Untangling Spiders' Evolutionary Web

Bijal P. Trivedi
National Geographic On Assignment
October 31, 2003
Chicago has a new resident—a species of recluse spider found elsewhere in Illinois but not in the city until recently, when a pest control company spotted it.

The spider is harmless to humans but invaluable to scientists embarked on a project called the Spider Tree of Life. The project's goal is to sketch evolutionary relationships among the 37,500 known species of spiders via a representative sampling of 500 species.

In his Origin of Species, in 1859, English biologist Charles Darwin wrote of "the great Tree of Life" that connects all organisms past and present and "covers the surface with its everbranching and beautiful ramifications."

In a contemporary manifestation of Darwin's vision, the National Science Foundation in Arlington, Virginia, has launched the Assembling the Tree of Life project and awarded U.S. $17 million in grants last year to 25 institutions to trace branches of flora and fauna evolution.

"Spiders are the dominant terrestrial carnivore," said Ward Wheeler, curator in the division of invertebrate zoology at the American Museum of Natural History in New York and lead investigator for the U.S. $2.7 million Spider Tree of Life project. "They are all predators—they have a huge influence on the ecosystem."

The Spider Tree of Life quest has led Petra Sierwald, assistant curator of zoology and insects at Chicago's Field Museum and a colleague of Wheeler, to prowl downtown basements. Recently she found a family of four females, four males and three "teenage" recluse spiders in an apartment building.

"The basement is a really nice place for these spiders," Sierwald said. "The heating ducts bring warmth, leaky pipes moisture, and the rats and cockroaches provide a great food delivery service."

From Chicago to Myanmar

Last summer Sierwald traveled much farther from home—to Myanmar. As a guest of the forestry department there, she taught courses on arthropod collection to local students and then found specimens of nursery web spiders and wolf and lynx spiders in a park near Yangon.

Different spiders required different capture techniques. For well-camouflaged webs, Sierwald uses an atomizer to spray cornstarch, which "paints" the webs. She finds the spider and either flicks it into a vial with a spoon or uses a suction device.

A flashlight turns up the wolf spider, whose eyes shine turquoise in the beam. "The reflections are so striking that you expect a very big spider—but often they are just little guys the size of the nail of my pinky finger," Sierwald said.

The need to retrace the Tree of Life stems from the belief that earlier methods of taxonomic classification prior to about 1970 were flawed.

"Taxonomy was more art than science and was theoretically misguided," said Jonathan Coddington, another colleague of Wheeler and a curator of arachnida and myriapoda at the Smithsonian's National Museum of Natural History in Washington, D.C.

For the past three decades, scientists have favored a new approach that compares specific morphological, behavioral, and genetic characteristics among species, taking advantage of computer power to tease out evolutionary links.

Hundreds of characteristics for each species are fed into a computer matrix that compares these with equivalent features from thousands of other species. Only recently have there been powerful enough computers to support such calculations.

Spider Silk

Museums contain archive arachnid collections but new specimens need to be collected so that the DNA can be extracted for analysis. Specialists are traveling the world to seek about 500 species of spiders, a sampling of most of the 108 spider families.

Jason Bond, an evolutionary biologist at East Carolina University in Greenville, North Carolina, specializes in trapdoor and funnel-web spiders, and tarantulas. The last two summers, Bond has hunted spiders in Australia and South Africa. With a small shovel and pick he digs up the trapdoor spider burrows, which vary in length from one inch to two feet (2.5 to 61 centimeters).

In Australia, Bond collected 32 types of trapdoor spiders, including a close relative, the funnel-web—the most lethal specimen to humans, but not fully understood.

"The selection of spiders represents diversity as well as focusing on particular species whose evolutionary history is controversial," Bond said.

The Spider Tree of Life is important to science—and to the silk industry, among others. Weight for weight, spider silk is stronger than steel and any manmade fiber like Kevlar.

"The two silk genes currently being used commercially are from two spiders that are not even particularly good at silk production," Coddington pointed out.

"If you want to produce silk using spider genes then you want to choose the spider whose life depends on the strongest silk—as it stands we don't know how far spiders have taken silk development." The Tree of Life could help determine the best silk-spinners.

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