How species separate to become separate species

From the BBC, an evolutionary puzzle explained:
Why one species branches into two is a question that has haunted evolutionary biologists since Darwin. 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. This process, called "reinforcement", prevents closely related species from interbreeding thus driving them further apart genetically and promoting speciation.

Although scientists have speculated about this mechanism for years, it has rarely been witnessed in nature. "The phenomenon of reinforcement is one of the very few mechanisms that has natural selection playing a role in speciation," said Harvard co-author Nikolai Kandul. "It might be very widespread but it is hard to find good evidence of it." For speciation to occur, two branches of the same species must stop breeding with one another for long enough to grow apart genetically.

The most obvious way this can happen is through geographical isolation. If a mountain range or river divides a population of animals for hundreds of generations, they might find that if they meet again they are no longer able to breed. But geographical isolation is not enough to explain all speciation. Clearly, organisms do sometimes speciate even if there is no clear river or mountain separating them.

The other mechanism that can theoretically divide a species is "reproductive isolation". This occurs when organisms are not separated physically, but "choose" not to breed with each other thereby causing genetic isolation, which amounts to the same thing. Reproductive isolation is much hazier and more difficult to pin down than geographic isolation, which is why biologists are so excited about this family of butterflies. The Harvard team made the discovery while studying the butterfly genus Agrodiaetus, which has a wide ranging habitat in Asia. The females are brown while the males exhibit a variety of wing colours ranging from silver and blue to brown.

Dr Kandul and his colleagues found that if closely related species of Agrodiaetus are geographically separate, they tend to look quite similar. That is to say, they do not display a distinctive "team strip". But if similarly closely related species are living side-by-side, the researchers noticed, they frequently look strikingly different - their "teams" are clearly advertised. This has the effect of discouraging inter-species mating, thus encouraging genetic isolation and species divergence. "This butterfly study presents evidence that the differences in the male's wing colouration is stronger [when the species share a habitat] than [when they do not]," said the speciation expert Axel Meyer, from Konstanz University in Germany. "This pattern would therefore support the interpretation that it was brought about by reinforcement, hence natural selection."

The reason evolution favours the emergence of a "team strip" in related species, or sub species, living side-by-side is that hybridisation is not usually a desirable thing. Although many of the Agrodiaetus species are close enough genetically to breed, their hybrid offspring tend to be rather weedy and less likely to thrive. Therefore natural selection will favour ways of distinguishing the species, which is why the clear markings exist.

"For me, this is a big discovery just because the system is very beautiful," said Dr Kandul. "As much as we can we are showing that [reinforcement] is the most likely mechanism."