Hawaiian Spiders on Different Islands Evolved Same Disguise in Parallel

In an unusual evolutionary twist, local stick spiders have come up with an almost identical repertoire of color morphs in multiple locations.

Mar 8, 2018
Catherine Offord

A gold morph of the Hawaiian stick spider (genus: Ariamnes)GEORGE RODERICKThe Hawaiian stick spider has evolved the same three color morphs on multiple different islands in parallel, according to research led by biologists at the University of California, Berkeley. The team’s findings, published today (March 8) in Current Biology, provide a rare example of evolution producing the same outcome multiple times and could throw light on the factors constraining evolutionary change.

“The possibility that whole communities of these spiders have evolved convergently is certainly exciting,” Ambika Kamath, a behavioral ecologist at the University of California at Santa Barbara who didn’t take part in the study, tells The Atlantic. She adds that the study provides insight into the “deterministic processes that shape the diversity of life.”

To study the stick spider’s biological past, evolutionary ecologist Rosemary Gillespie of UC Berkeley and her colleagues collected samples of the most prominent color morphs—each camouflaged for a different habitat—on Hawaii’s four largest islands. “You’ve got this dark one that lives in rocks or in bark, a shiny and reflective gold one that lives under leaves, and this one that's a matte white, completely white, that lives on lichen,” Gillespie explains in a statement.

Yet when the researchers sequenced the arachnids’ DNA, they discovered that each morph showed more in common with different color morphs from the same island than with the same color morph from different islands. The phylogenetic tree created using the data suggests that the same color patterns must have evolved multiple times as the stick spiders spread to new habitats.

“They arrive on an island, and boom! You get independent evolution to the same set of forms,” Gillespie says in the statement. “Most radiations just don’t do this,” she adds—although her team has previously reported a similar pattern of convergent evolution in another group of spiders in Hawaii, the spiny-legged Tetragnatha. “Now we’re thinking about why it’s only in these kinds of organisms that you get this sort of rapid and repeated evolution.”

One possibility raised in the paper is that predation has constrained the range of color options available to this camouflaged species. The team has yet to identify the genes responsible for the color changes in the spiders, but an understanding of this process could add insight into the role of predation on evolution in prey, Dolph Schluter, an evolutionary biologist at the University of British Columbia in Canada, who was not involved in the study, tells Science. “This underscores how a rich environment having few other species spurs rapid evolution in the few [organisms] that by chance managed to get there.”

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