The soft, blinking lights of fireflies aren’t just beautiful—they may also play a role in creating new species. A new study shows that using light-up powers for courtship makes species split off from each other at a faster pace, providing some of the clearest evidence yet that the struggle to find mates shapes the diversity of life.
The firefly’s glow, like the enormous claws of fiddler crabs and the elaborate dances of manakins, was sculpted by the struggle for sex. Scientists have long thought that this kind of mating-driven natural selection—called “sexual selection”—could make species split into two. Say females in two populations prefer different color patterns in males: Even if the populations have the same needs in every other way, that simple preference could make them split into species with males of separate colors. “A lot of closely related species differ in sexual traits,” says Emily Ellis, an evolutionary biologist at the University of California (UC), Santa Barbara.
But actually linking this kind of evolution to species proliferation is a hard idea to test. “So many people have looked at this and found differing results,” she says—possibly because they looked at smaller groups, like birds, rather than across the whole tree of life.
That’s where bioluminescence comes in. Many groups of living organisms, from insects to fish to octopuses, emit light, whether to ward off predators, dazzle prey, or attract mates. It’s a trait that has evolved more than 40 times across the animal kingdom, Ellis says.
She and her collaborator Todd Oakley, also at UC Santa Barbara, combed through scientific papers and came up with the family trees of 21 bioluminescent groups. Some of the groups use their glowing powers for sex, whereas some others—all marine creatures—use it for a type of camouflage: By lighting up their bellies, they match the light streaming down from the water’s surface, effectively becoming invisible to predators lurking in deeper waters.
Oakley and Ellis zoomed in on a specific kind of fork in the tree of life: On one branch is a group that uses bioluminescence for courtship; on the other, a nonglowing “sister group” descended from the same common ancestor. The researchers identified 10 such forks, and then compared the amount of species on the two branches.
They also identified 11 forks consisting of a branch bearing bioluminescent species that glow only for camouflage and a nonglowing sister branch. Once again, they compared the number of species on each branch in order to determine whether the bioluminescence itself or specifically its sexual use drives the changed rate.
Sure enough, the researchers reported online yesterday in Current Biology, the glowing mate-seekers have flourished. Every one of the groups that use bioluminescence for courtship contained more species than its sister group—about nine times as many, on average. For example, bioluminescent sharks known as “lanternsharks” comprise 37 species, compared with just one species in their sister group. Groups that use their glowing powers for camouflage, on the other hand, don’t tend to contain more or fewer species than their closest relatives.
In a second analysis, Ellis and Oakley calculated how quickly a group would be expected to rack up new species, based on the timing and species number of the even larger group that contains it. Four out of seven groups that use glowing courtship outperformed expectations, whereas not a single one of the eight groups using bioluminescence for camouflage did—another indication that evolution that happens because of sex can make light-up species split more quickly.
Until this point, there hasn’t been a lot of evidence to support the idea that sexual selection could make species more diverse, says Fredric Vencl, an evolutionary biologist at the State University of New York at Stony Brook. “This study goes a long way to remedying that.” He praises what he calls their “conservative” approach: By calculating estimated rates based on the larger groups, the authors specifically made it so that they would be less likely to see an effect. And they saw one anyway.
Beyond addressing an important question among scientists, the study showcases the wild diversity of bioluminescent life: From fireflies to glowing sharks to weird marine worms, creatures all over the world have figured out ways to use light to woo mates. Even Vencl, who studies fireflies himself, says he was surprised at the number of different groups that have evolved bioluminescent courtship. “They looked, and they found,” he says. “That’s the ingenuity of this research.”