The species problem is the set of questions that arises when biologists attempt to define what a species is. Such a definition is called a species concept; there are at least 26 recognized species concepts. A species concept that works well for sexually reproducing organisms such as birds is useless for species that reproduce asexually, such as bacteria. The scientific study of the species problem has been called microtaxonomy.
One common, but sometimes difficult, question is how best to decide which species an organism belongs to, because reproductively isolated groups may not be readily recognizable, and cryptic species may be present. There is a continuum from reproductive isolation with no interbreeding, to panmixis, unlimited interbreeding. Populations can move forward or backwards along this continuum, at any point meeting the criteria for one or another species concept, and failing others.
The current meaning of the phrase "species problem" is quite different from what Charles Darwin and others meant by it during the 19th and early 20th centuries. For Darwin, the species problem was the question of how new species arose. Darwin was however one of the first people to question how well-defined species are, given that they constantly change.
|“||[...] I was much struck how entirely vague and arbitrary is the distinction between species and varieties||”|
|— Charles Darwin, On the Origin of Species|
The idea that an organism reproduces by giving birth to a similar organism, or producing seeds that grow to a similar organism, goes back to the earliest days of farming. While people tended to think of this as a relatively stable process, many thought that change was possible. The term species was just used as a term for a sort or kind of organism, until in 1686 John Ray introduced the biological concept that species were distinguished by always producing the same species, and this was fixed and permanent, though considerable variation was possible within a species. Carolus Linnaeus (1707–1778) formalized the taxonomic rank of species, and devised the two part naming system of binomial nomenclature that we use today. However, this did not prevent disagreements on the best way to identify species.
"The traditional view, which was developed by Cain, Mayr and Hull in the mid-twentieth century, claims that until the ‘Origin of species’ by Charles Darwin both philosophy and biology considered species as invariable natural kinds with essential features. This ‘essentialism story’ was adopted by many authors, but questioned from the beginning by a minority … when Aristotle and the early naturalists wrote about the essences of species, they meant essential ‘functions’, not essential ‘properties’. Richards pointed out [Richard A. Richards, The Species Problem: A Philosophical Analysis, Cambridge University Press, 2010] that Linnaeus saw species as eternally fixed in his very first publication from 1735, but only a few years later he discovered hybridization as a modus for speciation.
From Darwin to Mayr
Charles Darwin's famous book On the Origin of Species (1859) offered an explanation as to how species evolve, given enough time. Although Darwin did not provide details on how species can split into two, he viewed speciation as a gradual process. If Darwin was correct, then, when new incipient species are forming, there must be a period of time when they are not yet distinct enough to be recognized as species. Darwin's theory suggested that there was often not going to be an objective fact of the matter, on whether there were one or two species.
Darwin's book triggered a crisis of uncertainty for some biologists over the objectivity of species, and some came to wonder whether individual species could be objectively real — i.e. have an existence that is independent of the human observer.
In the 1920s and 1930s, Mendel's theory of inheritance and Darwin's theory of evolution by natural selection were joined in what was called the modern synthesis. This conjunction of theories also had a large impact on how biologists think about species. Edward Poulton anticipated many ideas on species that today are well accepted, and that were later more fully developed by Theodosius Dobzhansky and Ernst Mayr, two of the architects of the modern synthesis. Dobzhansky's 1937 book articulated the genetic processes that occur when incipient species are beginning to diverge. In particular, Dobzhansky described the critical role, for the formation of new species, of the evolution of reproductive isolation.
Mayr's Biological Species Concept
Ernst Mayr's 1942 book was a turning point for the species problem. In it, he wrote about how different investigators approach species identification, and he characterized their approaches as species concepts. He argued for what came to be called the Biological Species Concept (BSC), that a species consists of populations of organisms that can reproduce with one another and that are reproductively isolated from other populations, though he was not the first to define "species" on the basis of reproductive compatibility. For example, Mayr discusses how Buffon proposed this kind of definition of "species" in 1753. Theodosius Dobzhansky was a contemporary of Mayr and the author of a classic book about the evolutionary origins of reproductive barriers between species, published a few years before Mayr's. Many biologists credit Dobzhansky and Mayr jointly for emphasizing reproductive isolation.
After Mayr's book, some two dozen species concepts were introduced. Some, such as the Phylogenetic Species Concept (PSC), were designed to be more useful than the BSC for describing species. Many authors have professed to "solve" or "dissolve" the species problem. Some have argued that the species problem is too multidimensional to be "solved" by any one concept. Since the 1990s, others have argued that concepts intended to help describe species have not helped to resolve the species problem. Although Mayr promoted the BSC for use in systematics, some systematists have criticized it as not operational. For others, the BSC is the preferred definition of species. Many geneticists who work on speciation prefer the BSC because it emphasizes the role of reproductive isolation. It has been argued that the BSC is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection.
Realism, in the context of the species problem, is the philosophical position that species are real mind-independent entities, natural kinds. Mayr, a proponent of realism, attempted to demonstrate species exist as natural, extra-mental categories. He showed for example that the New Guinean tribesman classify 136 species of birds, which Western ornithologists came to independently recognize:
"I have always thought that there is no more devastating refutation of the nominalistic claims than the above mentioned fact that primitive natives in New Guinea, with a Stone Age culture, recognize as species exactly the same entities of nature as western taxonomists. If species were something purely arbitrary, it would be totally improbable for representatives of two drastically different cultures to arrive at the identical species delimitations."
Mayr's argument however has been criticized:
"The fact that independently observing humans see much the same species in nature does not show that species are real rather than nominal categories. The most it shows is that all human brains are wired up with a similar perceptual cluster statistic (Ridley, 1993). On this view we [humans] might have been ‘wired’ differently and different species might now be wired differently from us, so that no one wiring can be said to be ‘true’ or veridical."
Another position of realism is that natural kinds are demarcated by the world itself by having a unique property that is shared by all the members of a species, and none outside the group. In other words, a natural kind possesses an essential or intrinsic feature (“essence”) that is self-individuating and non-arbitrary. This notion has been heavily criticized as essentialist, but modern realists have argued that while biological natural kinds have essences, these need not be fixed and are prone to change through speciation. According to Mayr (1957) reproductive isolation or interbreeding "supplies an objective yardstick, a completely non-arbitrary criterion” and "describing a presence or absence relationship makes this species concept non-arbitrary". The BSC defines species as "groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups". From this perspective, each species is based on a property (reproductive isolation) that is shared by all the organisms in the species that objectively distinguishes them.
Some philosophical variants of nominalism propose that species are just names that people have assigned to groups of creatures but where the lines between species get drawn does not reflect any fundamental underlying biological cut-off point. In this view, the kinds of things that people have given names to, do not reflect any underlying reality. It then follows that species do not exist outside the mind, because species are just named abstractions. If species are not real, then it would not be sensible to talk about "the origin of a species" or the "evolution of a species". As recently at least as the 1950s, some authors adopted this view and wrote of species as not being real.
A counterpoint to the nominalist views in regard to species, was raised by Michael Ghiselin who argued that an individual species is not a type, but rather an actual individual, an actual entity. This idea comes from thinking of a species as an evolving dynamic population. If viewed as an entity, a species would exist regardless of whether or not people have observed it and whether or not it has been given a name.
A popular alternative view, pragmatism, espoused by philosophers such as Philip Kitcher and John Dupre states while species do not exist in the sense of natural kinds, they are conceptually real and exist for convenience and for practical applications. For example, regardless of which definition of species one uses, one can still quantitatively compare species diversity across regions or decades, as long as the definition is held constant within a study. This has practical importance in advancing biodiversity science and environmental science.
Language and the role of human investigators
The nominalist critique of the view that kinds of things exist, raises for consideration the role that humans play in the species problem. For example, Haldane suggested that species are just mental abstractions.
Several authors have noted the similarity between "species", as a word of ambiguous meaning, and points made by Wittgenstein on family resemblance concepts and the indeterminacy of language.
- to categorize and identify organisms;
- to understand the evolutionary processes that give rise to species.
Under the first view, species appear to us as typical natural kinds, but when biologists turn to understand species evolutionarily they are revealed as changeable and without sharp boundaries. Hey argued that it is unrealistic to expect that one definition of "species" is going to serve the need for categorization and still reflect the changeable realities of evolving species.
Pluralism and monism
Many approaches to the species problem have attempted to develop one single common conception of what species are and of how they should be identified. It is thought that, if such a monistic description of species could be developed and agreed upon, then the species problem would be solved. In contrast, some authors have argued for pluralism, claiming that biologists cannot have just one shared concept of species, and that they should accept multiple, seemingly incompatible ideas about species. David Hull however argued that pluralist proposals were unlikely to actually solve the species problem.
"No term is more difficult to define than "species," and on no point are zoologists more divided than as to what should be understood by this word." Nicholson (1872).
"Of late, the futility of attempts to find a universally valid criterion for distinguishing species has come to be fairly generally, if reluctantly, recognized" Dobzhansky (1937).
"The concept of a species is a concession to our linguistic habits and neurological mechanisms" Haldane (1956).
"An important aspect of any species definition whether in neontology or palaeontology is that any statement that particular individuals (or fragmentary specimens) belong to a certain species is an hypothesis (not a fact)" Bonde (1977).
"The species problem is the long-standing failure of biologists to agree on how we should identify species and how we should define the word 'species'." Hey (2001).
"First, the species problem is not primarily an empirical one, but it is rather fraught with philosophical questions that require — but cannot be settled by — empirical evidence." Pigliucci (2003).
"We show that although discrete phenotypic clusters exist in most [plant] genera (> 80%), the correspondence of taxonomic species to these clusters is poor (< 60%) and no different between plants and animals. ... Contrary to conventional wisdom, plant species are more likely than animal species to represent reproductively independent lineages." Rieseberg et al. (2006).
- Wilkins, John S. (1 October 2006). "A List of 26 Species Concepts". Science Blogs.
- Mayr, Ernst (1982). "Chapter 6: Microtaxonomy, the science of species". The Growth of Biological Thought: Diversity, Evolution, and Inheritance. Belknap Press of Harvard University Press. ISBN 9780674364462.
- Robson, G. C. (1928). The Species Problem: an Introduction to the Study of Evolutionary Divergence in Natural Populations. Edinburgh: Oliver and Boyd.
- Darwin, C. (1859). On the origin of species by means of natural selection. London: Murray. p. 48. ISBN 978-84-206-5607-6.
- Wilkins, John S. (2006). "Species, Kinds, and Evolution". Reports of the National Center for Science Education. Retrieved 2009-09-24.
- Wilkins, John S. (May 10, 2009). "The first biological species concept : Evolving Thoughts". Archived from the original on May 15, 2009. Retrieved 2009-09-24.
- Britton, N.L. (April 1908). "The taxonomic aspect of the species question". The American Naturalist. 42 (496): 225–242. doi:10.1086/278927.
- Mayr, E. (1982). The Growth of Biological Thought. Cambridge, MA: Harvard University Press. ISBN 978-0-674-36445-5.
- Haveman, R. (2013). "Freakish patterns – species and species concepts in apomicts". Nordic Journal of Botany. 31 (3): 257–269. doi:10.1111/j.1756-1051.2013.00158.x.
- Johnson, D.S. (April 1908). "[Introduction]". The American Naturalist. 42 (496): 217. doi:10.1086/278925.
- Bailey, L.H. (December 1896). "The philosophy of species-making". Botanical Gazette. 22 (6): 454–462. doi:10.1086/327442.
- Mallet, James (December 2003). "Perspectives Poulton, Wallace and Jordan: how discoveries in Papilio butterflies led to a new species concept 100 years ago". Systematics and Biodiversity. 1 (4): 441–452. doi:10.1017/S1477200003001300.
- Dobzhansky, T. (1937). Genetics and the Origin of Species. New York: Columbia University Press. p. 310. ISBN 978-0-231-05475-1.
- Mayr, Ernst (1942). Systematics and the origin of species from the viewpoint of a zoologist. New York: Columbia University Press. ISBN 978-0-674-86250-0.
- Mallet, James (November 2001). "The speciation revolution" (PDF). Journal of Evolutionary Biology. 14 (6): 887–888. doi:10.1046/j.1420-9101.2001.00342.x.
- Coyne, Jerry (1994). "Ernst Mayr and the origin of species". Evolution. 48 (1): 19–30. doi:10.2307/2409999. JSTOR 2409999.
- Pigliucci, Massimo (June 2003). "Species as family resemblance concepts: The (dis-)solution of the species problem?". BioEssays. 25 (6): 596–602. doi:10.1002/bies.10284. PMID 12766949.
- Ghiselin, Michael (December 1974). "A radical solution to the species problem". Systematic Zoology. 23 (4): 536–544. doi:10.2307/2412471. JSTOR 2412471.
- de Queiroz, K. (December 2005). "Different species problems and their resolution". BioEssays. 27 (12): 1263–1269. doi:10.1002/bies.20325. PMID 16299765.
- Hey, J. (1997). "A reduction of "species" and a resolution of the species problem". Rutgers University, Department of Genetics. Retrieved 2007-12-25.
- Ridley, Mark (January 1989). "The cladistic solution to the species problem". Biology and Philosophy. 4 (1): 1–16. doi:10.1007/BF00144036.
- Stamos, D.N. (2003). The species problem: Biological species, ontology, and the metaphysics of biology. Lanham: Lexington Books. ISBN 978-0-7391-0503-0.
- Vrana, P.; Wheeler, W. (1992). "Individual organisms as terminal entities: Laying the species problem to rest". Cladistics. 8 (1): 67–72. doi:10.1111/j.1096-0031.1992.tb00051.x.
- Hey, J. (2001). Genes categories and species. New York, NY: Oxford University Press. ISBN 978-0-19-514477-2.
- Endler, J.A. (1989). "Conceptual and other problems in speciation". In Otte, D.; Endler, J.A. (eds.). Speciation and its consequences. Sunderland, Mass.: Sinauer Associates. pp. 625–648. ISBN 978-0-87893-658-8.
- de Queiroz, K. (1998). "The general lineage concept of species: Species criteria and the process of speciation". In Howard, D.J.; Berlocher, S.H. (eds.). Endless forms: Species and speciation. New York: Oxford University Press. pp. 57–75. ISBN 978-0-19-510901-6.
- Miller, W. (December 2001). "The structure of species, outcomes of speciation and the 'species problem': Ideas for paleobiology". Palaeogeography, Palaeoclimatology, Palaeoecology. 176 (1): 1–10. Bibcode:2001PPP...176....1M. doi:10.1016/S0031-0182(01)00346-7.
- Hey, J. (August 2006). "On the failure of modern species concepts". Trends in Ecology & Evolution. 21 (8): 447–450. doi:10.1016/j.tree.2006.05.011. PMID 16762447.
- Hull, D.L. (1999). "On the plurality of species: Questioning the party line". In Wilson, R.A. (ed.). Species: New Interdisciplinary Essays. Cambridge, MA: MIT Press. pp. 23–48. ISBN 978-0-262-73123-2.
- Wheeler, QD; Meier, R (2000). Species concepts and phylogenetic theory: A debate. New York: Columbia University Press. ISBN 978-0-231-10143-1.
- Zink, R.M.; McKitrick, M.C. (1995). "The debate over species concepts and its implications for ornithology". The Auk. 112 (3): 701–719.
- Levin, D.A. (April 1979). "The nature of plant species". Science. 204 (4391): 381–384. Bibcode:1979Sci...204..381L. doi:10.1126/science.204.4391.381. PMID 17757999.
- Sokal, RR; Crovello, TJ (March–April 1970). "The biological species concept: A critical evaluation". The American Naturalist. 104 (936): 127–153. doi:10.1086/282646.
- Coyne, Jerry; Orr, HA (2004). Speciation. Sunderland, Mass.: Sinauer Associates. ISBN 978-0-87893-089-0.
- Hopf, F.A.; Hopf, F.W. (1985). The role of the Allee effect on species packing. Theor. Pop. Biol. 27, 27-50.
- Bernstein, H.; Byerly, H.C.; Hopf, F.A.; Michod, Richard E. (December 1985). "Sex and the emergence of species". J. Theor. Biol. 117 (4): 665–90. doi:10.1016/S0022-5193(85)80246-0. PMID 4094459.CS1 maint: Uses authors parameter (link)
- Bernstein, Carol; Bernstein, Harris (1991). Aging, sex, and DNA repair. Boston: Academic Press. ISBN 978-0-12-092860-6.
- Michod, Richard E. (1995). Eros and evolution: a natural philosophy of sex. Reading, Mass: Addison-Wesley. ISBN 978-0-201-44232-8.
- Mayr, Ernst (1988). Toward a New Philosophy of Biology. Harvard University Press. p. 317. ISBN 9780674896666.
- Stamos, D. N. (2003). The Species Problem. Lexington Books. p. 95.
- Okasha, S. 2002. Darwinian metaphysics: Species and the question of essentialism. Synthese. 131: 191–213.
- Gregg, J.R. (November–December 1950). "Taxonomy, language and reality". The American Naturalist. 84 (819): 419–435. doi:10.1086/281639.
- Burma, B.H. (1954). "Reality, existence, and classification: A discussion of the species problem". In Slobodchikoff, C.N. (ed.). Concepts of species. Stroudsburg, PA: Dowden, Hutchinson & Ross. pp. 193–209.
- Ghiselin, M.T. (1997). Metaphysics and the origin of species. Albany, NY: State University of New York Press. ISBN 978-0-7914-3468-0.
- Dupré, J. (2001). "In defence of classification". Studies in History and Philosophy of Biological and Biomedical Sciences. 32: 203–219.
- Haldane, J.B.S. (1956). "Can a species concept be justified?". In Sylvester-Bradley, P.C. (ed.). The species concept in paleontology. London: Systematics Association. pp. 95–96.
- Hull, D.L. (September 1978). "A matter of individuality" (PDF). Philosophy of Science. 45 (3): 335–360. doi:10.1086/288811.
- Jardine, N. (March 1969). "A logical basis for biological classification". Systematic Zoology. 18 (1): 37–52. doi:10.2307/2412409. JSTOR 2412409.
- Hey, J. (July 2001). "The mind of the species problem". Trends in Ecology and Evolution. 16 (7): 326–329. doi:10.1016/S0169-5347(01)02145-0. PMID 11403864.
- Dupré, J. (1999). "On the impossibility of a monistic account of species". In Wilson, R.A. (ed.). Species: New Interdisciplinary Essays. Cambridge, MA: MIT Press. pp. 3–22. ISBN 978-0-262-73123-2.
- Mishler, B.D.; Donoghue, M.J. (December 1982). "Species concepts: A case for pluralism". Systematic Zoology. 31 (4): 491–503. doi:10.2307/2413371. JSTOR 2413371.
- Ereshefsky, M. (December 1992). "Eliminative pluralism". Philosophy of Science. 59 (4): 671–690. doi:10.1086/289701.
- Pigliucci, Massimo (2005). "Wittgenstein Solves (Posthumously) the Species Problem". Philosophy Now. Retrieved 2 December 2016.
- Nicholson, H.A. (1872). A manual of zoology. New York: Appleton and Company. p. 20.
- Bonde, N. (1977). "Cladistic classification as applied to vertebrates". In Hecht, M.K.; Goody, P.C.; Hecht, B.M (eds.). Major Patterns in Vertebrate Evolution. New York: Plenum Press. pp. 741–804.
- Rieseberg, L.H.; Wood, T.E.; Baack, E.J. (2006). "The nature of plant species". Nature. 440 (524–527): 524–527. Bibcode:2006Natur.440..524R. doi:10.1038/nature04402. PMC 2443815. PMID 16554818.
|Wikispecies has information related to Species|