Spider webs present an intriguing challenge to evolutionary biologists.
These finely honed death traps come in many forms, from the trampoline-like construction of the sheet web spider, to the instantly recognizable filigree of the orb weaver. Orb-style webs are made by diverse spiders, however, and there are two types, one that’s sticky and one that’s not. Ever since biologists began to sort out how tens of thousands of different species of spiders are related to one another, sketching a very large, many-legged family tree, they have wondered: Did spiders evolve to spin the orb web only once? Or multiple times?
It’s an important distinction, and one that scientists who study the evolution of spiders have fiercely debated.
If it evolved only once, then all those who weave it today are descended from a single, common ancestor. But there could have been a very different path of evolution in which different spider lineages independently arrived at the design. A new study published Thursday in Current Biology supports this hypothesis for spider and web evolution, using genetic data from 159 spider species to draw a new family tree containing multiple distinct branches of orb-weaving spiders.
In the late 1980s and early ‘90s, scientists believed this question was satisfactorily answered, said Gustavo Hormiga, a professor at George Washington University and an author of the paper. Before evolutionary biologists were using DNA sequencing regularly, the consensus was that the two groups that make different versions of the orb web had a common orb-weaving ancestor.
DNA has complicated that picture, however. In the past few years, 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. 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, 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.
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. 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,” 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. 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.”
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