What is a species?

Alligator snapping turtle (Macrochelys temminckii)

Before you read any further, I want you to take a minute and try to answer the question in the title. Go ahead and write down (or at least think about) the definition that scientists use to determine whether or not two organisms are members of the same species. Now that you have hopefully done that, I am going to burst your bubble and tell you that if you wrote down a definition, then no matter what definition you wrote, you’re wrong, or at the very least, incomplete. You see, there is no one universally agreed upon definition of a species. Rather, there are numerous “species concepts” and scientists debate endlessly about what constitutes a species. Further, taxonomic revisions happen constantly and it is extremely common for one “species” to get split up into multiple “species” while other “species” get lumped together into a single “species.” For example, the alligator snapping turtle (Macrochelys temminckii) was historically thought of as a single species, but in 2014, Thomas et al. proposed that it should be split into three species: M. temminckii, M. apalachicolae, and M. suwanniensis. Then, in 2015, Folt and Guyer argued that M. apalachicolae and M. suwanniensis were not actually different enough to be considered distinct species and should, therefore, be lumped back together, resulting in just two species of alligator snapping turtle (M. temminckii and M. suwanniensis).

These types of revisions happen frequently, and scientists routinely disagree about how to separate species. Indeed, when you get right down to it, the whole concept of distinct species is an artificial one that we use to categorize things. It is not an actual property of nature, which is why it is so amorphous. Nevertheless, discussions about what constitutes a species are important, and they have strong implications for topics like GMOs and evolution. Therefore, I want to discuss several of the more common species concepts, how scientists go about deciding what to call a species, and why the concept of a species is artificial and ultimately somewhat arbitrary.

The biological species concept

Let’s start with what is probably the most well-known species concept: the biological species concept. This concept defines species based on the inability of different groups to breed with each other. In other words, if two natural groups of organisms can interbreed, then, according to this concept, they are the same species, whereas if they can’t interbreed, they are considered to be members of different species. Unfortunately, this is probably the concept that you learned in high school, and I say “unfortunately,” because this is a pretty terrible species concept, and scientists don’t really use it much anymore.

One of the most obvious problems is simply that it cannot deal with asexual species (i.e., species where a single individual can clone itself rather than relying on a mate). Asexual species are actually pretty common in nature, and even occur in various groups of vertebrates (e.g., some lizards), but according to this concept, every single asexual individual should be its own species because it cannot interbreed with other individuals, or, at best, this concept simply has no way to define asexual species.

The second problem is that hybrids are extremely common in nature, even among organisms that everyone agrees represent different species. Plants are infamous for their ability to hybridize across species (this happens in nature, not just in horticulture) but animals frequently do it as well. For example, consider the Mallard (Anas platyrhynchos). This is the ubiquitous green-headed duck that you can find at ponds, lakes, and rivers throughout the US. Everyone agrees that it is its own species, but it can hybridize with multiple other species of duck (e.g., American Black Ducks [Anas rubrics], Pacific Black Ducks [Anas superciliosa], Northern Pintails [Anas acuta], etc.). Further, in many cases, those hybrid offspring are not sterile and can reproduce (as can their descendants). Thus, according to the biological species concept all of those ducks should be one species, but based on every other species concept, they should be different species, and no scientist (or birder) in the world would suggest that we should lump them all together. Finally, I need to emphasize that this is not an isolated example. Hybrids are everywhere in nature, and I could show you examples of hybrids in other birds, snakes, lizards, frogs, turtles, fish, mammals, etc. This is a very common phenomenon.

The morphological species concept

This is another very old species concept, but it is still sometimes used today. It proposes that species should be defined based on whether or not they can be distinguished morphologically. In other words, if scientists can look at two groups of organisms and visually distinguish them (or distinguish them by their calls) then they are different species, but if they are indistinguishable, then they are the same species. So, for example, all of the ducks that I mentioned earlier can be visually distinguished, thus, based on this concept, they are different species.

This concept sounds good, but it has multiple problems. First, there are quite a few “cryptic species” (reptiles have tons of these). These are species that are impossible (or at least extremely difficult) to distinguish visually or audibly, yet when we look at things like genetics, it is clear that they are very different from each other and represent different groups (i.e. species). It is also worth mentioning that in many cases, we may not be able to distinguish them, but the organisms themselves may be able to distinguish each other based on traits like pheromones that we are pretty bad at picking up.

The other problem with this species concept is that many species have tremendous amounts of variation across their range. Take the Northern Flicker (Colaptes auratus), for example. This woodpecker is found all over the US, and in eastern half of the country it has beautiful yellow feathers. In the western half, however, those feathers are red. In other words, there are two morphologically distinct groups of this species, yet we consider it to be a single species. As with the biological species concept, I could give you tons of examples like this, and I would even go as far as saying that regional variations within a species are the norm, not the exception. A good way to think about this problem may be simply to ask the question, “how different do two groups need to be before we consider them to be different species?” There is obviously no objective answer to that, which makes the definition rather arbitrary.

Two color morphs of the Northern Flicker (Colaptes auratus): Red-shafted (left) and Yellow-shafted (right). They are morphological distinct, but are still considered to be the same species.
Images are from BioQuick News and Ewebarticle.

The genetic species concept

This concept is in many ways just a modern version of the morphological species concept, but instead of morphology, it uses genetics. Thus, two groups that are genetically distinct are considered to be different species. As with the morphological species concept, however, the key question is, “how different is different enough to be separate species?” Once again, the answer is, “no one knows, and it is arbitrary.” Various levels of genetic similarity have been proposed and applied, but there is no one universally accepted answer. Further, there are frequently disagreements between the morphological species concept and the genetic species concept. As a result, as I mentioned earlier, it is extremely common for scientists to disagree about whether or not two groups are different species, and species frequently get lumped and split. Indeed, I was recently at a conference where one researcher somewhat facetiously suggested that we should start using the terms “gecies” to refer to genetic species, and “mecies” to refer to morphological species.

The phylogenetic species concept

The final species concept that I want to talk about (though there are many of lesser importance that I did not discuss) is basically an extension of the genetic species concept. Once again, it uses genetic patterns, but this time, instead of simply using the degree of difference among groups, it attempts to look at evolutionary history. Thus, a species is a genetically distinct group with a shared evolutionary history that differs from the other groups. This is certainly useful, and is probably the most widely used concept today, but as with the other concepts, it is still largely arbitrary because there are no universally accepted criteria for determining when an evolutionary history is divergent enough to constitute a separate species.

Which concept is correct?

As you’ve hopefully gathered by now, scientists disagree about the answer to this question. In practice, we tend to try to look for a convergence of multiple species concepts, but again, scientists frequently disagree about whether two things are different enough to be considered distinct species, and proposed taxonomic revisions are often highly contested.

To add another layer of complexity to this, scientists often add additionally sub-categories. For example, many species contain multiple “subspecies.” These are geographical subgroups of a species that are different enough to be noteworthy, but similar enough that they don’t merit species status (the two flickers that I mentioned earlier are subspecies). Here again, however, scientists often disagree about the boundary between species and subspecies, and it is common for subspecies to get elevated to full species and for multiple species to get lumped and demoted to subspecies.

In other cases, scientists prefer to talk about “evolutionarily significant units” rather than dwelling on the species/subspecies distinction. These are simply groups (usually populations) that are genetically (usually phylogenically) distinct enough that they need to be discussed and managed separately, regardless of whether you want to label them as separate species. Similarly, in the world of microbiology, it is common to abandon the label “species” altogether and instead use OTUs (operational taxonomic units) which are narrow taxonomic groups that are distinguished by an arbitrary threshold of similarity (97% similarity is often used).

Why isn’t there a universally accepted species concept and how does this relate to evolution and GMOs?

Finally, and I think most importantly, I need to explain why scientists disagree about how to define a species. It’s not simply that scientists are an argumentative and ornery bunch (though that plays into it). Rather it is because there is actually no such thing as a species. The concept of a species in an entirely artificial one that we invented to help us make sense of the world. It is not an actual construct of nature.

You see, as I have explained before, evolution is a spectrum, not a series of distinct blocks. Thus, nature does not immediately form distinct and obvious species. Rather, a group of organisms splits and gradually evolves in separate directions with each generation resulting in more and more differences. At some point, those two groups become different enough that we consider them to be different species, but where we draw that line is arbitrary. It is a judgement call that we are making, rather than an actual property of nature.

Further, it is important to realize that this also applies to other levels of taxonomic classifications (especially genera and families). Like species, it is often not clear how to demarcate these, and taxonomic revisions are common. For example, a few years ago, the frog genus Rana was split into multiple different genera, with most of the ranids in the USA moving into the new genus Lithobates. Also, just like with species, there are subcategories (subfamilies, supergenera, etc.) and scientists often disagree about where to draw the line. Again, this is because these taxonomic divisions are artificial. They are useful tools for us to understand life on planet earth, but evolution produces a spectrum, not distinct blocks, and when we see distinct blocks, they only exist because the rest of that spectrum died out. Imagine, for example, looking at a rainbow and picking the point where the red stops and the yellow begins. You can’t really do it.

In addition to everything that I have said so far, it is also worth noting that nature is kind of a freak, and it does some really weird things that are nigh impossible to categorize. One example that I am quite fond of is the fact that several thousand years ago, sweet potatoes stole DNA from a bacteria and incorporated that DNA into their own genome (thus essentially making them a natural GMO). Similarly, some butterfly species incorporated viral DNA into their genomes (Gasmi et al. 2015). What do you do with that as far as assigning taxonomy? What do you do when a species forms by stealing DNA from a totally different group of organisms?

Further, those examples aren’t even the most extreme. There is, for example, a salamander in the genus Ambystoma that does something known as kleptogenesis (Bogart et al. 2009). This salamander is unisexual (i.e., only has females), but it is not asexual (i.e., it isn’t capable of reproducing on its own). Rather, they take sperm from up to four other Ambystoma species and use that sperm to fertilize their eggs. Thus, each generation is formed by stealing sperm from a different species (sometimes multiple species) and incorporating part of their genome. What do you do with something like that? It defies classification, which brings me back to my central point that taxonomic classifications are simply imperfect constructs for helping us to understand the world, rather than actual properties of nature.

So, what does all of this have to do with evolution and GMOs? Well, when it comes to evolution, I think that the implications are pretty clear. Evolution predicts a spectrum, and that is exactly what we see. I often hear creationists talk about species or families as totally distinct obvious groups, but that’s just not reality. The distinctions are extremely fuzzy and often arbitrary.  For GMOs, the implications are a bit more nuanced, but important nonetheless. Like creationists, anti-GMO activists often make a big deal about species being distinct and find the notion of moving a gene from one species into another to be abhorrent, but as you can hopefully now see, that view makes little sense once you understand how nature actually works. Organisms exist as a spectrum, not as extremely distinct units, and we all share the same DNA. To be clear, something like a fish and a tomato are obviously distinct, but that is only because they are far apart on the spectrum. Go back to the rainbow analogy for a minute. The pure yellow and pure red are clearly distinct from each other, but you’d do yourself a disservice by treating them as if they are totally separate and detached from one another, because they both fall along the same spectrum. Further, as I explained above, nature does not respect the arbitrary labels that we put on things, and it even moves genes between highly divergent groups (e.g., a sweet potatoes and bacteria).

My point in all of this is simple, organisms exist as a spectrum, not distinct blocks, and categories like “species” and “family” are artificial constructs that we created to help us understand the world around us. So before you make a big deal about different families and species for topics like GMOs and evolution, keep in mind that those categories are simply tools that we use, not actual properties of nature.

Note: The GMO tomato with “fish genes” never went to market.

Literature Cited 

  • Bogart et al. 2009. Sex in unisexual salamanders: discovery of a new sperm donor with ancient affinities. Heredity 103:483–493.
  • Folt and Guyer. 2015. Evaluating recent taxonomic changes for alligator snapping turtles (TestudinesL Chelydridae). Zootaxa 3947:447–450.
  • Gasmi et al. 2015. Recurrent domestication by Lepidoptera of gens from their parasites mediated by bracoviruses. Plos Genetics 11:e1005470.
  • Thomas et al.. 2014. Taxonomic assessment of Alligator Snapping Turtles (Chelydridae: Macrochelys), with the description of two new species from the southeastern United States. Zootaxa 3786:141–165.
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22 Responses to What is a species?

  1. charles says:

    “The Northern Flicker (Colaptes auratus): Red-shafted (right) and Yellow-shafted (left)”. Yes, I can certainly see the difficulty.


    • Sebastián says:

      They may be different, but the point is that is difficult to determine when this differences are enough to classify them as separate species. Would you consider a blonde a different species when comparing with a brunette?
      At the end, what the article says is that we try to put labels on nature, but nature won’t always fall within our definitions because there are not distinct lines between the spectrum.


    • Sebastián says:

      Wait. Are the images on the wrong sides?


  2. Paul Robertson says:

    There is an error under the flicker photo. It should read that they are considered the same species, which is how the text reads. Sorry, too many years proof reading.


  3. Herman Mays says:

    Species are in fact real biological entities. They are not merely arbitrary distinctions. They are independent metapopulation lineages and we can identify them by applying population genetic principles. Higher taxonomic groups are more arbitrary but current species concepts incorporate numerous other concepts (namely the biological and phylogenetic concepts) in a single unified framework.


    • Fallacy Man says:

      “we can identify them by applying population genetic principles” except of course for all the times that we can’t, all of the numerous disagreements about where to draw the line, bizarre exceptions like kleptoogenesis, etc. That’s the point. As explained in the post, if we take something like the phylogenetic approach, we are still left in a position where we have to decide when two lineages have diverged enough that they deserve to be called separate species. It is a judgement call that we are making, and scientists constantly disagree about where to draw that line.

      “in a single unified framework.” haha, that is truly hilarious. Taxonomists absolutely do not have a single unified framework. They debate constantly about what framework should be being applied and how things should be classified. I have been to numerous conferences where I have gotten sit back and watch taxonomist debate about whether or not two groups were different species, and those debates happened because each of them had a different framework that they wanted to apply.

      I am a big fan of the Socratic method, so let me try this as a question. Explain to me at what non-arbitrary point two things are divergent enough to be two different species? All phylogenetic methods involve thresholds that we set, so pick a threshold and defend that threshold as a real, distinct, biologically prescribed point rather than simply a number that we decided means that things are different enough that we should probably treat them separately.

      Liked by 1 person

    • Fallacy Man says:

      Just to be 100% clear here, I’m not saying that there aren’t biologically distinct groups (usually separated by phylogeny) that behave different from each other and are worth talking about as separate. Rather, my point is that the precise point at which we define those groups as “species” is arbitrary and determined by us, not nature. Nature clearly does not respect the species boundaries that we define as is clearly evidenced by all the hybrids and other exceptions.

      Liked by 1 person

      • Herman Mays says:

        The claim was that the concept of species is arbitrary. This is untrue. Species may be considered independent metapopulation lineages. These independent lineages are the result of a process, speciation. Identifying species boundaries is often incredibly difficult, especially when their divergence is recent, but it’s not impossible and it doesn’t mean species are artificial or arbitrary. There are numerous methodologies grounded in population biology to determine if two lineages are independent.

        As an analogy a cake is the end result of a process, baking. That fact doesn’t mean our definition of a cake is entirely arbitrary. When does one start calling the batter a cake? Of course it’s difficult to distinguish two lineages before they have evolved evolutionary independence while they are in the process of speciation. They simply aren’t two separate lineages.

        There is a lot of current research in species delimitation such as unified lineage-based concepts of deQuieroz or coalescent-based multi-locus analyses from Carstens and others that were entirely ignored in this post and as such it really doesn’t reflect the modern thinking on species concepts and species delimitation.

        Full disclosure my colleagues and I work in this field.


      • Herman Mays says:

        I might get add, you said the following.

        “As explained in the post, if we take something like the phylogenetic approach, we are still left in a position where we have to decide when two lineages have diverged enough that they deserve to be called separate species.”

        Delimitation of a single lineage from two independent lineages ultimately relies on a population genetics approach and not strictly speaking a phylogenetic approach. Delimiting species is an application of population genetic principles. It’s again not some entirely arbitrary guess.


        • Fallacy Man says:

          I think that in many ways we are talking past each other here. I completely agree with you that we can apply these techniques to distinguish different groups, look at the evolutionary histories that resulted in those distinctions, etc. Also, to be clear, I am not in anyway trying to attack or belittle your work or the work of any taxonomists. Indeed, I think taxonomy has an important role in biology (especially in conservation). Rather, my point is simply that the exact position at which we draw the line and declare something to be its own species is a decision that we are making, rather than an inherent property of nature, and it is not a point that nature respects (thus all the hybrids).

          Let me try this. Let’s take a step back from species for a second and talk instead about genera. We obviously like to make nice, monophyletic genera where everything in a genus shares a common ancestor. Yet within a genus, there are very often subgroups, where multiple species cluster out together as having a common ancestor within a genus in addition to the common ancestor of the genus itself. What do we do with those sub-categories? At what point do we decide that a cluster deserves its own recognition, especially when there are multiple monophyletic sub-clusters inside other sub-clusters? That’s obviously highly contested and taxonomies constantly get revised.

          The same type of thing happens in populations. I can’t tell you how many population genetics talks I’ve been two where two populations are isolated from each other, with diverging phylogenies, yet no one would suggest that they are different species because they haven’t diverged “enough,” whatever that means. Indeed, in the frogs that I am studying at the moment, we have populations that we know have been essentially isolated for a few thousand years (following an initial founder event; they are not currently meta-populations), we know that there are genetic differences and we can make trees showing population distinctions based on ancestry, and in some cases there are even morphological differences (for some populations, I can easily distinguish their individuals from the individuals of other populations). Nevertheless, despite all of that no one would say that these populations represent different species or even subspecies because the divergence isn’t “deep” enough. Currently, they are certainly evolutionarily significant units, and they deserve to be talked about separately, but no one thinks their species. If we came back 20,000 years from now, maybe we would say they are species, but that just comes back to my central point. Where do we draw the line? How divergent do they need to be? Even if you could get everyone on board with a precise answer to that it would still be a line we decided to draw, not something nature prescribed. To put that another way, as far as nature “is concerned” these things are already separated. The habitat that once connected them is gone (because of natural causes, in this case), and the now disjointed populations came from ancient founder events (thus you have shared ancestor within a population), but they aren’t species because they have not met the threshold that we decided on.

          I don’t really think my system is the exception either. For example, a while ago I saw a talk by Craig Moritz (who I assume you know or at least know of given your field), and he put up a slide of the genetics of a wide-ranging gecko and there were tons of different phylogentic clusters. His comment was essentially, “you could potentially argue that each of these is its own species, but that depends entirely on how you define a species, and who really knows?”


  4. This sounds like a similar problem with classifying what celestial bodies are moons and which aren’t. I just watched a video about that. Arbitrary distinctions are useful in some aspects so long as we remember they *are* arbitrary, based on human observation, which can be flawed. My species definition was the biological one then I remembered some dogs can interbreed with wolves even though the former is Canis domesticus and the later Canis lupus. I believe…correct me if I’m wrong. Either way they’re a different taxonomical species yet they have that caveat.


    • Fallacy Man says:

      Interestingly, dogs/wolves were actually re-classified so that each are now subspeieces (Canis lupus lupus and Canis lupus familiaris), which only reinforces the fact that taxonomy is constantly in flux. In general though, you are correct that there are many groups that are considered separate species, yet they interbreed.

      Liked by 1 person

  5. Balanescu Dan says:

    I start from your affirmation: The concept of a species in an entirely artificial one that we invented to help us make sense of the world. It is not an actual construct of nature.” My question is: can you give a single example of “concept” who is a construct of nature? I suppose you want to say that “concept of a species” not follow any natural order, or something like that. Tell me if I understand correctly and in other order of thought, can we talk about science like a seeker and follower of a natural order, or science like a cultural machine who conceive linguistic tools for (exactly how you say) a coherent worldview? Can we talk here about a science who reveal the truth, meaning the scientific truth gaining weight directly proportional to the degree of objectivity vis-à-vis the observed phenomenon?


    • Fallacy Man says:

      I don’t think that I follow your question. There are plenty of scientific concepts that are actual properties of nature. Relativity, evolution, plate tectonics, etc. these are all real properties of nature that we discovered. In contrast, the concept of a species is just a category that we put things into to help us make sense of things.


  6. Thanks for this post – very helpful!

    What are your thoughts on how/where to begin addressing this in high school? Teach all the concepts with pros & cons? Skip them altogether? Mention their existence and why they’re problematic, but then move on?


    • Fallacy Man says:

      That’s a good question, and I wish I had a good answer. Personally, if I was teaching a group of high schoolers, I would probably introduce the well known ones (like the biological species concept) as well as the more widely used ones (like the phylogentic concept), and explain the pros and cons and why it is so difficult to actually classify things at that level, then try to root that in evolutionary theory. Figuring out exactly how much information to give to avoid total confusion is difficult though. I do, at the very least, think it is important to explain the problems with the biological species concept, because almost everyone seems to think that it is still the accepted definition of a species, and for most taxa it really isn’t (though there are a few exceptions like some branches of entomology).

      After publishing this post, I thought of an illustration that I at least think works pretty well and you’re welcome to use if you find it helpful. It would simply be two arrows branching down from a common point (making an upside down “Y”) where the common branch is orange and one arrow gradually transitions to red while the other gradually transitions to yellow. You can use that as a proxy for speciation driving two groups in separate directions with each group gradually becoming more and more different, and you can make the point that eventually the differences between them become clear, but it is very difficult to decide on the exact point at which they are both different enough that you should say that they are truly different colors rather than simply different shades of orange. That’s obviously a major oversimplification of how scientists merge the different species concepts together and look at phylogentic histories, but I think it would be a useful way for students to visualize it and think about it.


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