Evolution tracks predictable ways of being

Ed Yong writes:

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. [Continue reading…]

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