The distinct blinking pattern of fireflies enables mate recognition and, thus, the perpetuation of the species. But it seems this flashy courtship ritual could be driving the formation of new species, too.
A new analysis of diverse animal lineages—from octopuses and deep-sea fishes to glowworm beetles and fireflies—found that groups that choose their breeding partners using bioluminescent pulses accumulate species at a faster pace than their non-glowing relatives.
The findings, described online June 23 in Current Biology, provide some of the strongest support to date for the idea that traits involved in securing a mate serve as an evolutionary engine for global biodiversity.
“It’s long been thought that sexual selection will increase speciation in animals, but if you actually look at the data in a lot of detail it’s very hard to find really convincing evidence,” notes Mike Ritchie, a speciation researcher at the University of St. Andrews in Scotland who was not involved in the study. “This is probably the cleanest and most convincing evidence I’ve seen,” he says. “The lovely thing about it is, firstly it’s clear cut, but it’s also so very, very well controlled.”
Light production has evolved independently more than 40 times, and, like the peacock’s tail or the stag’s antlers, males often use it as a signal to win over mates. But bioluminescence can also have other functions in nature, including in a type of anti-predator defense in which animals emit light to match the background.
These two distinct ecological uses for light emission — courtship and camouflage — provide what W. Leo Smith of the University of Kansas describes as “a perfect example for a clear test” of the hypothesis that sexual displays lead to more species splitting through reproductive isolation and diversification.
In 2014, Smith and his colleagues reported that small marine lanternfishes that use bioluminescence in mate identification had a greater concentration of species than other deep-sea fishes that use bioluminescence for defensive purposes. And last year, researchers in Europe found something similar in sharks. But no one had ever looked to see whether the pattern held up across diverse and distantly related animal groups living on sea and land.
So, Todd Oakley and his graduate student Emily Ellis from the University of California, Santa Barbara, combed the scientific literature for well-characterized evolutionary trees with branches containing animals that glow in some way. They identified 10 with lineages that use light for courtship and another 11 with lineages that use light for camouflage. These included a wide range of marine and terrestrial taxa, including insects, crustaceans, cephalopods, segmented worms, and fishes.
The researchers counted up the number of species in each bioluminescent clade and found that all of the groups with light-courtship displays had more species and faster rates of species accumulation than their non-luminous relatives or ancestors. In contrast, those groups that used bioluminescence for predator avoidance had a diminished rate of species richness on average.
The bottom line, says Oakley: the evolutionary theory was correct. “Sexual selection does actually impact diversity.”