Hawaii: Where Evolution Can Be Surprisingly Predictable - The Atlantic - 0 views
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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.
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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.
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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.
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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.
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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.
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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.”