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May 22, 2013 | By:  Sedeer el-Showk
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Do Species Really Exist?

"Nor shall I here discuss the various definitions which have been given of the term species. No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species."
- Charles Darwin, On the Origin of Species, Chapter 2.

"I have just been comparing the definitions of species [...] It is really laughable to see what different ideas are prominent in various naturalists' minds, when they speak of 'species'; in some, resemblance is everything and descent of little weight - in some, resemblance seems to go for nothing, and Creation the reigning idea - in some, descent is the key — in some, sterility an unfailing test, with others it is not worth a farthing. It all comes, I believe, from trying to define the undefinable."
- Charles Darwin, in a letter to Joseph Hooker, 1856.

Faced with the rich diversity of living beings around us, humans have proven unable to resist the temptation to try to organize and categorize them. We have a natural tendency to classify things, a habit that's deeply rooted in our cognition and use of language. Our brain excels at recognizing patterns (and thus finding meaning where it doesn't exist), an ability that allows us to interact with the world using names — like "chair" — that we might be hard-pressed to properly explain. In fact, it's surprisingly difficult to define even a seemingly straightforward word like "chair" in a way that would let us recognize everything that should be included (from office chairs and recliners to stools and wheelchairs) but nothing that shouldn't (like tables, tree stumps, or other things we might decide to sit on).

Despite these difficulties, we've been classifying organisms throughout the history of human thought, from Aristotle's division between plants and animals to modern scientific nomenclature. The modern classification system is based on grouping organisms into units called 'species'; species, in turn, group together into a larger units called genus, family, order, and so on through the nested hierarchy of life. What make a species, though? Why should a particular group of organisms be thought of as a unit and given a distinct name? How do we decide which organisms make up a species?

Biologists have struggled with these questions (the "species problem") since before Darwin's time. Over the years, they've come up with a cornucopia of different answers, or species concepts. It's important for students of the natural world to appreciate and consider the different ways that "species" is used, since its meaning can change in different contexts. To me, it often seems that no single definition will ever suffice to capture the variety of ways that wildly different sorts of creatures (like plants and animals) manage to maintain their identity as a species or to form a new one.

The classical typological species concept is perhaps the most intuitive and familiar; according to this approach, organisms which look similar are grouped into a species. The typological species concept falls short, however, when faced with two clearly separate species which look very similar to one another or when the males and females of a single species look very different from each other, such as in the Eclectus parrot.

In biology courses, students are often taught another definition of species proposed by evolutionary biologist Ernst Mayr in 1942, the biological species concept. According to Mayr, a species is a group of organisms which can or do reproduce with each other but not with other such groups. Since Mayr's work, species concepts have multiplied as the debate about what constitutes a species has waxed and waned. Over 20 different species concepts were put forward in the second half of the century, including the evolutionary species concept, which defines a species as an evolutionary lineage that maintains its role and integrity, the cohesion species concept, in which a species is a group of organisms with cohesion mechanisms that preserve their identity, and the ecological species concept, according to which a species is a lineage which occupies its own niche and evolves separately from other lineages.

Each species concept represents an effort to capture the crucial similarities and differences that group organisms into species, but none has proven entirely satisfactory to date. Most of them stumble when applied to asexual life forms such as bacteria, which is particularly problematic if our goal is to understand life on Earth since it's really their planet, after all. Asexual reproduction is also common amongst plants, fungi, and some animals, so we shouldn't disregard it in our characterization of the living world. Another potentially confounding phenomenon is the transfer of genes between different species ("horizontal gene transfer", which will likely be the subject of a future post on Accumulating Glitches). Horizontal gene transfer can play havoc with the isolation or cohesion mechanisms crucial to many species concepts and has proven more prevalent than previously thought — at least 8% our own genome comes from viruses! Other problems can arise with even seemingly well-behaved species. In the San Gorgonio pass of the Sierra Nevada mountains in California live two subspecies of Song Sparrow which are different enough in their behaviour, appearance, and genetics to qualify as separate species. However, the two populations are connected to each other by a series of different subspecies which can interbreed, creating a continuous chain of genetic exchange. Ring species like the Song Sparrow or the Greenish Warbler pose a challenge to many species concepts, since it's unclear whether they should count as one species (since neighbours in the ring can interbreed) or several (since the ends can't). Finally, different species concepts can lead researchers to group organisms into different species, often in ways that are mutually incompatible.

Given these difficulties, some have argued that species don't really exist, reducing the problem to a debate about an arbitrary classification system. The persistence of the species problem shows that many aren't willing to dismiss it so lightly, though. Likewise, the fact that species are born and can go extinct seems to suggest that they are somehow real, since an abstract class (like "oxygen atoms") can't die out. If all of the oxygen atoms somehow disappeared, oxygen wouldn't 'go extinct'; as soon as a new atom with eight protons was formed, there would be oxygen again. The same can't be said of a species. Once the last dodo died, the species became extinct. Even a creature cloned from recovered dodo cells wouldn't be a dodo, since it would develop and exist in a very different environment. Philosophers Michael Ghiselin and David Hull have argued that species aren't really classes with members (like oxygen) but rather individuals made up of organisms (just as organisms are individuals made up of cells). Species are born, reproduce, and die;. Like individual organisms, they use various mechanisms to maintain their identity throughout their life, which is restricted in both space and time. Other philosophers of science, such as Mark Ereshefsky, have argued for a pluralistic approach to species and species concepts, advocating the existence of different types of species corresponding to various species concepts.

Many people have suggested that the problem with 'species' comes from confusing two different goals: (1) defining what constitutes a species; and (2) delineating how we recognize one. Perhaps the heart of the issue isn't determining what a species is but rather understanding how one comes into being — the very question that motivated Darwin. Many species concepts directly or indirectly address the mechanisms or consequences of speciation, the process by which new species arise. Speciation (which will also be the subject of a future post) can happen in a variety of ways in different groups of organisms, which may help to explain the proliferation of different species concepts. Understanding the factors involved in speciation and their influence on patterns of macroevolution is an important goal of evolutionary biology. Our continuing interest in the origin and extinction of species shows that many biologists implicitly think of them as real entities, even if we can't quite define exactly what they are.

Some questions to consider

  • What do you think 'species' means? Why does it matter?
  • Are species real things or just arbitrary groupings? Are they classes or individuals?
  • In terms of conservation, which is more important: the number of species or the amount of difference between them? How would you measure the difference?
  • Should we abandon the term 'species' and instead talk about 'ecological unit', 'reproductive unit', etc, according to different species concepts?

Further reading
Coyne, JA and Orr, HA Species: Reality and Concepts in Speciation. Sinauer Associates, Sunderland, Mass., 2004. ISBN 978-0-87893-089-0.
Ereshefsky, M. (1992) Eliminative pluralism. Philosophy of Science 59: 671-690.
Hey, J. (2009) Why Should We Care about Species? Nature Education 2(5).
Hull, David (1976) Are species really individuals? Systematic Zoology 25: 174-191.
Patten, Michael and Pruett, Christen (2009). The Song Sparrow, Melospiza melodia, as a ring species: patterns of geographic variation, a revision of subspecies, and implications for speciation. Systematics and Biodiversity 7: 33-62.

Image Credits:
Ecletus parrots (by user Doug Janson) and the ring species schematic (by user g_ambrus) are both from Wikimedia Commons.

May 25, 2013 | 02:38 PM
Posted By:  Sedeer el-Showk
John, I understand your point about the "typological species concept" and I see why you object to the way I've phrased things in the article.

On the other hand, even if it was never really a formal "concept" (in the same way as, say, the biological or cohesion concepts), isn't it useful to use "typological species concept" to describe the mental framework that underlies sorting specimens by observable morphology? That idea -- the link between morphology and "kind" which justifies morphological classification -- is certainly older than Mayr. In the quote at the top of this article, Darwin says "[for] some, resemblance is everything and descent of little weight" -- isn't that basically a typological approach to defining species? Doesn't it also correspond to how many people probably conceive of "species"?

I'm not trying to be difficult or argumentative; I really appreciate having expert input and I hope we can have an enlightening discussion. Thanks for joining us!
May 25, 2013 | 12:51 PM
Posted By:  Sedeer el-Showk
Thanks, Diana! There's definitely a parallel with the way 'gene' is used and, though there may be less of a question about their existence, I think they may be even harder to really define. I've been meaning to write a "What is a gene?'' post on Inspiring Science for a while but haven't gotten around to it yet.

You're right; part of the problem is the way language & science have developed together. I think another aspect is the difference/confusion between the technical and everyday uses of a word. Sometimes the common usage can colour how we handle the technical term, since we want it to correspond to our intuition.

Thanks for sharing your thoughts!
May 24, 2013 | 12:30 AM
Posted By:  John Wilkins
The idea that there ever was a "typological" species "concept" is a mistake. What Mayr did was take what museum taxonomists do, that is, put specimens in drawers according to their observable morphology, which is a practice rather than a "concept" and hypostatise it into a theoretical conception. But no matter what "concept" of species researchers have, including Mayr, they all use this practice to store and retrieve their specimens. It had little theoretical importance until he and others tried to set it up as a contrast to their preferred definition of "species" in order to promote the latter.

I have studied the history of concepts of species (and - warning, self promotion - published a book on the topic) and found that the pre-evolutionary and pre-genetic definitions of species were a lot more complex and subtle than the "essentialism story" that Mayr promoted suggests.
May 23, 2013 | 03:21 PM
Posted By:  Diana Cousminer
Great debut post, Sedeer!

This also brings to mind the big bag of mish-mash concepts that the word "gene" has grown to encompass-- from an intangible unit of inheritance, all the way to a clearly physically defined portion of the genome that encodes a concrete protein. I suppose you can't take the analogy as far as to wonder if genes really exist in the way that species may or may not actually exist, but the actual thing we define with the word "gene" has become rather complex and broad in a similar way. Part of the problem is the co-development of scientific concepts and language to reflect those concepts. Looking back on what we know of science today, we might label or name things differently than history did.

On a side note, I tend to agree with your comment that the categories we place parts of nature into actually reflect something about nature. But maybe I'm just one of those humans who likes things to be neat and orderly. :)
May 23, 2013 | 08:31 AM
Posted By:  Sedeer el-Showk
O.R.: Sure, things don't come in neat little boxes. It's all a messy continuum out there, but I don't think it's necessarily true that we're create the divisions. If we do our job well, the divisions should reflect something about what's actually out there. I would say that chemistry becomes biology when the components aren't just reacting, but also evolving. A new order of complexity emerges, and I don't think that's just something about how we're carving up the world. Different kinds of interactions appear with their own dynamics. The fact that a mouse and a magpie can't mate isn't strictly a chemical problem, nor is flourishes after a mass extinction while another dwindles. I don't think these distinctions are just epistemic; they can also reflect qualitative differences in the world around us. So I guess the question I'm asking is whether 'species' are arbitrary classifications or reflect those kinds of "actual" differences.

Thanks for the thought-provoking comment! :)
May 23, 2013 | 08:28 AM
Posted By:  Sedeer el-Showk
Thanks, Ilona, I'm glad you liked it! That's a great analogy; I wish I'd thought of it for the article. :) I've always been fascinated by stories about neurological damage that makes them unable to recognize objects or people, or able to name and object but not use it (or vice versa).
May 22, 2013 | 07:19 PM
Posted By:  O.R. Pagan
Congrats to you both! I regularly correspond with Sedeer, and now I'll make a point to check out Sarah's blog too!

About the species concept...

This is a situation that happens with science in general. It is a little bit like saying that you are going to examine a cell from a biological vs a chemical point of view. At which point chemistry becomes biology? This is not to say that classification schemes are not useful; quite the contrary.

I guess that what I am trying to say is what I say every semester at the very first lecture of my general Biology class for Bio majors: "Did you know that science does not exist? Nature does, and people invented science so we can divide nature into manageable parts so that we can have an easier time studying it".

May 22, 2013 | 03:05 PM
Posted By:  Ilona Miko
Great post, Sedeer. I couldn't help but think this sounds a lot like the way we think about sensory neuroscience--distinguishing between seeing or hearing something and recognizing/identifying it.
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