Scientists Draw New Spider Web Family Tree

The art of the orb-style spider web has perplexed evolutionary biologists for a long time.

Before DNA sequencing revolutionized the field, they had concluded that spiders who weave silky orbs evolved the ability to do so just once—that is, all creatures in this category had a common ancestor, the ancient master orb-builder.

But now that genetic data is available, experts are threading new clues to solve this decades-long debate. A new study in Current Biology that surveys 159 spider species (and 2,500 genes) suggests that there are actually multiple separate branches of orb-weaving spiders.

A garden orb-weaver spider sits upon its hard-won throne.

This would mean, contrary to what scientists originally thought, that spiders evolved this talent more than once—a tale by the name of “convergent evolution.”

Here’s Veronique Greenwood, reporting for The New York Times:

The resulting spider tree shows a massive network of species whose ancestors began to branch away from each other hundreds of millions of years ago. Over here are the wolf spiders; over there the builders of underground funnel webs, as well as the orb weavers and black widows. Because the researchers could draw on so many more genes and species than in previous studies, they are able to state the relationships among spiders with greater confidence than in the past, Dr. [Gustavo] Hormiga [a professor at George Washington University and an author of the paper] said.

To use the tree to study the evolution of webs built to catch prey, the researchers assigned each species a status: This one made an orb web to hunt, that one made a horizontal web, this one didn’t build a web at all, and so on. Then they asked what the most logical way for those traits to have arisen, looking for the most likely route from ancestors with various different webs to those that spiders build today.

Hormiga and his colleagues used this information to deduce that orb-weaving evolving more than once. In particular, spiders that make sticky orb webs appear to be closely related, while spiders that make non-sticky orb webs have a common ancestor that didn’t use webs for hunting.

Some scientists are expressing doubts, though. They argue that weaving is too complex to have arisen independently as many times as this research implies. Further approaches will be necessarily to construct the definitive family tree for the era of DNA sequencing.