A team of researchers who bred a species of brown African butterfly in the lab were shocked to discover that the offspring could turn purple in just six generations, or about a year.
The butterfly Bicyclus anynana before it was bred to turn purple. [Photograph courtesy of Antónia Monteiro]
Scientists have long known that butterflies are sensitive to changes in their environment and can evolve new colors if it suits them. But until now, no one knew how the fragile insects were able to pull this off.
Such a speedy evolution is “amazing,” said study co-author Hui Cao, a physicist at Yale University. “Within one year they look purple—I could not believe it, to be honest.”
In the laboratory, the team bred the butterfly Bicyclus anynana, which lives for about two months. They used microscopes to determine which individual butterflies’ wings best reflected light of the wavelength that produces purple.
When the scientists examined these purple wings under the microscope, they discovered that each scale had evolved to have the specific thickness that allowed it to reflect violet light. In other words, the wing scales had become perfect for reflecting purple.
After six generations of breeding, B. anynana developed scales in its wings that reflected purple (right to left) [Photograph courtesy of Antónia Monteiro]
The team then compared B. anynana and two distantly related species of butterfly in the same genus that had naturally evolved to be purple. They discovered that the same mechanism they’d observed in the lab had been at work in nature: All three species had wing scales of the same thickness.
A Butterfly of a Different Color
There are only two ways an animal can change its color: by altering its pigment, which requires the body to do some heavy lifting, or by tweaking the structure of its wings or other body parts that reflect light, Cao noted.
By changing their wings, butterflies had chosen the easier route. “That’s why they can evolve much faster and easier to really adapt to environmental change,” Cao said.
Scientists not involved with the study noted that the research advances science by showing both the speed and function of color evolution in butterflies.
“This paper nicely shows that underlying structural colors can evolve faster than pigment,” said Mark Scriber, professor emeritus in entomology at Michigan State University in East Lansing. “How quickly the response is to selection [for purple] was probably the most amazing thing.”
Butterfly expert Andrei Sourakov added by email, “This study contributes significantly to our understanding of an important trait frequently found in butterflies—iridescent coloration.”
Sourakov, of the Florida Museum of Natural History, suspects that many butterflies have evolved ways to change colors fast but using different mechanisms: “Nature has a way of achieving similar goals in multiple ways.”
For instance, common buckeye butterflies, which are usually almost entirely brown in Florida, are bred for the size of their iridescent blue patch, and can turn from brown to blue in six years, he noted.