Rethinking Spiders' Evolutionary Web

A massive study of spider genetics is showing that orb weavers - with their unique webs - may not have all descended from the same ancestor. Instead, orb weaving itself may just be a common and often necessary evolutionary option.

Experts have thought for decades that spider evolution followed a predictable path - one based on the theory of inheritance. That is, when the first spiders developed their own separate traits to adapt to changing ecosystems, those traits became the standard blueprint. Now we have spiders that trap, spiders that "fish" the skies, and spiders that lie in and wait on cleverly hidden webs.

When the first orb weaver spread out across the world, its trait of orb weaving stuck with it; and now, it has hundreds of descendants with the ability to weave orb-like webs in many different ways that are fine-tuned to their environments.

This theory sounds perfectly reasonable, but now researchers from Harvard University claim that it's just plain wrong.

In the largest phylogenic study of spiders ever conducted, researchers found that the two largest groups of orb weavers do not share a common ancestor.

"There are two possible explanations for this," researcher Gonzalo Giribet said in a statement. "One is that the orb web evolved far back in the lineage of the two groups, but has been lost in some groups. The other option is that the orb web evolved independently in these two groups. We still haven't resolved that question yet - we need to sample many more of these intermediate groups before we can say which option is correct."

Giribet added that he suspects it is the latter of these two options, in which orb weaving developed simply because it was necessary for survival - not because it was an inherited trait from a long-lost ancestor.

For the two largest groups, while the structures of their webs are similar, this may just be a coincidence, as their strategies for making them and using them are remarkably different.

"Their behaviors are very different, the types of silk are very different  - it's my bet that these would be two separate evolutions of a similar structure of web," Giribet said.

Still, more analysis will be required to arrive at any concrete conclusions. The researchers plan to map out the genetics of as many as 150 spider species in the future.

The study was published in Current Biology.