Why does evolution sometimes repeat itself? Spider-eating spiders may hold the answer

Why does evolution sometimes repeat itself? Spider-eating spiders may hold the answer

Discovering something for the second time might sound like a letdown. Not for ecologists in Hawaii, who have found that spider-eating spiders on four islands there independently evolved the same colors: gold, black, and white. This rare example of parallel evolution, which has also been seen in one other Hawaiian spider, could help clarify one of biology’s biggest mysteries: how and when evolution repeats itself.

“It’s one of the coolest hidden [examples] of animals evolving new species,” says Robert Fleischer, a conservation genomicist at the Smithsonian Conservation Biology Institute in Washington, D.C., who was not involved with the work.

Hawaii is a great place to study evolution. Any animal that washes up or flies there may be separated from its kin by thousands of kilometers, a great opportunity for the formation of new species. And every island within the archipelago is yet another opportunity to diversify as new arrivals move into habitats not yet taken over by other organisms.

So it is with a genus of spiders known as Ariamnes. The up-to-2-centimeter-size forest dwellers, which can camouflage themselves to look like sticks, probably arrived in Hawaii within the past 5 million years. Since then, they have differentiated into numerous species spread out over four islands.

To get a better sense of how these species evolved, Rosemary Gillespie, an evolutionary ecologist at the University of California, Berkeley, and her colleagues obtained DNA from individuals of each species. All four islands—Oahu, Kauai, Maui, and the Big Island of Hawaii—are home to shiny gold and dark species of the spider, and two have an additional dull white version. Gillespie’s team identified four entirely new species, making 15 in all.

Logic would dictate that the spiders that look most alike—i.e., those of the same color—would be the 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 today in Current Biology. Thus, each island was likely colonized by one spider that then diversified into the different-colored species.

The researchers suspect that the first Ariamnes was dark or gold, and that it landed on one of the oldest islands to form, perhaps Kauai, before spreading somehow to Oahu and eventually to the younger Maui and the Big Island. That first spider likely lived on the webs of other spiders, stealing snagged prey and sometimes chowing down on the web’s owner. Once in Hawaii, Ariamnes began to roam and hunt.

The results parallel another study of Hawaii’s “spiny leg” Tetragnatha spiders, also by Gillespie. In one group of this genus of long-jawed spiders, where and what they hunt determines their colors: green, maroon, or brown.

But evolution did not repeat itself in a different group of Tetragnatha spiders that also diversified in Hawaii. Now, Gillespie thinks she knows why. That other group of Tetragnatha spiders are typical, web-building spiders that don’t have to find a place to hide from birds during the day. Webless spiders like Ariamnes and the spiny leg Tetragnatha must very rapidly develop the protective coloration that matches where they hide out. The white species typically rest on pale lichens and the gold ones on the undersides of leaves, where their shininess makes them reflect the leaf’s colors. And the dark ones live under dead ferns or in moss.

“This underscores how a rich environment having few other species spurs rapid evolution in the few [organisms] that by chance managed to get there,” 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 had thought it was primarily competition for food that drove species to diversify and adapt. But here, predation seems key, he adds, and may have even been a “crucial step” in the origin of species that are heavily hunted. Matching the background is so important, that evolution does tend to repeat itself in those situations.

The results may even help scientists predict how other organisms might evolve in parallel to evade predators, the researchers say and help explain why evolution repeats itself sometimes but not other times. But what would be really interesting, says Fleischer, is if Gillespie’s team can identify the genes and the mutations responsible for the color changes. “That would be the Holy Grail.”

Source: Science Magazine

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