What Makes a Species? And Why it Matters.

A species should be an easy thing to define. Right?

Well, actually no. Not one bit.

Species are not always totally separate entities from their brethren. In many cases “species” are more like a gradient blending from one into another, without clear boundaries. As a result, there are ongoing discussions in the scientific world about how we define what does and does not qualify as a full species. Actually, to call it a “discussion” is a bit of an understatement. In fact, it’s more of a battle waged in the pages of scientific journals and in conference presentations across the world.

A few years ago, I was attending a meeting of the American Ornithologists Union, and sat in on a talk by a well known avian-geneticist who was arguing for the adoption of the Phylogentic Species Concept (more on this later). During the Q&A afterward, he was attacked by a series of avian ecologists who, in no uncertain terms, called the PSC a heap of bulls—t and requested that the speaker might learn more by shoveling said bulls—t than working in a lab. It quickly devolved into an extended shouting match (which was greatly enjoyed by the many graduate students in the room who only wished there was popcorn).

Speciation stories, that is how one species divided into two or more, are full of weird happenings, which muddies the water of species concepts. One that really throws a wrench into the gears is the “Ring Species”. The classic example of a ring species is the Ensatina escholtzii salamanders of California. Here, an ancestral population along the coast made it over the mountains where natural selection took hold resulting in a new subspecies (still able to interbreed with the source population). Over time the new subspecies expanded further south eventually making it back over the Sierra mountains to the coast, continuing to split into additional subspecies along the way. Finally, it expanded back north along the coast where it overlapped with the ancestral population. And here’s the rub: despite all the subspecies being able to interbreed back up the line, the ancestral population and the evolutionary newbie couldn’t. Weird right? And cool, I think.

A decade later the argument continues unabated, and a new paper in the journal The Auk throws yet another concept into the works with the introduction of the Mitonuclear Compatibility Species Concept.

But let’s back up a bit. First, what is a species concept? Simply, it’s how we define what is and is not a full species. Some of these rely on the behavior, distribution, and appearance of the taxon in question. Others examine the genetics, using math to decide, yes or no to species designation.

Here is a quick breakdown of a few of the two most important:

The Biological Species Concept: This is perhaps the most straight-forward and easy to understand (and why I like it). Proposed by the famous ecologist Ernst Mayr in 1942, the BSC says that if two taxa are in contact with one another and do not breed then they are separate species. Simply: Reproductive isolation (as opposed to geographic isolation) defines a species.

Phylogenetic Species Concept: Proposed by Joel Cracraft in 1983, the PSC posits that a species is the smallest diagnosable cluster of organisms that share a pattern of parental ancestry and descent. This concept relies on the genetics of the organisms and, conveniently can be defined very simply by looking at how much genes differ between taxa. Yes to this, no to that, and you never need to step outside the lab.

There are more (cladistic species concept, ecological species concept, evolutionary species concept, typological species concept, etc.) but most either were merged into or are very similar to the BSC or PSC.

I got to thinking about all this as I was reading the new paper in the ornithological journal The Auk, which proposes a new species concept call the “Mitonuclear Compatibility Species Concept”. That is a mouthful of a title, so let’s just go with an acronmym shall we? The MCSC, like the Phylogenetic Species Concept relies on genetic tools, but this one, includes mitonuclear DNA as the primary measuring tool. (Mitonuclear DNA is information shared by both the Nuclear DNA and Mitochondrial DNA, and is a small subset of the entire genetic code.)

Without boring you all to tears in an explanation I barely understand, this concept, while perhaps cleaner than either the PSC or BSC, is still a quantitative lab-based technique, and holds many of the same challenges of the PSC. Which is to say, the MCSC may divide species where none are obvious in appearance, or range, simply as a result of genetic differences within the population, resulting in the designation of many new species, though they may be entirely unrecognizable in the field.

Empidonax flycatchers like this Alder Flycatcher are already very similar to others in the genus. Can you imagine justifying conservation measures if these were split further making them entirely inseparable in the field?

And here, is where I spout my opinion:

These “cryptic species”, real or not, are problematic. Use of genetic tools, rather than ecological/physiological/anatomical measurements can result in division of existing taxa into multiple, perhaps many, different species. It’s convenient to have a simple equation to split them, but the outcome gets incredibly confusing.

Look, science and conservation are under attack. If we start claiming that there aren’t 700 some species of birds in North America, but actually 1000, or 1500, or 2000, with no clear way for a casual observer to tell the difference, it becomes very difficult to justify conservation efforts for any one taxa. I can hear the political shouting already: “They all look the same! What’s the difference! Who cares?”

Science is already seen as esoteric and this… well this is confusing for me and I have a graduate degree in this field! To make science approachable, compelling, and believable we have to make it understood. And deciding on species boundaries based on DNA arrangement rather than how animals behave in the field, where they occur, or even how they look makes it even more challenging.

Whether two taxa interbreed in nature, as used in the Biological Species Concept, is such a clear and easy to understand concept why mess with it?

As scientists, we need to look at the impact of our decisions, and deciding that species should be defined in the lab, rather than in the field, makes the increasingly important work of outreach and conservation much more difficult.

OK Rant over.

If you want to read about this new species concept and find references to just about all the rest of this stuff, check out the new paper in The Auk:
Hill, Geoffrey E. 2017. The mitonuclear compatibility species concept. The Auk. Vol 134 pp. 393-409. (DOI: 10.1642/AUK-16-201.1.