"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.