LINDA WERTHEIMER, HOST:
Butterfly wings come in all sorts of colors - dull grays and browns, iridescent blues and greens, bright red and yellow. And there's a reason for the variety. The colors help attract mates and fend off predators, but how those hues evolved has been something of a mystery to scientists until now. NPR's Rae Ellen Bichell reports.
RAE ELLEN BICHELL, BYLINE: Zoom down to the surface of a soft, smooth-looking butterfly wing and biologist Antonia Monteiro says it's actually covered in rugged, textured scales.
ANTONIA MONTEIRO: That overlap like shingles on a roof.
BICHELL: Zoom in even more to the nanoscale, and you'd find a labyrinth of hard, transparent architecture - pillars, ridges, archways, sometimes even spiral loop-de-loops.
MONTEIRO: And all kinds of other intricate things, all made out of chitin.
BICHELL: That's the same material that makes crab shells so tough. In butterflies, the way those chitin structures bend and reflect light is what creates such an extraordinary range of bright colors. Beams of light come in and ricochet off the scaly maze in all directions.
MONTEIRO: As they bounce off these structures, they go through a process of constructive interference or destructive interference.
BICHELL: It all has to do with geometry and spacing. Tweak those, and you change the wavelengths reflecting back out, which gives different colors. The question was, what exactly would have to change in those tiny, transparent structures to make a dull-looking butterfly brilliant? And how long would it take? Monteiro decided there was only one way to find out - to try to do it in the lab.
MONTEIRO: To try and evolve color.
BICHELL: Monteiro, who's now at the National University of Singapore, teamed up with some Yale physicists and picked a butterfly species called the squinting bush brown. It lives up to its dull name - goes well with tree bark. But it has some flashy close cousins with streaks of blue and violet on their wings.
MONTEIRO: So we wondered whether this specific species that was not showing any of those colors could also evolve those colors if we force it to through artificial selection.
BICHELL: So here's what they did; Some of the dull-brown butterfly wings did reflect slightly shorter wavelengths of light.
MONTEIRO: Meaning towards the bluer wavelength of the light spectrum. And we mated those individuals with each other.
BICHELL: One year and six generations later, they had bred squinting bush browns that were sporting purpley streaks across their wings. Monteiro was surprised to find that the new decor was the result of only very slight changes to the scales. The chitin layer had become a smidge thicker.
MONTEIRO: It seems to be incredibly easy to evolve these new colors in butterflies.
BICHELL: So should this modest, brown butterfly species need to adapt, it has a powerful color technology in its back pocket. Tweak a little chitin and boom - from brown to brilliant. Monteiro's work was published this week in the journal Proceedings Of The National Academy Of Sciences. Rae Ellen Bichell, NPR News. Transcript provided by NPR, Copyright NPR.