Why can reproductive isolation lead to directional or disruptive selection?
It is the other way round actually. In both disruptive and directional selection, population with extreme phenotypic and genetic traits is selected over the intermediary one. To understand extreme or intermediary traits, we can take an example where a species has red and white flowers, and breeding of red and white flowers produces pink flowers. In this case, red and white color become extreme phenotypic traits and pink color becomes the intermediary one (Note that this example is hypothetical). In directional selection (or selection in one direction), only one of the traits, i.e. red or white would be selected. In disruptive selection (that results in diversification), however, both the extreme traits would be selected. But the traits that lie in between the two are favored in none of the two. There could be many reasons for such a selection. This might happen, for example, in cases where population with moderate traits doesn’t fit to thrive in a particular environment. And when this happens, the population with intermediary traits gets lost gradually. It is, then, possible that the two types of populations of such a species with extreme traits might not interbreed. If that happens, over the time, the species becomes incipient with their extreme traits getting diverged. This could lead to reproductive isolation eventually. Note that reproductive isolation doesn’t occur in all the cases of disruptive and directional selection. Besides, this takes a very long time. And many a times, the “ability” to reproduce doesn’t get lost.
Please note that it is not necessarily "the other way around." Reproductive isolation can refer to the example given, wherein intermediate traits are selected against due to extremes being more environmentally fit, but it can also refer to geographic or environmental problems; for example, it is virtually impossible for African elephants and cheetahs to interbreed with their Asiatic counterparts.
Let's suppose that a population of animals live near a river. One side of the river is a forest, the other is a mountain. Over time, the river grows deeper, broader, faster, and fills with crocodiles. Crossing the river becomes more and more dangerous, making it more and more difficult for the mountain and forest populations to breed, and even when they do, their offspring may not necessarily be more fit. Adaptations to a mountain environment will be different than those for a forest; this is where directional and disruptive selection come into play. As the genes of the two populations are physically prevented from mixing by the river, the two populations begin to reflect more and more extreme samples of that original genetic dictionary.
So, it is possible for selection to lead to reproductive isolation, largely as a result of different behaviors. However, reproductive isolation can also lead to selection, largely as a result of the populations not being able to physically interact.
Isolation results in a limited gene pool. Because of this only certain traits will be passed on.
When the genes most suited to the environment persist the result is directional selection, as these will become the dominant traits.
When there is less necesity for a particular trait to persisit, the limited gene pool will result in a blending of the genotypes for a trait (or incomplete dominance). This is called disruptive selection.