Nature presents us with a tantalizing array of dazzling colors to feast our eyes on; from fiery red birds to emerald green butterflies, canary yellow flowers to electric blue jellyfish. These spectacular colors have three main sources: pigments, structural colors and bioluminescence.
Structural color, which is found in many birds such as the male Peacock’s beautifully iridescent tail feathers, arises from the interference of light by microscopic surface structures. These nanostructures refract visible light, splitting it into rich, component colors, much like a prism.
Despite years of research, scientists know little about how structural colors in nature evolved. In an attempt to find out more, Yale scientists turned to butterflies. Butterfly wings also have structural color which arises from the intricate nanostructures found on their microscopic wing scales.
The researchers wanted to see whether they could change the color of the wings on butterflies over generations. For the study, which has been published in PNAS, they selected the species Bicyclus anynana, which has brown wings with eyespots (as shown in header image). They chose this particular butterfly because some of its relatives have evolved violet colors on their wings twice independently.
The researchers measured the wavelengths of light reflected from the wings of B. anynana specimens and selected those that reflected UV light from their brown scales. They then bred these butterflies and continued this process of artificial selection over several generations. To their amazement, the researchers found that the brown scales evolved to violet in just six generations, a process that took less than one year.
B. anynana scales before (left) and after (right). Credit: Antónia Monteiro
“What we did was to imagine a new target color for the wings of a butterfly, without any knowledge of whether this color was achievable, and selected for it gradually using populations of live butterflies,” said lead researcher Antónia Monteiro in a news-release.
B. anynana wing after artificial selection. Image credit: Antónia Monteiro
The scientists found that this shift in color production was due to an increase in the thickness of the lower lamina scales. When they studied the scales of closely related species, B. sambulos and B. medontias, which naturally reflect violet light, they found that scale thickness was similar to that of the newly violet B. anynana wings.
The scientists conclude that these data indicate that butterfly populations harbor a lot of genetic variation that can lead to the rapid evolution of structural color through minor modifications in the physical dimensions of the scales.
Findings such as this may be useful in the field of biomimetics, where scientists attempt to recreate a system or element found in nature. For example, the researchers believe this information could assist the design of devices that tune color or trap light.