DNA has complicated that picture, however. In the last few years, Dr. Hormiga’s lab and others have built detailed family trees by sequencing small sections of spiders’ DNA. In these trees, spiders that have similar genetic markers are deemed more closely related to one another than to spiders whose markers are different. In order to have more points of comparison, the team behind the new paper used a more recently developed approach to compare approximately 2,500 genes.
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. Hormiga 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.
Dr. Hormiga and colleagues say based on this analysis that the ability to make orb webs must have arisen multiple times. On the tree, spiders that make sticky orb webs are all closely related, but the makers of non-sticky orb webs have an ancestor that didn’t use a web for hunting at all. Dr. Hormiga and colleagues write that the hypothesis that the orb web evolved once and was simply passed down “crumbles” under this evidence.
“It puts it on very solid ground that the orb web evolved more than once,” Dr. Hormiga said.
Not everyone agrees. Jason Bond, a professor at Auburn University who, with collaborators, has also published genealogies for spiders in recent years, says the tree itself is an admirable, solid piece of work, and rises above earlier efforts.
But he expressed doubts about the researchers’ conclusions. Dr. Bond said that because of the way the researchers categorized the data on web architecture, their analysis creates, among other things, the impression that the use of webs for hunting evolved independently more than 10 times among spiders. He said that seemed implausible, given the complexity of this particular way of spinning a web.
“I would say these authors have climbed pretty far out on to a limb,” he said. “It will be interesting to see if it holds.”