From BBC News:
Given our planet’s rich biodiversity, “speciation” clearly happens regularly, but scientists cannot quite pinpoint the driving forces behind it. Now, researchers studying a family of butterflies think they have witnessed a subtle process, which could be forcing a wedge between newly formed species. The team, from Harvard University, US, discovered that closely related species living in the same geographical space displayed unusually distinct wing markings.
These wing colours apparently evolved as a sort of “team strip”, allowing butterflies to easily identify the species of a potential mate.
More here.