Two wolf spiders, whose names are Pardosa lapponica and Pardosa concinna, run across open ground all over northern Canada. Here’s the problem: these two species of spiders live in a lot of the same places, and they look very similar. Katie Sim, a grad student working with Chris Buddle and me here at McGill, asked the obvious question: are these spiders really separate species? Katie’s insights on that question were just published in the journal Zootaxa.
As taxonomists, we can use multiple kinds of evidence to determine species limits. This includes things like morphology, genetic sequence data, geographic distribution, and ecology. These two species were originally described from widely separated areas: P. lapponica from Lapland, and P. concinna from Colorado. But since then they’ve been found in many more sites and we now know that their ranges overlap in northern North America.
The other long-accepted way of distinguishing between these two species was a small morphological difference between their reproductive structures (many closely related arthropods look very similar externally, but if there are differences, we often see them in the genitalia. “Why?” is a topic for another post).
As Katie collected spiders as part of our Northern Biodiversity Program fieldwork in northern Canada, she realized that the morphological differences between the two species weren’t that clear-cut, once you take variation into account. Based on careful measurements of specimens from all across the north, Katie found overlap in almost all morphological characters, even genitalic characters that had been used in the past. There was only one small piece of the complex male mating structures (the terminal apophysis, for the spider fans reading along) that seemed to hold up as a difference between the species (and only the males, obviously). Question marks started to appear.
Katie’s next step was to delve into the genetic differences between the two species. Even though species can look very similar externally, DNA sequence data sometimes uncovers fine differences between them. This is especially helpful with closely related, or recently diverged species. Katie used the DNA barcode, a section of the mitochondrial gene CO1, which has proven pretty useful for distinguishing animal species. And the DNA results showed some interesting patterns, some of which were unexpected.
The figure above is a haplotype network. Each circle is a little island of genetic similarity, connected to other islands by the lines. We’d expect different species to be part of separate “islands”, but that didn’t happen here. Pardosa lapponica (in light gray) and P. concinna (in black) sometimes share the same haplotype, and each of the two has multiple haplotypes. That means there’s more genetic variation within a “species” than between them. But wait! There’s more!
After a suggestion from one of the reviewers on an earlier version of the paper (this back-and-forth of suggestions is one of the strengths of peer-reviewed science), Katie looked at the CO1 barcode sequences of P. lapponica specimens from northern Europe, where it was originally described. Unexpectedly, the Russian specimens (the dark gray circles without numbers in the figure above) were genetically distinct, by a good margin, from the North American specimens of P. lapponica.
So what does this all mean, taxonomically? First, the spider we call “Pardosa lapponica” in North America seems not to be the same species as “Pardosa lapponica” from northern Europe (which “owns” the name, because it was described from there first). Our North American P. lapponica may, in fact, be the same species as the spider we’ve been calling Pardosa concinna, but before we can make the final decision on that, it would be necessary to study additional North American specimens, especially from Colorado (the “type locality”, or collection site of the original P. concinna), to confirm this.
And that’s how taxonomy often works: good, careful research will answer one question, and in the process, new questions pop up. Sometimes, you think you know a spider, and sometimes, you realize you really don’t.
Sim, K.A., C.M. Buddle, and T.A. Wheeler. 2014. Species boundaries of Pardosa concinna and P. lapponica (Araneae: Lycosidae) in the northern Nearctic: morphology and DNA barcodes. Zootaxa: 3884: 169–178.