Biologists say they have found a population of tropical butterflies that may be on its way to a split into two distinct species. The cause of this particular break-up? A shift in wing color and mate preference.

In a paper published this week in the journal Science, the researchers describe the relationship between diverging color patterns in Heliconius butterflies and the long-term divergence of populations into new and distinct species.

"Our paper provides a unique glimpse into the earliest stage of ecological speciation, where natural selection to fit the environment causes the same trait in the same population to be pushed in two different directions," says Marcus Kronforst, one of the paper's co-authors. "If this trait is also involved in reproduction, this process can have a side effect of causing the divergent subpopulations to no longer interbreed. This appears to be the process that is just beginning among Heliconius butterflies in Ecuador."

Previous studies of species formation have focused on the characteristics of well-differentiated species, and the health and viability of their hybrids in particular, in an effort to identify how the species may have emerged and how they stay distinct.

Heliconius provides a model for a different kind of study. The researchers considered species emergence from the opposite end, studying populations that have yet to diverge into separate species in order to identify the role of mate choice in the potential emergence of new species.

Having identified color-based mate preference in Heliconius, the researchers used a battery of genetic markers to compare the genomes of the white and yellow varieties, showing that they are genetically identical except for their different colors and preferences.

Their work suggests that the genes for color and preference are very close to one another in the genome; the two traits could even be caused by the same gene. Their next step is to identify the gene (or genes) responsible for the differences in color and mate preference.

Citation: Nicola L.,Chamberlain,Ryan I. Hill, Durrell D. Kapan, Lawrence E. Gilbert, Marcus R. Kronforst1, 'Polymorphic Butterfly Reveals the Missing Link in Ecological Speciation', Science, 2009, Vol. 326. no. 5954, pp. 847 - 850 DOI: 10.1126/science.1179141