According to the theory of Evolution, the traits of living organisms change over generations to adapt to their surroundings. Members of the same species, living under different circumstances, evolve in different ways. Over time, the species splits into two (or more) branches. This process is known as Speciation.
For this speciation to occur, the branches must refrain from interbreeding. The most obvious way this can happen is through Geographical Isolation (the branches are separated by a river, mountain, etc.), but it can also happen even when the branches are living side-by-side! Why it could be so was a mystery, until now:).

Agrodiaetus iphigenia nonacriensis (Courtesy: Sommerfugle)Even when the branches are living together, they might choose not to interbreed, a process termed Reproductive Isolation. Researchers from Harvard University, while studying a family of butterflies, think they have witnessed a subtle process, which could be forcing a wedge between such newly formed branches/species.
They found that closely related butterfly species (of genus Agrodiaetus, in Asia) living in the same geographical space displayed unusually distinct wing markings. These wing colors apparently evolved so as to allow the butterflies to easily identify the species of a potential mate.
It seems that if closely related species of Agrodiaetus are geographically separate, they tend to look quite similar. But when they are living side-by-side, they look strikingly different! This has the effect of discouraging inter-species mating, thus encouraging genetic isolation and species divergence:):).
However, at least to me, it is not obvious what is cause and what is effect. It could very well be, that because the species has developed the distinct markings while living in close quarters, that speciation was able to happen, rather than the other way round! Perhaps more research is needed to pin that one down:):).