For most forms of life, coloration is synonymous with the presence of a pigment that absorbs some wavelengths and reflects others. But butterflies and birds are different. Their wings and features have incredibly fine, microscopic features that can help channel light and create interference patterns that enhance some wavelengths while suppressing others.
Now, researchers have tested just how readily these complex optical systems can evolve. In just six generations of selection, they took a brown butterfly and shifted it to a rich, purple color.
Turning a "brown bush" purple
The species chosen for this work was Bicyclus anynana, an African species that also goes by the name of "squinting brown bush." As the name implies (and the image above shows), the majority of its pigmentation is brown, with a few prominent eye spots.
Several traits made this species a desirable choice for the work, not the least of which was practicality—it's easy to grow them in dense colonies in the lab. But pigmentation also mattered. "Squinting brown bush" coloration is a bit more complex than the human eye would lead us to believe, in that the butterfly's wings also readily reflect ultraviolet light. And two members of the genus have evolved purple pigmentation in the past, suggesting that a color change was within the realm of possibility.
The researchers started by checking the absorption spectrum of their existing lab strain of butterflies. This showed a peak of reflection at 300nm wavelengths, well within the UV range of the spectrum. But the peak was broad and varied from individual to individual, so the researchers selected the males and females that had the peak shifted closer to the visible spectrum, then mated them to produce the next generation. They repeated the process of mating and measuring reflection for five additional generations.
After six generations of selective breeding, the peak reflection had shifted well into the purple at 400nm.
(Six generations in this species take less than a year. The experiment actually involved eight generations total, though, because two generations saw low numbers of offspring and were simply allowed to mate randomly to build up the numbers again.)
The researchers then put the butterfly wings under the microscope. The wings are a complicated, two-layer affair, with upper scales held on a network of pillars and interconnections, plus a lower scale attached to the body of the wing. Pigments are present in each, and both scale layers reflect and redirect light that reaches them.
The researchers found that both layers of scales evolved the ability to reflect more violet light, but the lower level of scales showed the most dramatic changes. Changes on the surface of these scales, notably a thinning of some ridges present on their surfaces, seem to account for much of the color difference (although there may also be some pigmentation changes). Scales also got significantly thicker in the animals with purple wings.
The team then turned to the two species in the same genus that had already naturally evolved purple pigmentation. Similar things had happened in those animals, but in different locations: the upper scales changed more in one species, while both layers changed in the other. Thus, coloration changes aren't necessarily limited to any specific pathway.
The results show that, although the individual structures are tiny and delicate, the butterfly's wing as a whole is remarkably robust and can easily undergo rearrangements that radically change its optical properties. In fact, as the authors point out, a bit of variability in these properties appears to be a normal part of the genetic background of these species. This natural variability means that evolution doesn't have to wait for a fortunate mutation to get to work.
(Ed. note: We spent significant time looking for a useable image of the resulting purple butterfly and have contacted Yale to try to get one. Until they come through, your imagination will have to do the heavy lifting on this one.)