The glow of a beetle has inspired an elegant bit of evolutionary detective work that appears in the Proceedings of the National Academy of Sciences. Americans like myself are familiar with fireflies, but in the tropics the night is also illuminated by beetles. When Darwin came to Brazil on the Beagle, he amused himself by noting how the beetles were “rendered more brilliant by irritation.” Naturalists have gotten a bit more sophisticated at studying beetles since then. They now know that the male beetles use the light organs on their underside to get the attention of females that are sitting in the trees and bushes; when the female sees a glow she likes, she registers her approval by flashing light organs on her back. (Fireflies do the same thing, but while they flash, the beetles give off a steady glow.) Scientists also know how the glow is made–a gene creates a protein called luciferase, which cuts up another protein called luciferin, releasing photons at a distinctive frequency. Depending on the species, the frequency is different.
The authors of the PNAS were attracted to the glow of one beetle in particular: a species that lives on Jamaica, Pyrophorus. plagiophthalmus. This species is peculiar, because its males can glow in a wide range of colors, from green to orange. Why so many colors? It’s all too easy to say, “Well, natural selection made it that way,” and leave matters at that. In fact, it’s possible that natural selection had no immediate role at all. Maybe Jamaica was colonized by a handful of beetles that just so happened to have some rare mutants in their midst, and they all proceeded to breed like crazy. Or perhaps it’s the females that have been evolving, and the genes they use for their own light organs also produce light in the males.
The scientists realized that the beetles offered a fabulous opportunity to study adaptation. Thanks to previous generation of beetle-loving scientists, they knew a lot about just about every link in the chain that joins the sequence of a gene to a living, breathing organism. They could even take the gene out of beetles and stick it into bacteria in a petri dish, where the gene would continue to produce light. By tinkering with each nucleotide in the gene, they could see exactly how the light changed as a result.
The scientists found that the colors of the male beetles don’t depend on genes shared with the females. Instead, they are the product of three different versions of the same gene (alleles). The alleles produce green, yellow, and orange light, and since each beetle can carry two copies of the gene, they can make various colors. The scientists then reconstructed the evolutionary history of the gene, by comparing the alleles to genes from beetles on neighboring islands. It turns out that the green allele is the oldest. It’s likely that the first colonists of Jamaica all glowed green. Then, with a few changes to the gene’s sequence, a new version emerged that produced yellow light. And then most recently, an orange gene emerged. In other words, the glow has steadily been shifting down through the spectrum towards the red end.
And the shift, the researchers showed, has taken place thanks to natural selection. Scientists can detect natural selection in DNA in several ways, one of which is to compare the number of differences between genes that lead to changes in their respective proteins to differences that cause no change. If the protein-changing mutations are significantly more common than the silent ones, natural selection must be at work.
The scientists don’t actually know why the beetles are turning orange. It may be, for example, that birds showed up on the island that have a harder time spotting orange beetles than green ones. Or maybe some extinct beetle also glowed green, leading to dead-end interbreeding for females who picked the wrong species. Whatever the answer, the scientists have shown that there is an answer out there beyond the random flux of wandering beetles. Now the scientists have to go out and find the last links in this evolutionary chain.
CORRECTION 11/21/03 1 PM: Thanks to Dough Gladstone for pointing out that fireflies are also beetles. Still, my childhood would have been subtly yet significantly different if I had spent it watching unblinking glows at night instead of the lazy winks of lightning bugs.