On the ballooning of spiders and deep evolutionary branches

To keep track of projects, I use a sophisticated app called Moleskine. But early on in grad school, when I had a new project, I created a disk for everything related to that project. Like this:

One of these disks was labelled “Ballooning Spiders.” I had an idea for a side project that I humored for a few days.

I thought the ballooning behavior of spiders was pretty awesome. I still think the ballooning behavior of spiders is pretty awesome. I imagined it was quite likely that spiders could balloon across entire oceans. (Twenty years later, we know that’s true.)

Aerial routes through the atmosphere are not reciprocal. Spiders from one location might be able to make it to another continent, but I imagine that spiders on that continent won’t be making their way back. For spider species with cosmopolitan distributions that might balloon (?), this would cause some funky gene flow.

I was more curious about the possibility of colonization-by-ballooning events, and how this might be detected through phylogenetic reconstruction. If the establishment of new populations from ballooning isn’t very frequent (like how birds or flies establish on oceanic archipelagos), then you can imagine that whole lineages might result in adaptive radiations predicted by atmospheric circulation cells. This might be seen in the relationships within, or among, families or maybe even more towards the base of a spider tree. I read up on this as much as possible, but wasn’t sure about the spider phylogenies back then.

So I emailed Jonathan Coddington (a well-known spider gent who was, and is, at the Smithsonian). He graciously replied promptly and said it was interesting, but there was absolutely none of the resolution in the evolutionary relationships of spiders to get at this question. It couldn’t be a side project, it couldn’t even be a dissertation.

Maybe things are different now? Well, a study from last year suggest it’s quite possible. Before that, a phylogeographic study within a single genus shows how spiders got from Africa to Madagascar within the last few million years. So, it’s not so crazy to think that with a deeper tree of spiders, that bigger intercontinental relationships might be observed.

My sister-in-law runs a letterpress shop in Columbus called Igloo Letterpress. (If you’re at OSU, it’s just up the road and totally worth checking out, by the way, the shop is incredible.) One Christmas they made a print for me, putting beautiful huge old wood type to use with a Richard Feynman quote, which now adorns my office:

SCIENCE IS THE BELIEF IN THE IGNORANCE OF EXPERTS.

I’m not keen on Feynman as a human being, but we do see eye to eye about how discoveries are made in science.

The spider idea has traveled around with me though seen no use, like that floppy disk of my fieldwork conducted in January 1996. Over the years, I think I’ve mentioned it to a few spider people or phylogeographers (though I don’t recall mentioning it to arachnologist Catherine Scott, contributing editor of this site). The general response I’ve gotten has been along the lines of, “Well, maybe?”

I’m not saying that this notion — that deep phylogenies of spiders are partially explained by ballooning among continents — is actually right. As a biogeographer-of-sorts, it seems quite possible in my view, but I have no idea if it meshes with what is known about the evolutionary history of spiders. I don’t know if we know more than we did when I emailed Coddington twenty years ago. I just want to pose this as an example of a naive question that experts might not consider in depth, even though it’s not demonstrably false. It’s easy to look back at history to identify specious notions that get poo-pooed by experts but end up being right. It’s harder to come up with such notions that haven’t been tested by time. What assumptions are we making about our own fields, that aren’t adequately supported by data, that get in the way of understanding the world?