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.