What The Hell Is A Species Anyway?

The Sumatran tiger, which is different from other subspecies of tiger for reasons (Image Credit: Bernard Spragg, CC0 1.0)

We’re only 3 months in, but 2022 has been a hell of a year for species-related controversy. Grolar and pizzly bears have come roaring into public consciousness, researchers proclaimed that the T-Rex we know and love is actually three different species, and soon-to-be minted Doctor Yi-Kai Tea has been sinking and raising some truly glorious fish species like nobody’s business (we call this taxonomic ha-wrasse-ment).

With the classification calamities flying thick and fast, it’s easy to wonder exactly what it is about naming a species that is so damn hard. So let’s have a quick runthrough of what a species is, why taxonomy is so damn complicated, and why it even matters.

Taxonomy is a natural by-product of humanity’s wont to put things in boxes. The species is the smallest of the boxes (unless you count sub-species and good golly gosh let’s leave them for Vanessa to deal with). Larger boxes include:

  • Genus – our species is Homo sapiens, which makes our genus Homo
  • Family – in our case, hominids, but others include the Canidae (dog-like carnivores) and the Nephropidae (the lobsters)
  • Order – primates for us, but rodents, decapods and Psittaciformes (most parrots) are also orders

Defining species has been an absolute riot ever since the first dinosaur fossil was labelled Scrotum humanum, having been thought to be a giant’s fossilised… you get it. For a long time, scientists went with the biological species concept, which defined a species by its reproductive capacity. If two individuals could mate and produce genetically viable offspring, they were the same species.

That sounds straightforward, but thanks to the delightful intrigue brought about by the concept of hybridisation, it’s not. Polar bears and grizzlies are undoubtedly genetically distinct, yet the offspring of the two are capable of reproducing. Greater Canada Geese are capable of interbreeding with multiple goose species. And let’s not forget that most humans have a touch of other hominid DNA in us, whether it’s from early humans interbreeding with Neanderthals or Denisovans, two different species of hominids we interacted with during our species’ early years.

So if the biological species concept is restrictive, what are the other options? Phylogenetic species are defined “as the smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals (semaphoronts)” (Nixon and Wheeler, 1990). Basically, you group a bunch of individuals that share a common ancestor together because they all share similar characteristics. This allows for interbreeding between species.

The problem here is that ‘similar characteristics’ can have vastly different meanings when you jump between groups of animals. Some groups are easy to pull apart owing to very different features. Others may look very similar, but have enough genetic or behavioural differences that they’ll be called separate. Some species end up on the opposite end of the spectrum, whereby they look completely different, but owing to environmental effects are almost genetically identical. 

Take the cold-water salmonid fish species, the Arctic charr. In some far north lakes, its the only fish species around, which means it often turns to cannibalism. This can lead to two very different ‘morphs’, with one morph feeding on other Arctic charr and growing larger, while a second ‘dwarf’ morph stays small, feeding on insects and larvae. This leaves us with two very different looking fish of the same species. The same often happens with plants at different altitudes. Limited oxygen and water can often leave plants at higher altitudes looking stumpier and more shrub-like than their low-altitude counterparts.

Three different morphs of Arctic charr that develop under different conditions (Skoglund et al., 2015)

The other thing that complicates matters is the human element. New taxonomists are almost always having to wade into someone else’s work and decide how to move on with it. That earlier work has resulted in a classification influenced by previous researchers available technologies, prominent teaching methods, and general attitude on life. As Kai puts it: 

“[E]ven with traditional taxonomy, where A and B are very clearly different species, it’s not just about sorting things and putting them into categories. A lot of taxonomic work builds upon previous taxonomic work, and in some groups – for example fungi, some groups of flies, some groups of fishes – there’s just been historically very scant literature. That makes it really hard to do work in the present. It means there’s a lot of missing parts to the puzzle you’re trying to put together.”

Dr Yi-Kai Tea (see the full interview here)

Kai at least gets to work with living specimens. So what about the researchers who spend their time trying to decide which versions of long-extinct animals get their own species name? This was brought into light two months ago, when three American researchers decided that the T. rex itself might actually need to be split up into different species. The claim caused a storm of conjecture, a term that (and this is just my take) seems to apply to much of palaeotaxonomy. There have been many claims in the past of juvenile dinosaurs as separate species – the equivalent of looking at a human baby’s skeleton and assuming it’s some sort of goblin.

I encourage vitriolic comments regarding the T. rex in the comments below (no, no I don’t, behave yourself palaeotwitter) (Image Credit: Peat Bakke, CC BY 2.0)

The T. rex claim was made based on differing bone proportions and teeth structure, but many palaeontologists, both professional and passionate amateurs, pushed back strongly. Ultimately no official changes were made based on the paper, but the suggestions are doubtless already being developed and debated within the discipline.

Does it matter?

If taxonomy is such a mire of subjectivity and grey space, why does it matter? Why get bogged down in the pedantry? 

There are a few reasons. Earlier I mentioned hybridisation. This may seem like a fun practice, crossing animals with each other and coming up with something in between. But the reality is that it’s actually a genuine threat to many species.

I first learned about the problem with hybridisation during an interview with Dr. Peter Unmack and Dr. Karl Roy in 2018. They’d been working with the Running River Rainbowfish, a fish which had been cut off from its close relatives for millenia in one small section of river, and had evolved into its own species. It’s a beautiful example of how isolation breeds speciation. The problem was that it was still similar enough to other Rainbowfish to interbreed, a process which could quickly wipe out the Running River Rainbowfish’s unique genetic code and morphology. Peter and Karl were able to save their Rainbowfish through a concerted translocation effort, but not all species are this lucky.

The gradual movement of species towards the poles and climate change drives migrations is bringing different species into contact with each other. And since this takes place in the form of slow, incremental movement, as opposed to the vast distances covered by a species being relocated by humans, often these species aren’t too dissimilar. It’s certainly the case with the grizzlies and polars.

The concept of a species may be a little more arbitrary than it first appears, but it’s a very useful classification tool, and allows us to make more informed conservation decisions. The designation of a group of animals as a species marks it as unique, and worth saving. 

Even the distinction of subspecies allows for this. Sure, I might not be able to tell the difference between the different subspecies of giraffe, lowland gorilla or tiger, but their designation as a unique group gives them cultural value. It’s a symbolic recognition of their unique evolutionary history that should not be lost. So yes, taxonomy is a murky and endlessly controversial business, but that does not detract from its crucial place in biology.


Dr. Sam Perrin is a freshwater ecologist who completed his PhD at the Norwegian University of Science and Technology who wishes he could stop writing about fish but they keep making themselves relevant. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.

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