Photo:Robert Lawton/WikimediaScientists find a population of butterflies that appears to be splitting into two speciesBreaking up may actually not be hard to do, say scientists who've 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 colour and mate preference.
In a paper published this week in the journal Science, the researchers describe the relationship between diverging colour 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, a Bauer Fellow in the Canter for Systems Biology at Harvard University who received his doctor's degree at The University of Texas at Austin. "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."
Heliconius butterflies display incredible colour pattern variation across Central and South America, with closely related species usually sporting different colours. In Costa Rica, for example, the two most closely related species differ in colour: One species is white and the other is yellow. In addition, both species display a marked preference to mate with butter-flies of the same colour.
The Ecuadorian population examined by Kronforst and his colleagues shows the same white and yellow variation found in Costa Rica but has not yet reached a level of strong reproductive isolation. The entire population lives in close proximity and individuals of both colours come in contact with – and mate with – each other.
But, by studying the Ecuadorian population in captivity, the scientists found the two colours do not mate randomly. Despite the genetic similarity between the groups – white and yellow varieties differ only at the colour-determining gene – yellow Ecuadorian individuals show a preference for those of the same colour. White male butterflies, most of which are heterozygous at the gene that controls colour, show no colour preference.
"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, who began studies of Heliconius colour pattern and behavioural genetics in the laboratory of Professor Lawrence Gilbert at The University of Texas at Austin.
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 colour-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 colours and preferences.
Their work suggests that the genes for colour 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 colour and mate preference.
"If we can identify this gene or genes, we can say conclusively how they influence both colour and mate choice," says Kronforst. "Subsequent work could elucidate exactly how changes in individual genes can, over long periods of time, lead to novel species."
"This study shows the great potential of the genus Heliconius as a model system for integrating genetics, development, behaviour, ecology and evolution," says Gilbert, professor in the Section of Integrative Biology. "It is the culmination of diverse contributions of the co-authors involving insectary, field and laboratory research over more than a decade."
