PBS has a YouTube featuring rapid evolution in Hawaiian crickets after contact with a parasitoid fly. The parasitoids use the sound of calling male crickets to locate hosts. Calling males are parasitized at a high rate affecting their ability to reproduce. The parasitoid makes silence in males more adaptive.
The cricket arrived on three islands, Kauai, Oahu and Hawaii. Both on Kauai and Oahu, males that produced no song became a large percentage of the population. Interestingly, the silencing mutants on Kawaii and Oahu have mutations to different genes.
Isolation of populations is important in the theory of how new species are generated. In this example, the two cricket populations on different islands are selected by the same parasitoid agent, but the genetic changes to the populations differ. The accumulation of many such changes over long periods of separations can result in populations becoming so genetically different, that individuals can no longer inter mate and produce viable offspring.