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Convergent Evolution Strikes Again: Orb-Weaver Spiders

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Orb-weaver spiders are one of the most common types of spiders. They're the ones you find in your garden, or when you're on a hike, building those beautiful big circular webs. Conventional evolutionary thinking assumed that they were monophyletic -- i.e. all orb-weaving spiders are descended from a single common ancestral orb-weaving spider. According to a new molecular phylogenetic study of spiders, orb-weaving spiders actually evolved independently. Science Daily explains:

The largest-ever phylogenetic study of spiders, conducted by [Harvard] postdoctoral student Rosa Fernández, Gonzalo Giribet, Alexander Agassiz Professor of Zoology [also at Harvard], and Gustavo Hormiga, a professor at George Washington University, shows that, contrary to long-held popular opinion, the two groups of spiders that weave orb-shaped webs do not share a single origin.
Or as Nature News puts it:
The spiders that spin orb-shaped webs belong to two main families -- araneoids, such as garden spiders, and the more obscure deinopoids, which include ogre-faced spiders. The two groups spin webs out of chemically distinct silks -- araneoids weave a sticky fibre that is made more efficiently than the dryer webs of deinopoids. But because the groups weave webs similarly, arachnologists have long put the two together on the spider family tree in a group called orbicularians.

But now, molecular family trees, made by comparing DNA differences in hundreds of matching genes in numerous spiders, paint a very different picture. Although the findings support many previous ideas about spider family relations -- such as the distinctiveness of trapdoor spiders, which build subterranean traps of dirt, silk and plants -- orb weaving araeonoids and deinopoids seem to be more distantly related than previously thought.

The technical paper in Current Biology includes a somewhat concerning comment that the convergent evolution of orb-weaving spiders was resisted because it contradicted "the predominant paradigm of spider evolution":
The notion of nonmonophyly of Orbiculariae based on molecular data is not entirely novel, but it is at odds with the predominant paradigm of spider evolution, as it contradicts most morphological and behavioral data analyses. For this reason, molecular studies have often dismissed results that do not recover monophyly of orbicularians as artifactual or even "false."

(Fernández et al., "Phylogenomic Analysis of Spiders Reveals Nonmonophyly of Orb Weavers," Current Biology, 24: 1-6 (August 4, 2014).)

If you're an evolutionist, it makes sense that you would resist this conclusion: complex, shared traits like typically suggest homology, or derivation from a common ancestor. This data provides yet another example of how biological similarity does not necessarily imply inheritance from a common ancestor.

Now obviously when confronted with these situations, there are generally two options: One is that orb-weaver spiders evolved twice. The other is that some ancient spider ancestor spun orb-webs, but many of its subsequent ancestors lost the ability to do so. The paper in Current Biology thus states:

These results imply independent origins for the two types of orb webs (cribellate and ecribellate) or a much more ancestral origin of the orb web with subsequent loss in the so-called RTA clade. Either alternative demands a major reevaluation of our current understanding of the spider evolutionary chronicle.

Nature News quotes Jason Bond, an evolutionary biologist at Auburn University who led the survey, offering his preference: "Bond, however, favours the idea of a very ancient web-weaving behaviour that was lost over the course of evolution. The suite of behaviours required for web-weaving seem too complicated to have evolved over and over again, he says."

Too complicated for convergent evolution? That hasn't stopped evolutionists from invoking such an explanation in other cases when they were faced with no other more palatable option.

Whatever option you choose, however, the problem remains: the data fail to fit the treelike pattern predicted by common descent.

Image source: Wikipedia.