Species Are Like Pornography, You’ll Know Them When You… Actually, Never Mind

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Rusty crayfish, screwing up our definition of a species
The rusty crayfish, ruining it for everyone
Photo by: Peterwchen, CC BY-SA 4.0 , via Wikimedia Commons

Everyone knows what a species is. Right? Of course you do. You look at your cat, and then at your dog, and you can tell that they’re two different species. Or, you look at your dog, and you look at an elm tree, and you can definitely tell that they’re two different species. It’s obvious. Nobody would get confused by this. Which means that biologists must have a pretty good grip on the definition of a species, right? Right?

(Insert deranged laughter)

Nope. Not even close. Basically, nobody knows what the fuck a species is. There’s a huge debate about how to define the term. In fact, we can’t even agree that distinct species actually exist. That’s right. There are some folks who would argue that, while the differences between your dog and an elm tree are pretty drastic, they’re not prepared to say they’re different species. Because, after all, they’re still sort of kind of related. You may have to go back to single celled organisms a bajillion years ago to find a common ancestor, but it’s not like your dog came from outer space or anything. Unless you have a chihuahua. The jury is still out on those guys. Anything with eyeballs that don’t fit in their skull, but still claims to be a domesticated wolf, is pretty sketch.

Point is, every time you think you’ve got the definition of a species nailed down, someone can easily tear holes in your scheme. Granted, a lot of the time, for ordinary folks doing ordinary things, it’s not really that important. But if you’re, say, trying to protect an endangered species, it suddenly becomes extremely important to know exactly what counts as that species. Likewise, if you’re trying to control an exotic invasive species.

Doing it old school

Let’s start with what you and I learned in high school biology class. Most likely, you learned that any group of individuals that can breed together and produce fertile offspring are the same species. This is usually illustrated by pointing out that, while horses and donkeys can reproduce, the offspring, called a mule, is sterile and cannot reproduce either with other mules or with horses or donkeys. Thus, even though they technically can breed together, horses and donkeys are different species by this definition. This definition is called the “biological species concept”, a name which sure sounds vague to me. Wouldn’t any definition of a species be “biological”? I mean, that’s all we’re talking about here, is biology! Biologists are obsessed with naming things, yet that’s the best we could come up with for a core concept of the field? You don’t see this in other fields. Physicists don’t talk about the “atomic model of the atom”. We don’t call our dominant theory explaining the beginning of the universe the “Beginning Theory”.

Anyway, this biological species concept works well enough for most of us, most of the time. It certainly works pretty good for horses and donkeys. We’ve got those two straightened out, right? Hybrids like the mule bend the rules but don’t break them. They’re sterile, so they don’t really count. It seems like a pretty solid system for figuring out what’s what out there in nature. And so I believed, quite happily, until one day when I was learning about the rusty crayfish.

Pack mules on the trail
The sturdy mule, a hybrid exception that proves the rule… a rule that’s broken anyway

Crayfish, if you’re not familiar, are those guys that look like little lobsters, only they live in freshwater streams and lakes. The rusty crayfish is… well, truth be told, it’s just a crayfish. I mean, if you know what you’re looking for, you can definitely tell them apart from other types of crayfish, and their behavior is a little different, too. But to most folks, its gonna look a lot like most other crayfish. The rusty is native to the United States, namely Indiana, Kentucky, and western Ohio. However, in the 1970’s it started showing up outside of its native range and quickly spread to the entire Great Lakes region and several other places around the United States. In the process, it started replacing native species of crayfish wherever it turned up. That sucks, but it’s sadly a pretty common story.

What’s this got to do with trying to define a species? Well, I heard about a little hiccup in the effort to stop the spread of the rusty crayfish. Turns out, part of the reason they were displacing native crayfish was because they could hybridize with a couple of other species. They were breeding with the local crayfish species. Not hard to see how this is a problem… if the natives are breeding with the aggressive invaders, then they’re not busy making more natives to help keep their numbers up. But one little thing about this story blew my mind.

The hybrid crayfish were breeding back with the rusty crayfish. They weren’t sterile like a mule. They were half rusty, but still making babies with full fledged rusty crayfish, so their offspring were 3/4 rusty. And then those individuals might breed with the native crayfish again. So in some places, you could find crayfish that were fully rusty, some that were fully a native species, and others that were only 15% rusty, or 90%, or anywhere in between. Their genetics were all mixed up, and nothing bad seemed to be happening. They weren’t sterile. They were just fine.

So, what IS a species, then?

I was shocked. That means they weren’t really hybrids then, right? These crayfish must have been the same species all along? But no, nobody was suggesting that these crayfish all be lumped in to the same species. To this day, they’re still considered separate species. They look a little different, they behave differently, they have slightly different habits. There’s good, practical reasons to keep them split up, at least as far as wildlife biologists are concerned. But this completely broke the definition of a species that I had learned. If a hybrid of two different species could have its own babies, then… what was a species?

This phenomenon isn’t restricted to crayfish, either. In parts of the western United States, there’s a problem in some of the trout streams. Native cutthroat trout are hybridizing with introduced rainbow trout. Rainbows are native to the western U.S. as well, but not everywhere, and they’ve been stocked over the decades in streams where the only trout species used to be cutthroat trout. And just like those damned crayfish, they’re all blending together in some areas. There’s pure rainbows, pure cutthroats, and many intergrades in between, all swimming around in the same stretch of river. And nobody, not fishermen, not biologists, not anyone, is calling for rainbow trout and cutthroat trout to be classified as one species. But, according to what we all learned in high school biology class, they should be.

Homo neanderthalensis, possibly more closely related to you than you thought
Photo by: Mettmann, CC BY-SA 4.0 , via Wikimedia Commons

Oh, and there’s one more example of this that I should mention: Us. Humans. Homo sapiens. Turns out, we’re also hybrids of hybrids of hybrids. See, unless your ancestry is 100% African, your genetic makeup is probably about 2% Neanderthal. How does one become only 2% of a species? Because at some point, there was someone with one parent who was Homo sapiens and one who was Homo neanderthalensis. 50-50. Then that person had babies with a full fledged Homo sapiens. And after that point they pretty much stayed away from Neanderthals. After a few generations their genetic makeup became mostly, but not entirely, Homo sapiens. Oh, and this happened at least three different times, too. Yeah, the stuff geneticists can figure out is crazy.

And did I mention the Denisovans? They’re not as well known as Neanderthals, but they’re another species of ancient human, and apparently our ancestors banged them as well. If you’ve got any ancestry from Australian Aborigines or native peoples of the South Pacific islands, you’re probably a few percent Denisovan. For the record, Denisovans and Neanderthals got it on, too. Basically, humans are sluts, is the main takeaway here.

Keep it in your pants, humans

So… wait…. does that mean that modern humans, Neanderthals, and Denisovans are all the same species? Meh. The jury is still out on that one, and it’s made a lot tougher by the fact that they’re extinct. Well, not entirely extinct. Some of their genes are still hanging around in us. Regardless, it’s much harder to figure this stuff out from fossils and remnant DNA than when you’ve got living individuals to study.

Which actually leads us to another problem with the biological species concept. If the way you define a species is by whether or not individuals can interbreed… what about all the species that we only know from the fossil record? You can’t tell shit about reproduction from fossils. We can’t prove that, for example, a triceratops and a T. rex couldn’t interbreed. Nobody believes they did, but we can’t prove it. You can put fossils in a room together with dim lighting, a bottle of wine, and lame saxophone music all you want, but you’ll never get any baby fossils that way. So, by the common definition of a species, we can’t really say that any of these extinct organisms are different species. Or the same species. Obviously, if two sets of bones look just alike and were found together in the same location, you’re gonna feel pretty comfortable saying they’re the same species, but since you can’t prove they could breed together… yeah.

Pair of T. rex skeletons in mating position
Nice try, but we still can’t prove these two could reproduce
Photo by: Usuario:NeGRa, CC BY-SA 2.5 ES , via Wikimedia Commons

Oh and hey, what about species that don’t have sexual reproduction? Plenty of single celled lifeforms reproduce asexually, as do many plants, invertebrates, and even a few vertebrates. These organisms are basically clones of their parents and grandparents and so on. No interbreeding at all. The standard definition doesn’t work for them, either. Yet when you see a dandelion, you know it’s a fucking dandelion. The fact that it’s reproducing asexually, that the seeds are all clones of the parent, doesn’t change that. So… what’s a species then?

Schrödinger’s species: It both is and isn’t

And I’m not even done. The more you keep picking at the biological species concept, the more it falls apart. Let’s look at the ensatina salamander. It’s a little salamander that’s found along the western coast of North America. Throughout most of their range, they are all pretty similar and are considered to be one large breeding population. However, as they range south into California, they break up into two arms, one spreading south near the coast and one that’s further inland. There’s a number of different subspecies strung along these arms, one after the other, each one a little different looking from the others and living in different habitat. A subspecies, by the way, is basically a regional variety, slightly distinct from the rest of the species, but essentially the same. Each ensatina subspecies in each arm can interbreed with their neighbors to the north and to the south, indicating that they’re all the same species.

Except for the two groups at the southern-most portion of the state, where the coastal and inland arms meet up again. Here, the large blotched ensatina salamander from the inland branch meets the Monterey ensatina, at the end of the coastal branch. These two groups are different enough from each other that they can not interbreed, even though they can breed with their other neighbors, which can each breed with their own neighbors. We call this a ring species, and it breaks the rules. The different groups literally form a ring, and interbreeding can happen all along the ring, back and forth… except for those two troublemakers. According to the biological species concept, this would mean that the Monterey ensatina and the large blotched ensatina are both the same species, because they theoretically belong to the same breeding pool, but also separate species, because they can’t breed with each other. So… which is it? Who knows!

Map showing the range of the various subspecies of the salamander Ensatina eschscholtzii along the western coast of North America.
Range of the ring species, Ensatina eschcholtzii. Members of E.e. eschscholtzii and E.e.klauberi cannot interbreed.
Illustration by: Thomas J. Devitt, Stuart J.E. Baird and Craig Moritz, CC BY 2.0 , via Wikimedia Commons

Ring species are thought to be an example of a kind of speciation, or new species being created. The ensatina salamander appears to be splitting in to two distinct species in the state of California. Eventually, if either of those two southern subspecies become incapable of breeding with their other neighbor, they’ll truly be a separate species. Maybe they’ll both split off and we’ll end up with three species total. It just hasn’t happened yet. And maybe it never will, who knows. But it sure makes a mess of our concept of what a species is.

So… if the standard definition of a species that we thought was just fine, is actually full of holes, then what’s the correct definition? Well, don’t worry, because you have TWENTY SIX other definitions to choose from. That’s right. I told you before… nobody can agree on any of this shit. Some folks think that all that matters is the evolutionary lineage of a species or group of species. Others insist we focus on the ecological niche that a group occupies. People disagree over to what extent genetics should play a role. Some don’t think there’s any such thing as a species. To some extent, everything is related to everything else and we’re all just at different points along the same genetic and evolutionary spectrum.

Spoiled for choice

So… is there a point to all of this migraine-inducing nightmare? Does it really matter what a species is, or is it just something for biologists to argue about after they’ve had a couple of beers? The maddening answer is, it depends.

Did you hear that? Oh, that was you? Pulling your own hair out? Okay, cool. That’s a pretty normal reaction to all this. But, it’s true. It depends on what you’re doing, what you’re studying, what your goals are. If you’re a paleontologist studying fossils of long extinct plants and animals, it’s almost a moot point. Your only real option is to group things together based on how similar their fossilized parts are to each other. If you’re lucky, you may have some genetic material to work with, but most of the time that’s going to be impossible. So a lot of this whole conversation is kind of meaningless in that case. But, if you’re a conservationist trying to save a rare species, you may want to preserve as much genetic diversity as possible. In which case you’re going to want to consider each population that has distinctive genetics as an important and separate group, even if to the naked eye they all look and act the same and would normally be considered the same species and subspecies.

Chuck wasn’t too worried about how to define a species, for the record

If you’re studying evolution, then you’re going to be more focused on how different groups are related to each other, what common ancestors they share. An ecologist may not care much about the genetics or evolutionary history of any species. They’re looking at things from a system level, and what they care about is the role a species plays in that system, not whether it’s the same species as a similar organism or not. Someone studying the Yellowstone ecosystem might think it’s interesting that a polar bear can hybridize with a grizzly bear, but since there’s no polar bears in the Yellowstone, they’re probably not going to spend much time worrying about whether that means they’re the same species or not.

A species by any other name is, like, whatever

When it comes down to it, most of the time, for most of us, the definition of a species that we learned in school is good enough, even though we know it’s flawed. Most of us don’t wrestle with bigger issues, like if an endangered subspecies is worth protecting even though it’s nearly identical to the larger population, or if it matters that members of two different populations are starting to breed together. Most of the time, if it’s not our jobs, then we’re just trying to figure out what that flower or bird is. And “daisy” or “sparrow” may be good enough for us, even though there’s many different species in each of those groups. But it’s worth understanding that the concept of a species is not set in stone, that even the experts struggle with it. It’s important to know that the natural world is so diverse, so fluid, so complex, that it constantly defies our attempts to pin it down, to apply a set of rules to it.

Some people argue that learning more about nature takes away from the wonder and mystery of it. But I, and many others, feel that as we understand more about biology and science in general, it only becomes more wondrous and amazing. On the one hand, you can have two crayfish that everyone agrees are different species, yet they can produce fertile offspring together. On the other hand, you can have two very similar salamanders that really ought to be able to breed together, yet they can’t because they’re halfway to splitting off into their own distinct species. Just knowing that this is how the natural world works, makes the world a more amazing place for me. It helps me to understand how much there is that we don’t understand. It makes me see how special it all is, and that it’s worth saving as much of it as possible, or else we’ll literally never understand what it was we lost.

Except for the chihuahuas. We can just push their eyeballs back into their sockets and send them back home in to outer space as far as I’m concerned. Domesticated wolf, my ass.

MORE READING:
What is a species? The most important concept in all of biology is a complete mystery
How Do We Define A Species?
How many species concepts are there?
Hybrid Trout Threaten Montana’s Native Cutthroats

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