What’s a species?

Good question. The designation “species” is the primary organizing label around which we personally encounter taxonomy (the organizing framework of how life manifests on the planet), but grabbing hold of precisely what constitutes a species can be slippery as a Seal Salamander in a Virginia creek (a creature I tried to grab hold of just this summer).

Take, as examples, the below 3 photographs of salamanders I photographed at different times and places this summer…

Slimy Salamander, Shenandoah National Park, Virginia (USA)

Slimy Salamander, Francis Marion National Forest, South Carolina (USA)

Slimy Salamander, Allegany County, New York (USA)

I’ve captioned all 3 as “Slimy Salamander,” unmistakable by the slick black body speckled flamboyantly with silvery white. For a long time, they’ve been understood as a single species, with the differences in size and density of markings attributed to naturally occurring variation within the species.

But the seeming similarities, and the naming conventions of the past, hide an underlying complexity that defies simple divisions between taxonomic categories. What was once considered a single species (Slimy Salamander, Plethodon glutinosus), or perhaps a few geographically divided subspecies, is now understood to be a complicated tangling of 10 or so species in a “Slimy Salamander Complex.” (I say “10 or so,” because there is still some disagreement over exactly how many species are distinguishable within the “complex.” The Savannah River Ecology Lab says 13. Salamander expert James Petranka says 3.)

In this newer understanding, the 3 photos above are 3 separate species: White-spotted Slimy Salamander (P. cylindraceus), South Carolina Slimy Salamander (P. variolatus), and Northern Slimy Salamander (P. glutinosus). Much of the differentiating details aren’t visible to a field observer, being mostly identified through genetic tests. But genes don’t (often) lie, and so now we know these species are distinct. Unless…

Unless you rely on the most basic definition of “species” we all learned in high school biology class: a group of organisms capable of interbreeding to produce fertile offspring. There are many assumptions wrapped up in that shorthand definition, and Slimy Salamanders contradict most of them because of their habit of interbreeding across species lines. Some texts actually refer to the area where species geographies meet as a “zone of uncertainty” (which sounds like a region of space traversed in Star Trek time travel episodes). This is where interbreeding takes place in ways that confuse species identification. If that weren’t complicated enough, many salamanders in the Slimy complex will hybridize with salamanders outside of the complex altogether, such as with the Jordan’s Salamander (P. jordani). And wouldn’t you know: Jordan’s Salamanders have their own complex of related but distinct species that also may interbreed with one another. All of this makes a neat and tidy definition of the word “species” impossible.

We all have a pretty strong sense of what self-evidently divides a cat from a dog. But it’s hard to find a precise definition which is applicable to all possible levels of differentness between organisms. I think the reason, as it is for most of the wonderful things we encounter and struggle with in biology, is probably evolution.

The radical nature of evolutionary explanations of the world is that categories that divide living things do not have immutable, absolute characteristics. History is a branching, tangled hedge of incremental but inexorable changes in what makes any organism “the way it is.”

Can we see what this means for the fullest understanding of what life is? That all living things are simply variations on a single theme, ultimately related in a single super-lineage whose individual generations are essentially all but indistinguishable in precise definition?

This is what I sense when I imagine myself looking backward along the mind-bendingly long branches of life’s tree. Yeah, I know, pretty deep thoughts that are possibly beyond the scope of what began as a blog reflection on naming  Slimy Salamanders. But I’m inevitably led there each time I turn over a log and wonder about the white-speckled body that scurries away from me underneath.

The varied panoply of all living things disguises the simplest but profoundest truth: all of it, every piece, from the scurrying body under the log, to the mold growing on the bark, to the log itself, to, of course, me, are all diverse expressions of the same phenomenon we call life.

What’s true is that we’re all very different from another while, simultaneously, in a very real sense, we’re the same.