Two morphologically identical Drosophila populations live in Lower Nahal Oren, Mount Carmel, Israel. There are two slopes of a valley that face each other and drosophila live on both slopes (they share a common ancestor). The environment is very different even though they are very close. In the center of the valley is an area in which flies from each side have an opportunity to mate. (an aside: This is frowned upon by fly society buy these tiny Montagues and Capulets persist). Evaluate each scenario:

These two populations of flies should probably express some phenotypic difference due to the environmental condition on each slope. Remember that the phenotype is the product of interaction between the genotype and the environment. However, the difference between populations would exist if they were isolated and separated by an ecological or biological barrier. The barrier would impede genetic flow, and each population would have its particular genetic charge and phenotype.

In the exposed situation, we know that each population inhabits a slope. And that the slopes differ in the environmental conditions. But in the center of the valley, these two populations get to meet and mate. The valley is acting as a corridor between both groups and allows the genetic flow to occur.

Let us remember that gene flow is an evolutive strength only if

there exists migration from one population to the other, and
the allelic frequency in one generation is different from the allelic frequency in the next generation.

Genetic flow acts homogenizing the allelic frequencies between the two populations, and it might introduce variability into the new one. For genetic flow to be possible, there needs not only the movement of the genes from one population to the other but also the reproduction process on the new population. Hence, it involves an interaction between the dispersion pattern and the reproductive system.

Individuals from both groups get to mate in the valley, leaving fertile offspring. The offspring, carrying genes from both populations, are included in one or the other group, where they will reproduce. In each population, the frequency of genes of the other group increase with time as the flies keep mating. Eventually, genetic flow homogenizes the variability between both populations, making them look similar.

Scenario Two:

It might be an early dispersion event in which the common ancestor of both populations used to inhabit the center of the valley. With time, and as the original population got bigger, dispersion took a significant role. Flies began moving into both slopes. Even though each area has different environmental conditions, there has not been enough time for these animals to adapt morphologically to each side, so they still look identical. Also, as it is an early event, these two new groups are not separated yet, so they have not developed biological barriers between them, and individuals can still mate in the center of the valley.