Evolution can be complicated, but that doesn't necessarily mean it's unpredictable.
A group of stick spiders in the Hawaiian Islands, for example, apparently evolves into the same three forms every time it colonizes a new island or region. These different species are known as "ecomorphs," a term for organisms that look similar and occupy the same kind of habitat, but aren't as closely related as they seem.
"This very predictable repeated evolution of the same forms is fascinating because it sheds light on how evolution actually happens," says University of California-Berkeley evolutionary ecologist Rosemary Gillespie, lead author of a new study about the spiders, in a statement. "Such outstanding predictability is rare and is only found in a few other organisms that similarly move around the vegetation."
The story of these bizarre spiders begins 2 to 3 million years ago, when an ancestor "sailed" across the Pacific Ocean on long strands of silk. (Yes, some spiders can aerially disperse across oceans.) It's not clear where these sailors came from, but they were pirates, getting food by stealing it from the webs of other spiders.
When they arrived in the Hawaiian Islands, however, they didn't find very many webs to raid. So they branched out a little, developing other ways to survive by not just raiding other spiders' webs, but by trapping and eating the spiders themselves.
A total of 14 new species evolved from these pioneers, each shaped by the ecological niche it learned to exploit. That's adaptive radiation, a phenomenon made famous by Charles Darwin's study of the way finches' beaks evolved in the Galapagos Islands. It's common on remote islands and archipelagos, and it's a key reason why places like the Galapagos and Hawaiian islands are such hotbeds of biodiversity.
In this case, however, something is different.
Evolutionary déjà vu
These 14 stick spiders live in native forests on the islands of Kauai, Oahu, Molokai, Maui and Hawaii, and at first glance, they may only seem to include three species. "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 says in another statement. These colorings let the spiders blend in with a specific habitat type on each island, helping camouflage them from their main predators, birds known as Hawaiian honeycreepers.
Yet despite their resemblances, they really do represent 14 different species. And because the species on each island evolved from one original colonizer, spiders on separate islands that look alike are not each other's closest relatives — for example, a white spider on Oahu is a closer relative to the brown spider on the same island than it is to a similar-looking white spider on Maui. "You can find these spiders in pretty much every habitat on each island," Gillespie says. "This really detailed and finely tuned repetition of evolution of the same form is really quite uncommon."
As Gillespie and her co-authors report in the journal Current Biology, this is a rare case of distinct physical forms evolving repeatedly on each island or region.
"They arrive on an island, and boom! You get independent evolution to the same set of forms," Gillespie says, noting these forms are roughly the same every time. "They don't evolve to be orange or striped. There isn't any additional diversification."
This could mean the spiders have some kind of preprogrammed switch in their DNA, Gillespie suggests, that can be quickly activated to help them evolve into these successful forms. Ecomorphs are relatively rare and not well-studied, though, so more research will be needed to investigate that possibility and reveal how it works.
Adaptive radiation typically produces a variety of styles, as with Darwin's finches or Hawaiian honeycreepers, not a small set of repeating forms. And convergent evolution — when two species independently evolve the same strategy to exploit a niche, like flying squirrels and sugar gliders — doesn't usually happen repeatedly like this. Such a fixed pattern of repetitive evolution has only been documented in a few cases, Gillespie says: the Hawaiian branch of long-jawed Tetragnatha spiders, the Anolis lizards of the Caribbean, and these 14 species of Ariamnes stick spiders.
"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. She's still investigating that question, but she notes these three lineages do have a few things in common. They all live in remote places with few predators, for example, and rely on camouflage to survive in a very specific habitat. They also range freely in the vegetation — neither of the two spiders groups are web-builders, instead actively seeking out prey.
By examining these shared traits, Gillespie hopes to "provide insight into what elements of evolution are predictable," she says, "and under which circumstances we expect evolution to be predictable and under which we do not."
'Weird and wonderful' creatures
That's a worthy goal, but it's not the only — or most urgent — thing she hopes to achieve with this research. Aside from shedding more light on evolution, Gillespie and her colleagues want to highlight the unique ecological power of Hawaii's native forests. The island chain is losing its biodiversity, earning the nickname "extinction capital of the world," but there is still time to protect what's left.
"This study provides insights into a fundamental question about the origins of biodiversity, but also presents a remarkable story that can call attention to the need for conserving nature in all of its forms," says co-author George Roderick, chair of the Department of Environmental Science Policy and Management at Berkeley.
"Often, I hear people saying, 'Oh, Hawaii's so well-studied. What else is there to look at?'" Gillespie adds. "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."