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Species Concepts

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The species concept “problem” has pervaded for many years and will not be resolved anytime soon, if ever. The problem, of course, being that no two scientists will agree on universal definitions of what the darn things are! Taxonomist are exceptional argumentative and someone will undoubtedly disagree with everything in this article!

Species concepts were first defined based on morphological traits. Linneaus, being limited by technology at the time, used the “eyeball method” to study things – meaning he looked at them and described what he saw. This is formalized as the morphological or typological species concept (Cracraft, 2000; Mayr, 1996), and many biologists are just fine with this. It looks different, ergo it is and any distinguishing characters that could be observed, counted and measured were enough to define new species.

Characters are delimited by the practicing taxonomist and thus not all-inclusive of the whole organism. Morphological characters are typically those most easily observed, although the level of observation (i.e. from external features to cellular features) can have large effects on species identification and definition. For instance, Winston (1999) describes a case where closer observation of a western Atlantic species of the hermit crab hitch-hiking hydroid Hydractinia echinata, typically found off the coasts of Europe, resulted in the description of two additional species based on previously “hidden”, or non-scrutinized, morphological and ecological characters (Buss and Yund, 1989). Similar case are all too common. The closer we peer, the more we find.

The typological species concept idealized a species into an individual that represented a character or suite of characters that differentiated it from all other individuals. Thus, followers of this concept were forced to ignore population level variation. This plasticity of traits causes confusion and obscures the nature of a species particular adaptations. An extreme, but really fascinating, example comes from deep-sea hydrothermal vents off the coast of the Pacific northwest. A large polychaete tubeworm, Ridgeia piscesae, was originally described as two species due to two very unique morphotypes (see image below). Hydrothermal vent tubeworms are known to harbor symbiotic bacteria which use hydrogen sulfide as a chemical energy source, which is readily abundant coming out of the vents.

Ridgeia piscesae, from Carney et al. 2007. a) generalized body form, b) short-fat morphotype, c) long-skinny morphotype.

Despite living within a stone throw of each other, populations of the “short-fat” and “long-skinny” tubeworm have completely different phenotypes. It was a few years later that with a suite of nuclear and mitochondrial genetic markers that it was realized that the two morphotypes were genetically indistinguishable. This phenotypic plasticity is the result of differential gene expression related to which environment the larvae settle in (Carney et al. 2007): active black smoker chimneys, which are characterized by higher hydrothermal flow, higher temperatures and greater sulfide concentrations or diffuse-flow hydrothermal fields. Even though the morphology is so different they were combined into one species.

Mayr (1942) brought the species concept from the individual level to the population level by defining species as discrete populations of individuals that are reproductively isolated, or unable to. A problem with this view of species is that data on inter-breeding are typically not known and museum specimens are often collected with disregard to such data (Wheeler, 1999). Mallet (1995) even went so as to call reproductive isolation a useless concept because it cannot be tested. Others have countered that all species concepts are inherently untestable by experimentation or observation (Coyne and Orr, 2004). Claims of biological species are often typological species in practice.

Mallet (1995) defined the genotypic cluster species concept to refute some of the pitfalls of the biological species concept and incorporate additional knowledge from genetics in terms of ‘identifiable genotypic clusters’ with no appreciable heterozygotes. Coyne and Orr (2004) argued that the genotypic clustering species concept focuses on identification of species and not the origin of species, is not conservative enough, and will over-recognize species in sympatry compared to the biological species concept. Furthermore, they argue that since the genotypic clustering concept is non-hierarchical, it doesn’t reflect the hierarchical nature of evolution and confuses polymorphic forms and Batesian mimics without introducing a reproductive criterion (Coyne and Orr, 2004).

While the biological species concept emphasizes isolating mechanisms which separate members of a species, the recognition (Patterson, 1985) and cohesion (Templeton, 1989) species concepts emphasize keeping members of a species cluster intact. The recognition species concept focuses on a shared fertilization system between individuals. Thus, it can only consider barriers to fertilization as modes of speciation. Coyne et al. (1988) considered this to be a subset of the biological species concept. Templeton (1989), on the other hand, contended that the advantage of the cohesion species concept was its emphasis on mechanisms that enforced gene flow between populations. This made it superior to the biological species concept in dealing with asexual and hybridized sympatric clusters that maintained their identities. Harrison (1998) brought a particular valid criticism to the approach of cohesion: “… life cycles and habitat associations have not been molded by selection for the purpose of ‘cohesion’.” (pg. 25). That is to say that selection appears to be a non-cohesive force by definition.

There are other concepts as well, Mayden (1997) lists 22-24 different conceptualizations and philosopher of science John Wilkins* lists 26. The above concepts view the species as the end-point of evolution, without considering the historical nature of the process of evolution. Hennig (1966) recognized this fact and argued for a temporal component to systematic theory which he termed phylogenetic systematics. Though many subsequent authors agree with Hennig in the use of a phylogenetic concept of species, several authors disagree on the particulars. This has led to differences in interpretation of what a species is and how species are related to one another. The Hennigian species concept incorporated the interbreeding model of a biological species concept, but with a historical component. This was modified by Willmann (1986) to specifically state that species are reproductively isolated and originate via a stem species branching off into two new species. The stem species, by definition, ceases to exist by way of either extinction of speciation. The latter point is important to proponents of this species concept because with dissolution of the stem species, monophyly (a species and all of its descendants) can be maintained.

Other authors have other interpretations of what a phylogenetic species concept is. The main concepts differ between whether species are viewed as irreducible clusters that are diagnostically distinct from other clusters (Wheeler and Nixon, 1990), as exclusive monophyletic units (de Queiroz and Donoghue, 1988), or as a group of organisms whose genes have more recently coalesced with each other relative to organisms outside that group and containing no exclusive group within it (Baum and Donoghue, 1995). At first glance, the species concept proposed by Baum and Donoghue (1995) appears to most accurately reflect evolutionary history. Upon closer inspection, it is nearly impossible to have complete knowledge of the evolutionary history of all genes in all the organisms in an analysis.

In practice, purveyors or this species concept have often used one or a few loci in delimiting species (Coyne and Orr, 2004). Shaw (2001) relaxed this extreme assumption to “greater than 50%”, meaning that a species is delimited if most of the genes have coalesced. While operationally useful, this definition may be just as arbitrary as using diagnostic morphological characters. Describing species as exclusive monophyletic units seeks to overcome such arbitrariness and potentially has the greatest power of all species concepts discussed here to represent a true phylogeny. But it is known that phylogenies based upon genes do not necessary reflect a species true phylogeny, which may never be known with certainty (Avise and Wollenberg, 1997).

Proponents of the evolutionary species concept claim theirs can be mos universally applied relative to all others. Wiley (1978) purported that a species concept must satisfy five criteria: universal validity, allow for testable hypotheses, include valid special case species definitions, specify what types of species origins are possible or not possible, and be “capable of dealing with species as spatial, temporal, genetic, epigenetic, ecological, physiological, phenetic, and behavioral entities” (pg. 18). Modifying an earlier concept from Simpson (1961), Wiley states: “A species is a single lineage of ancestral descendant populations of organisms which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate.” Wiley’s modification removed the need for species to be changing, as originally defined by Simpson (1961). The criticisms levied against the evolutionary species concept appear to be more about lack of operational criteria to delimit species spatio-temporally (Wheeler and Meier, 2000).

An additional outcome of the species concept debate is the view that only populations are real and that species are artifacts (Brooks and McLennan, 1999). Darwin (1859) believed that species were arbitrary constructs of the taxonomist for convenience, while Mayr believed that species were real entities (Mayr, 1996). Levin (1979) championed the view that species are the empirical units of evolution and ecology, while evolutionary species concept supporters argued that if monophyletic groups are real then so are species (Wiley and Mayden, 2000b). Other interpretations span the range between arbitrary constructs and representing real natural entities. Furthermore, it seems as if every taxonomist is trying to find that one perfect species concept that works for all scenarios and types of organisms (Hey, 2001; Hey, 2006; Wheeler and Meier, 2000).

Several authors have advocated for pluralism, or the use of multiple species concepts (Mayden, 1997; Mayden, 1999). Different situations or questions may call for using different species concepts. Hey (2006) cautions against this though, stating it doesn’t help to settle anything regarding the species debate. Fitzhugh (2006) treads close to a pluralistic view species in his advocacy for a “requirement of total evidence” approach to systematics. This requirement suggests that any evidence relevant to the species question needs to be considered. Total evidence could include morphological character information, genetic characters, behavioral traits and more. While perhaps not setting out to satisfy multiple species concepts, the requirement for total evidence may do just that along the way.

As with many biologists studying biodiversity and other taxonomists, I feel unsatisfied by the current plethora of species concepts. Those attempting to be generally applied, such as the phylogenetic, Hennigian and evolutionary species concepts, tend to inflate biodiversity by elevating subspecies, or perhaps even distinct populations to species status. I’m unsure whether this inflation is due to the flexibility in the definitions, viewing species as lineages or clusters, or the taxonomic practice of the practitioners.

Those attempting to restrict the definition or discount evolutionary processes, like the biological species concept, tend to underestimate biodiversity. Additionally, the biological, recognition and cohesive species concepts cannot satisfactorily deal with asexual organisms and are unable to be applied broadly within only the animal kingdom. While reproductive isolation may be an important criterion for speciation to occur, other mechanisms are known such as hybridization, recombination, horizontal gene transfer (can occur between a eukaryote and prokaryote as shown in recent research: see Hotopp et al., 2007) to name but a few. Reproductive isolation may also be a product of speciation and not a causal mechanism (Mishler and Donoghue, 1982; Wiley and Mayden, 2000a).

As do the proponents of the evolutionary species concepts, I believe that species are real, are individuals and ancestral species do not need to become extinct during a speciation event. I view the act of formally describing a species as formulating a hypothesis about that species’ unique suite of characters and the evolutionary history of the retention, loss or modification of those characters over time. I believe the evolutionary species comes closest to my views of what species are. I agree that species are entities of organisms that maintain its identity throughout time and space from other entities. This is a key factor to species being operationally and pragmatically useful. I understand this might not sit well with philosophers and some other evolutionary biologists. Some of the phylogenetic species concepts recognize too many species, whereas some of reproductive isolationist concepts ignore asexual and allopatric species. The latter is unacceptable and the former may give a misleading foundation for other areas of study (i.e. biodiversity ecology) to test hypotheses on.

* Though I haven’t read the book, John Wilkins is an authority on species concepts and recently published Species: A History of an Idea, which promises to be informative and interesting!


Avise, J. C., and K. Wollenberg. 1997. Phylogenetics and the origin of species. PNAS 94:7748-7755.

Baum, D. A., and M. J. Donoghue. 1995. Choosing among alternative phylogenetic species concepts. Systematic Botany 20:560-573.

Brooks, D. R., and D. A. McLennan. 1999. Species: turning a conundrum into a research program. Journal of Nematology 31:117-133.

Buss, L. W., and P. O. Yund. 1989. A sibling species group of Hydractinia in the north-eastern United States. Journal of the Marine Biological Association of the UK 69:857-874.

Carney, S.L., J.F. Flores, K.M. Orobona,D.A. Butterfield, C.R. Fisher, S.W. Schaeffer. 2007. Environmental differences in hemoglobin gene expression in the hydrothermal vent tubeworm, Ridgeia piscesae. Comparative Biochemistry and Physiology, Part B 146:326–337.

Coyne, J. A., and H. A. Orr. 2004. Speciation. Sinauer Associates, Inc., Sunderland.

Coyne, J. A., H. A. Orr, and D. J. Futuyma. 1988. Do we need a new species concept? Systematic Zoology 37:190-200.

Cracraft, J. 2000. Species concepts in theoretical and applied biology: a systematic debate with consequences. Pages 3-14 in Species Concept and Phylogenetic Theory: A Debate (Q. D. Wheeler, and R. Meier, eds.). Colombia University Press, New York.

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Fitzhugh, K. 2006. The ‘requirement of total evidence’ and its role in phylogenetic inference. Biology & Philosophy, 21:309-351.

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Levin, D. A. 1979. The nature of plant species. Science 204:381-384.

Mallet, J. 1995. A species definition for the modern synthesis. Trends in Ecology & Evolution 10:294-299.

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Kevin Zelnio About the Author: Kevin has a M.Sc. degree in biology from Penn State, a B.Sc. in Evolution and Ecology from University of California, Davis, and has worked at as a researcher at several major marine science institutions. His broad academic research interests have encompassed population genetics, biodiversity, community ecology, food webs and systematics of invertebrates at deep-sea chemosynthetic environments and elsewhere. Kevin has described several new species of anemones and shrimp. He is now a freelance writer, independent scientist and science communications consultant living near the Baltic coast of Sweden in a small, idyllic village.

Kevin is also the assistant editor and webmaster for Deep Sea News, where he contributes articles on marine science. His award-winning writing has been appeared in Seed Magazine, The Open Lab: Best Writing on Science Blogs (2007, 2009, 2010), Discovery Channel, ScienceBlogs, and Environmental Law Review among others. He spends most of his time enjoying the company of his wife and two kids, hiking, supporting local breweries, raising awareness for open access, playing guitar and songwriting. You can read up more about Kevin and listen to his music at his homepage, where you can also view his CV and Résumé, and follow him twitter and Google +. Editor's Selection Posts on EvoEcoLab!

Follow on Twitter @kzelnio.

The views expressed are those of the author and are not necessarily those of Scientific American.

Comments 8 Comments

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  1. 1. yarberry 3:43 pm 04/20/2012

    I tend to side more with Darwin in that the concept of species is an arbitrary construct. I would take it further in that our ‘need’ to classify is culturally based and the resultant categories are culturally prejudicial. Our (Western European) worldview has become dominant and so our classification of the world pushes others aside. What would the idea of species look like if it was based upon a Navajo worldview or Coastal Salish?

    We have difficulty making our categories work because they are artificial in many respects. Sure the science is good and does a nice job of identifying differences between populations, but it is still based upon a particular view of our world.

    How important is it to identify various species? Is it just to make it easier to track how many are driven to extinction?

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  2. 2. Kevin Zelnio in reply to Kevin Zelnio 5:22 pm 04/20/2012

    I agree to an extent. Species are arbitrary constructs, which is why I iterated the operational utility aspect. It is very useful for an ecologist to quantify species and leads to many insights about how the world works. Every culture has classified the flora and fauna around them throughout history. I think the fact that the western classification scheme is dominant is also in itself arbitrary. The great point in my opinion is that scientists are all on same page. The linnaean system works because we want it to. The threshold for allowing new classifications in is very high because so many scientists are on the same page now. Phylocode is a more recent example, being greatly ignored because the inertia for change is stacked against them.

    I think it is very important to be able identify species. Species are an indicator of many things in ecosystems. Understanding how diverse a particular patch is helps us to make predictions on how changes will affects that patch and surrounding systems. And there is many practical reasons we need to be interested in diversity (another post perhaps). How go extinct is only a small part of the equation.

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  3. 3. StephenJ 7:45 pm 04/20/2012

    What is the question again? Is this like arguing about what makes a planet? What is the operational consequence of calling a particular group of organisms a species or just part of some greater group that constitutes a species? The different criterion you mention simply reflect different ways of using the word species to serve different purposes. The difficulty seems to be due to the fact that relatedness creates a bush kind of pattern of descent, compounded by the twigs recombining and branches apparently terminating. A population may all be of common descent and therefore share a lot of features. But the key to evolution is the production of diversity. So if there is any point where the idea of species actually touches reality, it is at the point where mutation occurs. The question is a not what to do with a horizontal gene transfer between a eukaryote and prokaryote. The question is whether that occurrence produces more diversity than existed previously. Where that added diversity becomes speciation is probably the point where the change is irrevocable.

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  4. 4. CherryBombSim 11:55 pm 04/20/2012

    Style criticism: Your last paragraph has too many “I believe…I agree…I understand.”

    On the whole, I liked the essay, though. The fundamental concept is that a species is whatever we want it to mean, as long as it is a useful generalization. Evolution is like the comic-strip Calvin, who makes up his own rules as he plays the game. It is just doing its own thing, and we are only pasting cladistic labels on it to organize our own thinking.

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  5. 5. Panimerus 2:06 pm 04/21/2012

    Interesting read!
    A couple of comments:
    I would argue that species are not arbitrary – there really is a diagnosable, discontinuous variation in organismal traits, and this is an empirical phenomenon. If it was not, we could expect different methods of species delimitation to produce very different hypotheses about species identity. This is not the case; evidence from morphology, DNA-taxonomy, morphometrics, ecological niche modelling etc. produce remarkably consistent results.
    Also, species are not necessarily monophyletic. See Rieppel (2010, DOI: 10.1111/j.1439-0469.2009.00545.x).

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  6. 6. Kevin Zelnio in reply to Kevin Zelnio 10:50 am 04/23/2012

    @StephenJ: I think I understand you right. Are you saying that Evolution is the product of there being diversity? Or that diversity is the product of evolution? Mutations arise every generation, but its those mutations with staying power within populations that are key to initiating the process of speciation. Some of these species concepts more or less view species as the end of point or evolution whereas other tends to view species along a continuum of evolution. Again though, it comes down to what we, as scientists, can detect!

    @CherryBombSim: Not sure why that is a criticism, since I wanted to share what my thoughts were, but yes I believe that the point is that it is a useful designation. And not just to evolutionary biologists, but ecologists, physiologists and all other specialities look to taxonomists and systematicists to define what species are for their own uses. Taxonomy is almost a service industry really! It’s too bad that funders and employers do not realize this though…

    @Panimerus: I agree that most animals this is the case. The marine invertebrates I work with are certainly unique species, from anything else ever described. But the species concept problem has difficulty when universally applied. Plants and bacteria, for instance, can be very problemsome for definitions based on mammals and fish! That is why I advocate for pluralism of species concepts, finding the magic fit is just not possible because life is so amazingly diverse! Also, for every “consistent result” you have an equal an opposite inconsistent result lol. For instance, many elephants or giraffes have been considered one species for a long time, but by applying a more phylogenetic species concept (or even a population genetics species concept), some authors think there are several species. (I don’t have an opinion either way, but tend to be more skeptical of concepts that encourage splitting).

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  7. 7. poihths 9:06 am 04/24/2012

    Concepts like “species” that have clear centers and fuzzy edges are always problematical. Is there any real value gained by noting that the third hair on the fifth leg of the bug is slightly differently shaped in two different bugs? Are we not better served, do we not get better value, from studying biological variations as continuous phenomena rather than discrete ones?

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  8. 8. Kevin Zelnio in reply to Kevin Zelnio 10:05 am 04/24/2012

    @poihths: I think the answer is yes to both questions. It’s never as easy as counting the ass hairs on a shrimp (as my former advisor would snicker at me), but a suite of morphological characters that argue for separation of an individual from known species and the aggregation of a number of individuals under a new or known species. The typological concepts cover this and define a range of acceptable variation to be enveloped under a species name. Other biologists needs a definition of what a species *is*. Evolutionary biologists are more comfortable with the continuous phenomena of species, though. But we do a disservice to the greater scientific community if we don’t give species a proper way to be quantified.

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