Here, speciation occurs because the original butterfly population gets separated into two populations, each facing a different set of environmental conditions. Initially, there was just a single population of butterflies in the valley and all butterflies were able to mate with each other. A fire destroyed their habitat so they had to migrate to other areas. Some butterflies migrated to eastern side of mountain and some to western side of mountain.
Now there were two butterfly populations and they faced different environment from each other. Over the time, the populations adapted to their local surroundings and started to accumulate changes in their genome. Their gene pools started to separate from each other. Eventually they evolved into two different species. When their old habitat was restored in the valley, they returned once again. However, this time they were not able to interbreed because they were now two different species.
