We don't usually expect evolution to be predictable. But Hawaiian stick spiders of the Ariamnes genus have repeatedly evolved the same distinctive forms, known as ecomorphs, on different islands, researchers report on March 8 in the journal Current Biology. Ecomorphs -- which look the same and live in the same kinds of habitats, but aren't as closely related as they appear -- are surprisingly rare, and the researchers hope that these newly described ones might help us understand what's behind this strange evolutionary pattern.
The stick spiders live in the forests of the Hawaiian archipelago, over 2,000 feet above sea level, on the islands of Kauai, Oahu, Molokai, Maui, and Hawaii. Although they're nocturnal arthropods that can't see well, they're still brightly and distinctly colored. "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," explains Rosemary Gillespie, an evolutionary biologist at the University of California, Berkeley.
These different colorings allow the spiders to camouflage themselves against specific similarly colored surfaces in their respective habitats and avoid their major predator, birds called Hawaiian honeycreepers. But what's remarkable is that as the spiders have moved from one island to the next during their evolutionary history, these same forms have evolved over and over again. This process produces new species that are more closely related to spiders of different forms on the same island than they are to lookalikes from other islands.
And it happens fast -- at least in evolutionary time. A dark spider that hops from an old island to a new one can diversify into new species of dark, gold, and white spiders before gold and white spiders from the old island have time to reach the new one. "They arrive on an island, and boom! You get independent evolution to the same set of forms," Gillespie says.
It's also important that these forms are the same each time. "They don't evolve to be orange or striped. There isn't any additional diversification," she says. This, she believes, suggests that the Ariamnes spiders have some sort of preprogrammed switch in their DNA that can be quickly turned on to allow them to evolve rapidly into these successful forms. But how that process might work is still unclear.
It hasn't really been studied, because ecomorphs aren't common. "Most radiations just don't do this," she says. Typical adaptive radiation, like with Darwin's finches, usually produces a wide diversity of forms. And convergent evolution, where two different species independently evolve the same strategy for fulfilling a certain niche, doesn't usually happen repeatedly. There are just a few good examples of this kind of fixed pattern of repeated evolution: the Ariamnes spiders, the Hawaiian branch of the Tetragnatha genus of long-jawed spiders, and the Anolis lizards of the Caribbean.
"Now we're thinking about why it's only in these kinds of organisms that you get this sort of rapid and repeated evolution," Gillespie says. While it's a question she's still working on, the three lineages do all live in remote locations, have few predators, and rely on their coloring to camouflage them in a very particular habitat. They are also all free living in the vegetation: neither of the two spider groups builds a web, which means that they, like the lizards, are free to move about and find the kind of habitat they require for camouflage. She hopes that examining what these groups have in common will "provide insight into what elements of evolution are predictable, and under which circumstances we expect evolution to be predictable and under which we do not."
She also hopes that this research will help the world to understand how much Hawaii's vulnerable forests still have to offer. "Often, I hear people saying, 'Oh, Hawaii's so well studied. What else is there to look at?' But there are all these unknown radiations that are just sitting there, all these weird and wonderful organisms. We need everyone to understand what's there and how extraordinary it is. And then we need to see what we can do to protect and conserve what still waits to be described."