582. How Did Spiders Diversify?

Darwin's Bark Spider web reaches two and half meters wide

By Sindya A. Bhanoo, The New York Times, November 7, 2011

There are thousands of species of orb-weaving spiders, found on every continent but Antarctica.

Now, researchers report that these spiders that build spiral-shaped webs first emerged about 230 million years ago, during the Middle Triassic period.

They then rapidly diversified, and a wide variety of orb weavers emerged by the middle of the Jurassic period, about 170 million years ago, according to a study in the journal Proceedings of the Royal Society. In the past, scientists thought that orb weavers diversified when flying insects did — that the spiders needed to specialize to catch different insects.

But the new research indicates that “there is no real direct relationship,” said the study’s first author, Dimitar Dimitrov, an evolutionary biologist at the University of Copenhagen. In fact, he said, flying insects diversified 100 million years after the orb weavers.

He believes that orb weavers diversified for another reason: to create specialized habitats. Some create webs ideal for tree branches, while others specialize in webs better suited for tree trunks and tall grasses.

“When you see them today they might be in the same place, at the same time, but occupying different niches,” he said. Dr. Dimitrov and his colleagues used DNA samples of existing spiders, along with fossil remains, to create an evolutionary tree and a timeline.

“We were able to find that the orbs have a single origin,” he said.