Something to discover for the second time may sound like a disappointment. Not for ecologists in Hawaii, who found that spider spiders on four islands independently developed the same colors: gold, black, and white. This rare example of parallel evolution, also seen in another Hawaiian spider, could help clarify one of biology's greatest secrets: how and when evolution repeats itself.
"It's one of the coolest hidden [examples] animals new species," says Robert Fleischer, a preservative genomist at the Smithsonian Conservation Biology Institute in Washington, DC, who was not involved in the work.
Hawaii is a great place to study evolution. Any animal that washes or flies can be separated from its relatives by thousands of miles, which presents a great opportunity for the creation of new species. And every island within the archipelago is another diversification opportunity, as newcomers move into habitats that are not yet occupied by other organisms.
So it is with a spider genus, known as Ariamnes . The up to 2 centimeters large forest dwellers, who can disguise themselves like sticks, have probably arrived in Hawaii in the last 5 million years. Since then, they have divided into numerous species on four islands.
Rosemary Gillespie, an evolutionary ecologist at the University of California, Berkeley, and her colleagues received DNA from individuals to better understand how these species evolved from each species. All four islands – Oahu, Kauai, Maui and the Big Island of Hawaii – are home to shiny gold and dark spider species, and two have an extra matte white version. Gillpie's team identified four completely new species, a total of 15.
Logic would dictate that the spiders that look most similar – d. H., The same color – most closely related. But when Gillespie's team built a family tree from the genetic data, the closest relatives were spiders living on the same island, the team reports in Current Biology . So each island was probably colonized by a spider, which then diversified to the different colored species.
The researchers suggest that the first Ariamnes was dark or gold, forming on one of the oldest landed islands, possibly Kauai, before somehow spreading to Oahu and eventually to younger Maui and the Big Island , This first spider probably lived on the nets of other spiders, stole prey and sometimes chewed on the owner of the net. Once in Hawaii Ariamnes began to hike and hunt.
The results go in parallel with another study of Hawaiis "prickly legs" Tetragnatha Spiders, also by Gillespie. In a group of this genus of longspider spiders, where and what they hunt determines their colors: green, reddish brown or brown.
But evolution has not repeated itself in another group of Tetragnatha spiders that also varied in Hawaii. Now Gillespie thinks she knows why. This other group of Tetragnatha spiders are typical web-forming spiders that do not need to hide from birds during the day. Webless spiders such as Ariamnes and the prickly leg Tetragnatha need to quickly develop the protective coloring that goes with their hideout. The white species typically rest on pale lichen and the golden ones on the undersides of the leaves where their luster reflects the colors of the leaf. And the dark live among dead ferns or moss.
"This underscores how a rich environment with few other species drives rapid evolution in the few [organisms] that came by accident," says Dolph Schluter, an evolutionary biologist at the University of British Columbia in Vancouver, Canada who was not involved in the study. According to Schluter, most researchers thought that it was above all the competition for food that drove species for diversification and adaptation. But here, predication seems to be the key, he adds, and may even have been a "decisive step" in the emergence of species that are heavily hunted. The consistency with the background is so important that evolution is repeated in these situations.
The findings could even help scientists make predictions on how other organisms could evolve in parallel to evade the predators, the researchers explain, explaining why evolution sometimes repeats but not other times. But what's really interesting, Fleischer says, is whether Gillpie's team can identify the genes and the mutations responsible for the color changes. "That would be the Holy Grail."