Butterfly wings come in many beautiful colors, but little is known about how these colors have come into existence. The evolution of butterfly wings and other colorful structures in nature has remained a mystery even though efforts have been made to determine how these natural wonders have developed.
A study recently published in the journal Proceedings of the National Academy of Sciences reported an experiment using artificial selection to shed light on the evolution of colorful butterfly wings. The study was carried out by scientists in the Department of Ecology and Evolutionary Biology and Department of Applied Physics at Yale University and also by scientists from Kookmin University in Seol, South Korea and the National University of Singapore in Singapore.
The coloration of an animal or plant can be based on one of two things: photopigments that reflect or absorb different wavelengths of light or variations in the minute structures of the body that can reflect light differently. The color of butterfly wings results from the interaction of light with nanoparticles in the butterfly wing scales. There are many different structures within the nanoparticle complement and the differences in the structures create different responses to light that result in various color perceptions. What the evolutionary mechanisms are in producing and arranging these nanoparticles in the scales of butterfly wing is the question that the study attempted to answer.
The researchers used artificial selection; that is, selective breeding by humans, to determine how many generations it took for butterfly wing colors to evolve. Butterflies that were in the Bicyclus anynana species were used and they were selectively bred so that violet wing scales were selected over ultra-violet brown wing scales. These butterflies were compared with two closely related species, Bicyclus medontias and Bicyclus sambulos, which evolved through natural selection to have violet blue scales.
The results showed that violet wing scales evolved from ultra-violet brown wing scales in only six generations as a result of artificial selection. Also, the study showed that the structural changes that were responsible for this evolution in the color of the butterfly wings involved changes in how thick the chitin lamina was in the wing scales.
The authors suggested that the results could be interpreted as indicating that there must be a large amount of background genetic variants that could be used for the speedy evolution of nanoparticle structural variation that resulted in changes in color. This background genetic variation could be “tapped” and the slight modifications in the structural composition of the wing scales means rapid evolution in the color appearance of the butterfly’s wings. If the butterfly wings had a color appearance based on photopigments, it would be high unlikely that the color of butterfly wings could evolve very fast. It would take the evolutionary process a much longer time for key mutations in the genes for photopigments to come into existence and be subjected to the forces of natural selection for alterations in color to occur.
Another interesting outcome of nanoparticle structural variations as the basis for color appearance is it allows for the presence of iridescent coloration. Iridescence is the property of objects changing color with different point of view. As the angle of viewing changes, the color appearance changes. This property is what makes butterflies and other insects seem to be more ethereal and otherworldly.
By Margaret Lutze