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These Curious Spiders Evolved the Same Way Over and Over and Over Again

A new study suggests the stick spider evolved the same way in multiple different places

New research sheds light on the stick spider's evolutionary history in Hawaii. (George Roderick)
smithsonian.com

The genus of spiders known as Ariamnes has always been known to have a few tricks up its sleeve. Measuring a max of just two centimeters in size, it can hide from predators by camouflaging itself to look like a stick in the forest. Now, scientists have discovered that these stick spiders also have a peculiar evolutionary history.

As Elizabeth Pennisi reports for Science, new research by Rosemary Gillespie, an evolutionary ecologist at the University of California, Berkeley, and her colleagues demonstrates parallel evolution in the genus — that is, spiders on different Hawaiian islands independently evolved species of three different colors: gold, black, and white.

They also found that since arriving in Hawaii, the original group of spiders has diversified, spreading out across five of the islands. That one species formed 14 more over the years, George Dvorsky reports for Gizmodo, creating a total of 15 different stick spider species still alive in Hawaii—four of which the researchers identified in this latest study. Their research was published Thursday in the journal Current Biology.

For the study, the researchers examined the genetics of the spiders on five of the Hawaiian islands where gold and dark species of the spider live. Two of the islands are also home to a white version. Using this data, they constructed a family tree to study the relationships between the creatures.

While it would make sense that spiders of the same color would be the most closely related, Gillespie’s team found that wasn't the case. Residents of a single island were most closely related, while like-colored critters of other islands were distant relatives. This suggests that each island was likely colonized by one spider that then evolved into different-colored species. 

“It’s one of the coolest hidden [examples] of animals evolving new species,” Robert Fleischer, a conservation genomicist at the Smithsonian Conservation Biology Institute, tells Pennisi.

Gillespie and her team think the first Ariamnes was dark or gold. It likely landed on the islands about 2 to 3 million years ago, inhabiting the oldest island of Kauai. In Hawaii, there weren’t many webs to steal food from, according to UC Berkeley's website. So the creatures started to trap and eat other spiders, as they still do today.

They each developed different colors where they could be effectively camouflaged in their own niches, Ed Yong reports for The Atlantic. One species became brown, living on rocks. Others became gold to hide on the undersides of leaves. Still others became a matte-white color, similar to lichen. ​They slowly hopped to the younger islands. And as they did, they would evolve.

What’s interesting is that throughout their history, the spiders would evolve over and over again in a predictable manner. While the different species of spiders have a similar body type, each has distinct physical traits. And their different colors allowed them camouflage themselves in their environments.

“There are only a certain number of good ways to be a spider in these ecosystems, and evolution repeatedly finds those ways,” Catherine Wagner, an evolutionary biologist at the University of Wyoming, who was not involved in the new study, tells Yong.

This diversification of species into different environments is not unexpected. Most famously, Charles Darwin witnessed this phenomenon with finches on the Galapagos Islands. But with the spiders, there isn’t any additional differentiation during each bout of diversification. "They don't evolve to be orange or striped," Gillespie says in a press release. She believes that this suggests the spiders are preprogrammed to evolve quickly to be successful, but how that works exactly isn’t clear.

The study finds similar conclusions to Gillespie's earlier work on Hawaii’s Tetragnatha spiders. The "spiny leg" Tetragnatha spiders developed colors tailored to their hunting habits. But in another group of Tetragnatha spiders, the evolution did not repeat itself. As Pennisi reports, the researchers believe it’s because this other group of Tetragnatha spiders are web-building spiders that don’t have to find a place to hide from birds during the day. 

Gillespie says in the statement that she hopes the research will provide insight into the predictable elements of evolution and the conditions that set the stage for such changes to occur. 

About Julissa Treviño

Julissa Treviño is a writer and journalist based in Texas. She has written for Columbia Journalism Review, BBC Future, The Dallas Morning News, Racked, CityLab and Pacific Standard.

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