exchange genes.

* Reproductive isolation is a key factor that prevents gene flow between different species. This isolation can be due to various mechanisms like geographical barriers, differences in mating behaviors, or incompatibility of genetic material.

While gene flow can occur between populations within a species, it is ultimately limited by the boundaries of the species itself.

However, there are some important nuances:

* Hybridization: In rare cases, gene flow can occur between closely related species through hybridization. This can lead to the introduction of new genetic material into a species, but it's not the typical way gene flow operates.

* Evolution: Over very long periods, species can evolve and diverge into distinct lineages. While gene flow was possible between the ancestral populations, it might not be possible anymore due to genetic incompatibilities.

Therefore, while the species is the largest unit where gene flow is possible, it's essential to remember that species are not static entities and their boundaries can shift through time.