Heliconius cydno butterflies in western Ecuador have two colour variations, white and yellow (Marcus Kronforst and Krushnamegh Kunte)

Researchers have found a population of tropical butterflies in Ecuador that might be on its way to splitting into two different species.

Marcus Kronforst of Harvard University and his colleagues found that variations in wing colour within a species of Heliconius butterfly in Ecuador is also tied to mate preference and could lead to a split in the species.

The butterfly Heliconius cydno comes in two wing-colour varieties — yellow or white — but the two varieties coexist and mate with each other. The pattern of yellow or white markings on the wings is controlled by a single gene.

However, when the researchers studied the butterflies in captivity, they found that they didn't mate randomly. Yellow male butterflies showed a preference for mates of the same colour while white male butterflies didn't show a colour preference.

The researchers compared this situation to another group of Heliconius butterflies in Costa Rica, where the white and yellow butterflies are separate but closely related species and show preference for butterflies of the same colour.

"This subtle difference in mate preference between the colour forms in Ecuador may be the first step in a process that could eventually result in two species, as we see in Costa Rica," says Kronforst.

The researchers say their work suggests that the genes for colour and sexual preference for colour are very close together on the genome and could even be caused by the same gene.

As a result, the white and yellow varieties of the butterfly species could become more and more isolated.

"This process can have a side effect of causing the divergent sub-populations to no longer interbreed," said Kronforst. "This appears to be the process that is just beginning among Heliconius butterflies in Ecuador."

The research appears this week in the journal Science.