Hawaii: Where Evolution Can Be Surprisingly Predictable
On each Hawaiian island, stick spiders have evolved into the same basic forms—gold, white, and dark. It’s a stunning example of how predictable evolution can be.
Most people go to Hawaii for the golden beaches, the turquoise seas, or the stunning weather. Rosemary Gillespie went for the spiders.
Situated around 2,400 miles from the nearest continent, the Hawaiian Islands are about as remote as it’s possible for islands to be. In the last 5 million years, they’ve been repeatedly colonized by far-traveling animals, which then diversified into dozens of new species. Honeycreeper birds, fruit flies, carnivorous caterpillars ... all of these creatures reached Hawaii, and evolved into wondrous arrays of unique forms.
So did the spiders. There are happy-face spiders whose abdomens look like emojis, and which Gillespie started studying in 1987. There are appropriately named long-jawed spiders, which caught her attention years later. Spiders have so repeatedly radiated on Hawaii that scientists often discover entirely new groups of species at once, allowing them to have some taxonomic fun. One genus was named Orsonwelles and each species is named after one of the director’s films; another group is named after all the characters from the film Predator. “The diversity is extraordinary,” says Gillespie, an evolutionary biologist at UC Berkeley.
The most spectacular of these spider dynasties, Gillespie says, are the stick spiders. They’re so-named because some of them have long, distended abdomens that make them look like twigs. “You only see them at night, walking around the understory very slowly,” Gillespie says. “They’re kind of like sloths.” Murderous sloths, though: Their sluggish movements allow them to sneak up on other spiders and kill them.
During the day, stick spiders hide, relying on their camouflage to protect them from the beaks of honeycreepers. Each of Hawaii’s islands has species of stick spider that come in three distinctive colors—shiny gold, dark brown, and matte white. Go to Oahu and you’ll find all three kinds. Head to East Maui and you’ll see the same trio. It would be tempting to think that the same three species of stick spider, one for each color, have traveled throughout the island chain. But the truth is much stranger.
Gillespie has shown that the gold spiders on Oahu belong to a different species from those on Kauai or Molokai. In fact, they’re more closely related to their brown and white neighbors from Oahu. Time and again, these spiders have arrived on new islands and evolved into new species—but always in one of three basic ways. A gold spider arrives on Oahu, and diversified into gold, brown, and white species. Another gold spider hops across to Maui and again diversified into gold, brown, and white species. “They repeatedly evolve the same forms,” says Gillespie.
Gillespie has seen this same pattern before, among Hawaii’s long-jawed goblin spiders. Each island has its own representatives of the four basic types: green, maroon, small brown, and large brown. At first, Gillespie assumed that all the green species were related to each other. But the spiders’ DNA revealed that the ones that live on the same islands are most closely related, regardless of their colors. They too have hopped from one island to another, radiating into the same four varieties wherever they land.
One of the most common misunderstandings about evolution is that it is a random process. Mutations are random, yes, but those mutations then rise and fall in ways that are anything but random. That’s why stick spiders, when they invade a new island, don’t diversify into red species, or zebra-striped ones. The environment of Hawaii sculpts their bodies in a limited number of ways.
The shiny gold spiders, for example, are perfectly camouflaged against the undersides of leaves. The dark brown ones are especially hard to spot against rocks and bark. And the matte-white ones blend in among the white lichens that festoon Hawaii’s forests. “There are only a certain number of good ways to be a spider in these ecosystems, and evolution repeatedly finds those ways,” says Catherine Wagner, an evolutionary biologist at the University of Wyoming, who was not involved in the new study.
These “ways to be a spider” can also be constrained by the creatures themselves, Wagner adds. For example, the golden spiders get the color from pale-yellow pigments that lie over a layer of guanine crystals on their bodies. If those crystals are thin plates, they produce shiny reflections that accentuate the overlying yellows, producing a metallic gold. If the crystals are small cubes, they scatter light instead of reflecting it, allowing the natural white color of the guanine to come through. And if the guanine crystals are absent altogether, the dark color of the spider’s underlying organs overwhelms the overlying pigments. Perhaps that’s why the spiders always come in the same basic trinity of hues: They’re the easiest to produce, with minimal tweaks.
In 1989, evolutionary biologist Stephen Jay Gould mused about what would happen if you replayed life’s tape—that is, if you rewound time and allowed history to play out again. Would the same species evolve from the same ancestors? “If each replay strongly resembles life’s actual pathway, then we must conclude that what really happened pretty much had to occur,” he wrote. Gould felt that was unlikely, but Hawaii’s spiders say otherwise. They’ve done a version of Gould’s experiment, and they’ve shown that under some circumstances, evolution can indeed be highly predictable.
It’s not just spiders, either. Luke Mahler and Jonathan Losos have shown that across the Greater Antilles, anole lizards have repeatedly converged on the same basic forms. Each island has a species that lives among twigs, another that dwells among grass, and yet another that hugs tree trunks. “If you weren’t an anole biologist and someone came down, blindfolded you and put you on a different island, you’d think: Oh yeah, those are the same lizards,” Mahler once told me. But they’re not. They’re the product of the same repeated convergent evolution that produced the gold, dark, and white stick spiders.
The question then is not whether evolution is predictable. Clearly, it can be. It’s more interesting to ask under what conditions it is predictable. Again, Hawaii has a lot to say on that. The stick spiders and long-jawed spiders have repeatedly evolved into the same physiques, but other lineages of Hawaiian spiders have not. Why did evolution go to town when shaping their bodies, while retreading its greatest hits when crafting the stick spiders, long-jawed spiders, and anoles? What do those three groups have in common?
Gillespie says that they’re all free-living animals that hide from predators during the day. They need to blend in, but there’s only a limited number of backgrounds for them to match. And since they live on islands, they only face a limited range of predators, with a narrow range of perceptual skills. All of this constrains their evolutionary fates.
Ambika Kamath from the University of California at Santa Barbara notes that the stick-spider story might change in the future as new species are described or redescribed. “The speed at which this radiation took place can make the classification of species tricky, and the taxonomy of this group may be in flux for a while,” she says. That said, “the possibility that whole communities of these spiders have evolved convergently is certainly exciting” and will provide important insights into the “deterministic processes that shape the diversity of life.”
Gillespie adds that there’s an urgency to this work. For millions of years, islands like Hawaii have acted as crucibles of evolution, allowing living things to replay evolution’s tape in the way that Gould envisaged. But in a much shorter time span, humans have threatened the results of those natural experiments. “The Hawaiian islands are in dire trouble from invasive species, and environmental modifications,” says Gillespie. “And you have all these unknown groups of spiders—entire lineages of really beautiful, charismatic animals, most of which are undescribed.”